These terms are commonly used in information about the MySQL database server. This glossary originated as a reference for terminology about the InnoDB storage engine, and the majority of definitions are InnoDB-related.
A
An acronym standing for atomicity, consistency, isolation, and durability. These properties are all desirable in a database system, and are all closely tied to the notion of a transaction. The transactional features of InnoDB adhere to the ACID principles.
Transactions are atomic units of work that can be committed or rolled back. When a transaction makes multiple changes to the database, either all the changes succeed when the transaction is committed, or all the changes are undone when the transaction is rolled back.
The database remains in a consistent state at all times -- after each commit or rollback, and while transactions are in progress. If related data is being updated across multiple tables, queries see either all old values or all new values, not a mix of old and new values.
Transactions are protected (isolated) from each other while they are in progress; they cannot interfere with each other or see each other's uncommitted data. This isolation is achieved through the locking mechanism. Experienced users can adjust the isolation level, trading off less protection in favor of increased performance and concurrency, when they can be sure that the transactions really do not interfere with each other.
The results of transactions are durable: once a commit operation succeeds, the changes made by that transaction are safe from power failures, system crashes, race conditions, or other potential dangers that many non-database applications are vulnerable to. Durability typically involves writing to disk storage, with a certain amount of redundancy to protect against power failures or software crashes during write operations. (In InnoDB, the doublewrite buffer assists with durability.)
See also atomic, commit, concurrency, doublewrite buffer, isolation level, locking, rollback, transaction.
An algorithm for InnoDB tables that smooths out the I/O overhead introduced by checkpoints. Instead of flushing all modified pages from the buffer pool to the data files at once, MySQL periodically flushes small sets of modified pages. The adaptive flushing algorithm extends this process by estimating the optimal rate to perform these periodic flushes, based on the rate of flushing and how fast redo information is generated. First introduced in MySQL 5.1, in the InnoDB Plugin.
See also buffer pool, checkpoint, data files, flush, InnoDB, page, redo log.
An optimization for InnoDB tables that can speed up lookups
using = and IN operators,
by constructing a hash index in
memory. MySQL monitors index searches for InnoDB tables, and if
queries could benefit from a hash index, it builds one
automatically for index pages
that are frequently accessed. In a sense, the adaptive hash
index configures MySQL at runtime to take advantage of ample
main memory, coming closer to the architecture of main-memory
databases. This feature is controlled by the
innodb_adaptive_hash_index
configuration option. Because this feature benefits some
workloads and not others, and the memory used for the hash index
is reserved in the buffer pool,
typically you should benchmark with this feature both enabled
and disabled.
The hash index is always built based on an existing InnoDB secondary index, which is organized as a B-tree structure. MySQL can build a hash index on a prefix of any length of the key defined for the B-tree, depending on the pattern of searches against the index. A hash index can be partial; the whole B-tree index does not need to be cached in the buffer pool.
In MySQL 5.6 and higher, another way to take advantage of fast
single-value lookups with InnoDB tables is to use the
memcached interface to InnoDB.
See Section 14.2.10, “InnoDB Integration with memcached” for details.
See also B-tree, buffer pool, hash index, memcached, page, secondary index.
Acronym for adaptive hash index.
See also adaptive hash index.
Acronym for asynchronous I/O. You might see this acronym in InnoDB messages or keywords.
See also asynchronous I/O.
The code name for the original InnoDB file format. It supports the redundant and compact row formats, but not the newer dynamic and compressed row formats available in the Barracuda file format.
If your application could benefit from InnoDB table
compression, or uses BLOBs or
large text columns that could benefit from the dynamic row
format, you might switch some tables to Barracuda format. You
select the file format to use by setting the
innodb_file_format option
before creating the table.
See also Barracuda, compact row format, compressed row format, dynamic row format, file format, innodb_file_format, redundant row format.
A set of functions or procedures. An API provides a stable set of names and types for functions, procedures, parameters, and return values.
When a backup produced by the MySQL
Enterprise Backup product does not include the most
recent changes that occurred while the backup was underway, the
process of updating the backup files to include those changes is
known as the apply step. It is
specified by the apply-log option of the
mysqlbackup command.
Before the changes are applied, we refer to the files as a raw backup. After the changes are applied, we refer to the files as a prepared backup. The changes are recorded in the ibbackup_logfile file; once the apply step is finished, this file is no longer necessary.
See also hot backup, ibbackup_logfile, MySQL Enterprise Backup, prepared backup, raw backup.
Metdata for ARCHIVE tables. Contrast with .ARZ file. Files with this extension are always included in backups produced by the mysqlbackup command of the MySQL Enterprise Backup product.
See also .ARZ file, MySQL Enterprise Backup, mysqlbackup command.
Data for ARCHIVE tables. Contrast with
.ARM file. Files with this
extension are always included in backups produced by the
mysqlbackup command of the
MySQL Enterprise Backup
product.
See also .ARM file, MySQL Enterprise Backup, mysqlbackup command.
A type of I/O operation that allows other processing to proceed before the I/O is completed. Also known as non-blocking I/O and abbreviated as AIO. InnoDB uses this type of I/O for certain operations that can run in parallel without affecting the reliability of the database, such as reading pages into the buffer pool that have not actually been requested, but might be needed soon.
Historically, InnoDB has used asynchronous I/O on Windows
systems only. Starting with the InnoDB Plugin 1.1, InnoDB uses
asynchronous I/O on Linux systems. This change introduces a
dependency on libaio. On other Unix-like
systems, InnoDB uses synchronous I/O only.
See also buffer pool, non-blocking I/O.
In the SQL context, transactions are units of work that either succeed entirely (when committed) or have no effect at all (when rolled back). The indivisible (“atomic”) property of transactions is the “A” in the acronym ACID.
See also ACID, commit, rollback, transaction.
Special instructions provided by the CPU, to ensure that critical low-level operations cannot be interrupted.
A property of a table column (specified by the
AUTO_INCREMENT keyword) that automatically
adds an ascending sequence of values in the column. InnoDB
supports auto-increment only for primary
key columns.
It saves work for the developer, not to have to produce new unique values when inserting new rows. It provides useful information for the query optimizer, because the column is known to be not null and with unique values. The values from such a column can be used as lookup keys in various contexts, and because they are auto-generated there is no reason to ever change them; for this reason, primary key columns are often specified as auto-incrementing.
Auto-increment columns can be problematic with statement-based
replication, because replaying the statements on a slave might
not produce the same set of column values as on the master, due
to timing issues. When you have an auto-incrementing primary
key, you can use statement-based replication only with the
setting
innodb_autoinc_lock_mode=1. If
you have innodb_autoinc_lock_mode=2, which
allows higher concurrency for insert operations, use
row-based replication rather
than statement-based
replication. The setting
innodb_autoinc_lock_mode=0 is the previous
(traditional) default setting and should not be used except for
compatibility purposes.
See also auto-increment locking, innodb_autoinc_lock_mode, primary key, row-based replication, statement-based replication.
The convenience of an
auto-increment primary key
involves some tradeoff with concurrency. In the simplest case,
if one transaction is inserting values into the table, any other
transactions must wait to do their own inserts into that table,
so that rows inserted by the first transaction receive
consecutive primary key values. InnoDB includes optimizations,
and the
innodb_autoinc_lock_mode
option, so that you can choose how to trade off between
predictable sequences of auto-increment values and maximum
concurrency for insert
operations.
See also auto-increment, concurrency, innodb_autoinc_lock_mode.
A setting that causes a commit operation after each SQL statement. This mode is not recommended for working with InnoDB tables with transactions that span several statements. It can help performance for read-only transactions on InnoDB tables, where it minimizes overhead from locking and generation of undo data, especially in MySQL 5.6.4 and up. It is also appropriate for working with MyISAM tables, where transactions are not applicable.
See also commit, locking, read-only transaction, SQL, transaction, undo.
B
A tree data structure that is popular for use in database
indexes. The structure is kept sorted at all times, enabling
fast lookup for exact matches (equals operator) and ranges (for
example, greater than, less than, and BETWEEN
operators). This type of index is available for most storage
engines, such as InnoDB and MyISAM.
Because B-tree nodes can have many children, a B-tree is not the same as a binary tree, which is limited to 2 children per node.
Contrast with hash index, which is only available in the MEMORY storage engine. The MEMORY storage engine can also use B-tree indexes, and you should choose B-tree indexes for MEMORY tables if some queries use range operators.
See also hash index.
Identifiers within MySQL SQL statements must be quoted using the
backtick character (`) if they contain
special characters or reserved words. For example, to refer to a
table named FOO#BAR or a column named
SELECT, you would specify the identifiers as
`FOO#BAR` and `SELECT`.
Since the backticks provide an extra level of safety, they are
used extensively in program-generated SQL statements, where the
identifier names might not be known in advance.
Many other database systems use double quotation marks
(") around such special names. For
portability, you can enable ANSI_QUOTES mode
in MySQL and use double quotation marks instead of backticks to
qualify identifier names.
See also SQL.
The process of copying some or all table data and metadata from a MySQL instance, for safekeeping. Can also refer to the set of copied files. This is a crucial task for DBAs. The reverse of this process is the restore operation.
With MySQL, physical backups
are performed by the MySQL Enterprise
Backup product, and logical
backups are performed by the
mysqldump command. These techniques have
different characteristics in terms of size and representation of
the backup data, and speed (especially speed of the restore
operation).
Backups are further classified as hot, warm, or cold depending on how much they interfere with normal database operation. (Hot backups have the least interference, cold backups the most.)
See also cold backup, hot backup, logical backup, MySQL Enterprise Backup, mysqldump, physical backup, warm backup.
The code name for an InnoDB file
format that supports compression for table data. This
file format was first introduced in the InnoDB Plugin. It
supports the compressed row
format that enables InnoDB table compression, and the
dynamic row format that
improves the storage layout for BLOB and large text columns. You
can select it through the
innodb_file_format option.
Because the InnoDB system tablespace is stored in the original Antelope file format, to use the Barracuda file format you must also enable the file-per-table setting, which puts newly created tables in their own tablespaces separate from the system tablespace.
The MySQL Enterprise Backup product version 3.5 and above supports backing up tablespaces that use the Barracuda file format.
See also Antelope, compact row format, compressed row format, dynamic row format, file format, file-per-table, innodb_file_format, MySQL Enterprise Backup, row format, system tablespace.
An early stage in the life of a software product, when it is available only for evaluation, typically without a definite release number or a number less than 1. InnoDB does not use the beta designation, preferring an early adopter phase that can extend over several point releases, leading to a GA release.
See also early adopter, GA.
A file containing a record of all statements that attempt to change table data. These statements can be replayed to bring slave servers up to date in a replication scenario, or to bring a database up to date after restoring table data from a backup. The binary logging feature can be turned on and off, although Oracle recommends always enabling it if you use replication or perform backups.
You can examine the contents of the binary log, or replay those statements during replication or recovery, by using the mysqlbinlog command. For full information about the binary log, see Section 5.2.4, “The Binary Log”. For MySQL configuration options related to the binary log, see Section 16.1.4.4, “Binary Log Options and Variables”.
For the MySQL Enterprise Backup
product, the file name of the binary log and the current
position within the file are important details. To record this
information for the master server when taking a backup in a
replication context, you can specify the
--slave-info option.
Prior to MySQL 5.0, a similar capability was available, known as the update log. In MySQL 5.0 and higher, the binary log replaces the update log.
See also binlog, MySQL Enterprise Backup, replication.
An informal name for the binary log file. For example, you might see this abbreviation used in e-mail messages or forum discussions.
See also binary log.
A special mode of full-text
search enabled by the WITH QUERY
EXPANSION clause. It performs the search twice, where
the search phrase for the second search is the original search
phrase concatenated with the few most highly relevant documents
from the first search. This technique is mainly applicable for
short search phrases, perhaps only a single word. It can uncover
relevant matches where the precise search term does not occur in
the document.
See also full-text search.
A portion of a system that is constrained in size or capacity, that has the effect of limiting overall throughput. For example, a memory area might be smaller than necessary; access to a single required resource might prevent multiple CPU cores from running simultaneously; or waiting for disk I/O to complete might prevent the CPU from running at full capacity. Removing bottlenecks tends to improve concurrency. For example, the ability to have multiple InnoDB buffer pool instances reduces contention when multiple sessions read from and write to the buffer pool simultaneously.
See also buffer pool, concurrency.
A shutdown operation immediately followed by a restart. Ideally with a relatively short warmup period so that performance and throughput quickly return to a high level.
See also shutdown.
A mechanism for managing different-sized pages in the InnoDB buffer pool.
See also buffer pool, page, page size.
A memory or disk area used for temporary storage. Data is buffered in memory so that it can be written to disk efficiently, with a few large I/O operations rather than many small ones. Data is buffered on disk for greater reliability, so that it can be recovered even when a crash or other failure occurs at the worst possible time. The main types of buffers used by InnoDB are the buffer pool, the doublewrite buffer, and the insert buffer.
See also buffer pool, crash, doublewrite buffer, insert buffer.
The memory area that holds cached InnoDB data for both tables and indexes. For efficiency of high-volume read operations, the buffer pool is divided into pages that can potentially hold multiple rows. For efficiency of cache management, the buffer pool is implemented as a linked list of pages; data that is rarely used is aged out of the cache, using a variation of the LRU algorithm. On systems with large memory, you can improve concurrency by dividing the buffer pool into multiple buffer pool instances.
Several InnoDB status variables,
information_schema tables, and
performance_schema tables help to monitor the
internal workings of the buffer pool. Starting in MySQL 5.6, you
can also dump and restore the contents of the buffer pool,
either automatically during shutdown and restart, or manually at
any time, through a set of InnoDB
configuration variables such as
innodb_buffer_pool_dump_at_shutdown
and
innodb_buffer_pool_load_at_startup.
See also buffer pool instance, LRU, page, warm up.
Any of the multiple regions into which the
buffer pool can be divided,
controlled by the
innodb_buffer_pool_instances
configuration option. The total memory size specified by the
innodb_buffer_pool_size is
divided among all the instances. Typically, multiple buffer pool
instances are appropriate for systems devoting multiple
gigabytes to the InnoDB buffer pool, with each instance 1
gigabyte or larger. On systems loading or looking up large
amounts of data in the buffer pool from many concurrent
sessions, having multiple instances reduces the contention for
exclusive access to the data structures that manage the buffer
pool.
See also buffer pool.
The built-in InnoDB storage engine within MySQL is the original form of distribution for the storage engine. Contrast with the InnoDB Plugin. Starting with MySQL 5.5, the InnoDB Plugin is merged back into the MySQL code base as the built-in InnoDB storage engine (known as InnoDB 1.1).
This distinction is important mainly in MySQL 5.1, where a feature or bug fix might apply to the InnoDB Plugin but not the built-in InnoDB, or vice versa.
The relationships and sequences of actions that form the basis of business software, used to run a commercial company. Sometimes these rules are dictated by law, other times by company policy. Careful planning ensures that the relationships encoded and enforced by the database, and the actions performed through application logic, accurately reflect the real policies of the company and can handle real-life situations.
For example, an employee leaving a company might trigger a sequence of actions from the human resources department. The human resources database might also need the flexibility to represent data about a person who has been hired, but not yet started work. Closing an account at an online service might result in data being removed from a database, or the data might be moved or flagged so that it could be recovered if the account is re-opened. A company might establish policies regarding salary maximums, minimums, and adjustments, in addition to basic sanity checks such as the salary not being a negative number. A retail database might not allow a purchase with the same serial number to be returned more than once, or might not allow credit card purchases above a certain value, while a database used to detect fraud might allow these kinds of things.
See also relational.
C
The general term for any memory area that stores copies of data for frequent or high-speed retrieval. In InnoDB, the primary kind of cache structure is the buffer pool.
See also buffer, buffer pool.
The number of different values in a table
column. When queries refer to
columns that have an associated
index, the cardinality of each
column influences which access method is most efficient. For
example, for a column with a unique
constraint, the number of different values is equal
to the number of rows in the table. If a table has a million
rows but only 10 different values for a particular column, each
value occurs (on average) 100,000 times. A query such as
SELECT c1 FROM t1 WHERE c1 = 50; thus might
return 1 row or a huge number of rows, and the database server
might process the query differently depending on the cardinality
of c1.
If the values in a column have a very uneven distribution, the
cardinality might not be a good way to determine the best query
plan. For example, SELECT c1 FROM t1 WHERE c1 =
x; might return 1 row when x=50 and
a million rows when x=30. In such a case, you
might need to use index hints
to pass along advice about which lookup method is more efficient
for a particular query.
Cardinality can also apply to the number of distinct values present in multiple columns, as in a composite index.
For InnoDB, the process of estimating cardinality for indexes is
influenced by the
innodb_stats_sample_pages and
the innodb_stats_on_metadata
configuration options. The estimated values are more stable when
the persistent statistics
feature is enabled (in MySQL 5.6 and higher).
See also column, composite index, index, index hint, persistent statistics, random dive, selectivity, unique constraint.
A metadata file used with the InnoDB
transportable tablespace
feature. It is produced by the command FLUSH TABLES ...
FOR EXPORT, puts one or more tables in a consistent
state that can be copied to another server. The
.cfg file is copied along with the
corresponding .ibd file, and
used to adjust the internal values of the
.ibd file, such as the
space ID, during the
ALTER TABLE ... IMPORT TABLESPACE step.
See also .ibd file, space ID, transportable tablespace.
A special data structure that records changes to
pages in
secondary indexes. These values
could result from SQL INSERT,
UPDATE, or
DELETE statements
(DML). The set of features
involving the change buffer is known collectively as
change buffering, consisting of
insert buffering,
delete buffering, and
purge buffering.
Changes are only recorded in the change buffer when the relevant page from the secondary index is not in the buffer pool. When the relevant index page is brought into the buffer pool while associated changes are still in the change buffer, the changes for that page are applied in the buffer pool (merged) using the data from the change buffer. Periodically, the purge operation that runs during times when the system is mostly idle, or during a slow shutdown, writes the new index pages to disk. The purge operation can write the disk blocks for a series of index values more efficiently than if each value were written to disk immediately.
Physically, the change buffer is part of the system tablespace, so that the index changes remain buffered across database restarts. The changes are only applied (merged) when the pages are brought into the buffer pool due to some other read operation.
The kinds and amount of data stored in the change buffer are
governed by the
innodb_change_buffering and
innodb_change_buffer_max_size
configuration options. To see information about the current data
in the change buffer, issue the
SHOW ENGINE INNODB
STATUS command.
Formerly known as the insert buffer.
See also buffer pool, change buffering, delete buffering, DML, insert buffer, insert buffering, merge, page, purge, purge buffering, secondary index, system tablespace.
The general term for the features involving the
change buffer, consisting of
insert buffering,
delete buffering, and
purge buffering. Index changes
resulting from SQL statements, which could normally involve
random I/O operations, are held back and performed periodically
by a background thread. This
sequence of operations can write the disk blocks for a series of
index values more efficiently than if each value were written to
disk immediately. Controlled by the
innodb_change_buffering and
innodb_change_buffer_max_size
configuration options.
See also change buffer, delete buffering, insert buffering, purge buffering.
As changes are made to data pages that are cached in the buffer pool, those changes are written to the data files sometime later, a process known as flushing. The checkpoint is a record of the latest changes (represented by an LSN value) that have been successfully written to the data files.
See also buffer pool, data files, flush, fuzzy checkpointing, LSN.
In InnoDB, a validation mechanism to detect
corruption when a page in a
tablespace is read from disk
into the InnoDB buffer pool.
This feature is turned on and off by the
innodb_checksums configuration
option. In MySQL 5.6, you can enable a faster checksum algorithm
by also specifying the configuration option
innodb_checksum_algorithm=crc32.
The innochecksum command helps to diagnose corruption problems by testing the checksum values for a specified tablespace file while the MySQL server is shut down.
MySQL also uses checksums for replication purposes. For details,
see the configuration options
binlog_checksum,
master_verify_checksum, and
slave_sql_verify_checksum.
See also buffer pool, page, tablespace.
In a foreign key relationship,
a child table is one whose rows refer (or point) to rows in
another table with an identical value for a specific column. The
corresponding row in the parent
table must exist before the row can be created in the
child table. The values in the child table can prevent delete or
update operations on the parent table, or can cause automatic
deletion or updates in the child table, based on the ON
CASCADE option used when creating the foreign key.
See also foreign key, parent table.
A page in the InnoDB buffer pool where all changes made in memory have also been written (flushed) to the data files. The opposite of a dirty page.
See also buffer pool, data files, dirty page, flush, page.
A shutdown that completes without errors and applies all changes to InnoDB tables before finishing, as opposed to a crash or a fast shutdown. Synonym for slow shutdown.
See also crash, fast shutdown, shutdown, slow shutdown.
A type of program that sends requests to a server, and
interprets or processes the results. The client software might
run only some of the time (such as a mail or chat program), and
might run interactively (such as the mysql
command processor).
The InnoDB term for a primary key index. InnoDB table storage is organized based on the values of the primary key columns, to speed up queries and sorts involving the primary key columns. For best performance, choose the primary key columns carefully based on the most performance-critical queries. Because modifying the columns of the clustered index is an expensive operation, choose primary columns that are rarely or never updated.
In the Oracle Database product, this type of table is known as an index-organized table.
See also index, primary key, secondary index.
A backup taken while the database is shut down. For busy applications and web sites, this might not be practical, and you might prefer a warm backup or a hot backup.
See also backup, hot backup, warm backup.
A data item within a row, whose storage and semantics are defined by a data type. Each table and index is largely defined by the set of columns it contains.
Each column has a cardinality value. A column can be the primary key for its table, or part of the primary key. A column can be subject to a unique constraint, a NOT NULL constraint, or both. Values in different columns, even across different tables, can be linked by a foreign key relationship.
In discussions of MySQL internal operations, sometimes field is used as a synonym, while row is the preferred term when discussing SQL.
See also cardinality, foreign key, index, primary key, row, SQL, table, unique constraint.
See also composite index, index.
When an index is created with a length specification, such as
CREATE INDEX idx ON t1 (c1(N)), only the
first N characters of the column value are stored in the index.
Keeping the index prefix small makes the index compact, and the
memory and disk I/O savings help performance. (Although making
the index prefix too small can hinder query optimization by
making rows with different values appear to the query optimizer
to be duplicates.)
For columns containing binary values or long text strings, where sorting is not a major consideration and storing the entire value in the index would waste space, the index automatically uses the first N (typically 768) characters of the value to do lookups and sorts.
See also index.
A SQL statement that ends a transaction, making permanent any changes made by the transaction. It is the opposite of rollback, which undoes any changes made in the transaction.
InnoDB uses an optimistic mechanism for commits, so that changes can be written to the data files before the commit actually occurs. This technique makes the commit itself faster, with the tradeoff that more work is required in case of a rollback.
By default, MySQL uses the autocommit setting, which automatically issues a commit following each SQL statement.
See also autocommit, optimistic, rollback, SQL, transaction.
The default InnoDB row format since MySQL 5.0.3. Available for tables that use the Antelope file format. It has a more compact representation for nulls and variable-length fields than the prior default (redundant row format).
Because of the B-tree indexes that make row lookups so fast in InnoDB, there is little if any performance benefit to keeping all rows the same size.
See also Antelope, file format, redundant row format, row format.
An index that includes multiple columns.
See also index, index prefix.
The compression feature of the MySQL
Enterprise Backup product makes a compressed copy of
each tablespace, changing the extension from
.ibd to .ibz. Compressing
the backup data allows you to keep more backups on hand, and
reduces the time to transfer backups to a different server. The
data is uncompressed during the restore operation. When a
compressed backup operation processes a table that is already
compressed, it skips the compression step for that table,
because compressing again would result in little or no space
savings.
A set of files produced by the MySQL
Enterprise Backup product, where each
tablespace is compressed. The
compressed files are renamed with a .ibz file
extension.
Applying compression right at the start of the backup process helps to avoid storage overhead during the compression process, and to avoid network overhead when transferring the backup files to another server. The process of applying the binary log takes longer, and requires uncompressing the backup files.
See also apply, binary log, compression, hot backup, MySQL Enterprise Backup, tablespace.
A row format that enables data
and index compression for
InnoDB tables. It was introduced in the InnoDB Plugin, available
as part of the Barracuda file
format. Large fields are stored away from the page that holds
the rest of the row data, as in dynamic
row format. Both index pages and the large fields are
compressed, yielding memory and disk savings. Depending on the
structure of the data, the decrease in memory and disk usage
might or might not outweigh the performance overhead of
uncompressing the data as it is used. See
Section 14.2.2.7, “InnoDB Data Compression” for usage details.
See also Barracuda, compression, dynamic row format, row format.
A feature with wide-ranging benefits from using less disk space, performing less I/O, and using less memory for caching. InnoDB table and index data can be kept in a compressed format during database operation.
The data is uncompressed when needed for queries, and re-compressed when changes are made by DML operations. After you enable compression for a table, this processing is transparent to users and application developers. DBAs can consult information_schema tables to monitor how efficiently the compression parameters work for the MySQL instance and for particular compressed tables.
When InnoDB table data is compressed, the compression applies to the table itself, any associated index data, and the pages loaded into the buffer pool. Compression does not apply to pages in the undo buffer.
The table compression feature requires using MySQL 5.5 or
higher, or the InnoDB Plugin in MySQL 5.1 or earlier, and
creating the table using the
Barracuda file format and
compressed row format, with the
innodb_file_per_table setting
turned on. The compression for each table is influenced by the
KEY_BLOCK_SIZE clause of the
CREATE TABLE and
ALTER TABLE statements. In MySQL
5.6 and higher, compression is also affected by the server-wide
configuration options
innodb_compression_failure_threshold_pct,
innodb_compression_level, and
innodb_compression_pad_pct_max.
See Section 14.2.2.7, “InnoDB Data Compression” for usage details.
Another type of compression is the compressed backup feature of the MySQL Enterprise Backup product.
See also Barracuda, buffer pool, compressed row format, DML, hot backup, index, INFORMATION_SCHEMA, innodb_file_per_table, plugin, table, undo buffer.
See composite index.
The ability of multiple operations (in database terminology, transactions) to run simultaneously, without interfering with each other. Concurrency is also involved with performance, because ideally the protection for multiple simultaneous transactions works with a minimum of performance overhead, using efficient mechanisms for locking.
See also ACID, locking, transaction.
The file that holds the option
values used by MySQL at startup. Traditionally, on Linux and
UNIX this file is named my.cnf, and on
Windows it is named my.ini. You can set a
number of options related to InnoDB under the
[mysqld] section of the file.
Typically, this file is searched for in the locations
/etc/my.cnf
/etc/mysql/my.cnf
/usr/local/mysql/etc/my.cnf and
~/.my.cnf. See
Section 4.2.3.3, “Using Option Files” for details about the search path
for this file.
When you use the MySQL Enterprise Backup product, you typically use two configuration files: one that specifies where the data comes from and how it is structured (which could be the original configuration file for your real server), and a stripped-down one containing only a small set of options that specify where the backup data goes and how it is structured. The configuration files used with the MySQL Enterprise Backup product must contain certain options that are typically left out of regular configuration files, so you might need to add some options to your existing configuration file for use with MySQL Enterprise Backup.
See also my.cnf, option file.
A read operation that uses snapshot information to present query results based on a point in time, regardless of changes performed by other transactions running at the same time. If queried data has been changed by another transaction, the original data is reconstructed based on the contents of the undo log. This technique avoids some of the locking issues that can reduce concurrency by forcing transactions to wait for other transactions to finish.
With the repeatable read isolation level, the snapshot is based on the time when the first read operation is performed. With the read committed isolation level, the snapshot is reset to the time of each consistent read operation.
Consistent read is the default mode in which InnoDB processes
SELECT statements in
READ COMMITTED and
REPEATABLE READ isolation
levels. Because a consistent read does not set any locks on the
tables it accesses, other sessions are free to modify those
tables while a consistent read is being performed on the table.
For technical details about the applicable isolation levels, see Section 14.2.4.3, “Consistent Nonlocking Reads”.
See also ACID, concurrency, isolation level, locking, MVCC, READ COMMITTED, READ UNCOMMITTED, REPEATABLE READ, SERIALIZABLE, transaction, undo log.
An automatic test that can block database changes to prevent data from becoming inconsistent. (In computer science terms, a kind of assertion related to an invariant condition.) Constraints are a crucial component of the ACID philosophy, to maintain data consistency. Constraints supported by MySQL include FOREIGN KEY constraints and unique constraints.
See also ACID, foreign key, relational, unique constraint.
A value that is incremented by a particular kind of
InnoDB operation. Useful for measuring how
busy a server is, troubleshooting the sources of performance
issues, and testing whether changes (for example, to
configuration settings or indexes used by queries) have the
desired low-level effects. Different kinds of counters are
available through
performance_schema tables and
information_schema tables,
particularly
information_schema.innodb_metrics.
See also INFORMATION_SCHEMA, metrics counter, Performance Schema.
An index that includes all the columns retrieved by a query. Instead of using the index values as pointers to find the full table rows, the query returns values from the index structure, saving disk I/O. InnoDB can apply this optimization technique to more indexes than MyISAM can, because InnoDB secondary indexes also include the primary key columns. InnoDB cannot apply this technique for queries against tables modified by a transactions, until that transaction ends.
Any column index or composite index could act as a covering index, given the right query. Design your indexes and queries to take advantage of this optimization technique wherever possible.
See also column index, composite index, index, secondary index.
MySQL uses the term “crash” to refer generally to any unexpected shutdown operation where the server cannot do its normal cleanup. For example, a crash could happen due to a hardware fault on the database server machine or storage device; a power failure; a potential data mismatch that causes the MySQL server to halt; a fast shutdown initiated by the DBA; or many other reasons. The robust, automatic crash recovery for InnoDB tables ensures that data is made consistent when the server is restarted, without any extra work for the DBA.
See also crash recovery, fast shutdown, InnoDB, redo log, shutdown.
The cleanup activities that occur when MySQL is started again after a crash. For InnoDB tables, changes from incomplete transactions are replayed using data from the redo log. Changes that were committed before the crash, but not yet written into the data files, are reconstructed from the doublewrite buffer. When the database is shut down normally, this type of activity is performed during shutdown by the purge operation.
During normal operation, committed data can be stored in the change buffer for a period of time before being written to the data files. There is always a tradeoff between keeping the data files up-to-date, which introduces performance overhead during normal operation, and buffering the data, which can make shutdown and crash recovery take longer.
See also change buffer, commit, crash, data files, doublewrite buffer, InnoDB, purge, redo log.
Acronym for “create, read, update, delete”, a common sequence of operations in database applications. Often denotes a class of applications with relatively simple database usage (basic DDL, DML and query statements in SQL) that can be implemented quickly in any language.
An internal data structure that is used to represent the result
set of a query, or other
operation that performs a search using an SQL
WHERE clause. It works like an iterator in
other high-level languages, producing each value from the result
set as requested.
Although usually SQL handles the processing of cursors for you, you might delve into the inner workings when dealing with performance-critical code.
See also query.
D
See DDL.
Metadata that keeps track of InnoDB-related objects such as tables, indexes, and table columns. This metadata is physically located in the InnoDB system tablespace. For historical reasons, it overlaps to some degree with information stored in the .frm files.
Because the MySQL Enterprise Backup product always backs up the system tablespace, all backups include the contents of the data dictionary.
See also column, .frm file, hot backup, index, MySQL Enterprise Backup, system tablespace, table.
The directory under which each MySQL
instance keeps the
data files for InnoDB and the
directories representing individual databases. Controlled by the
datadir configuration option.
See also data files, instance.
The files that physically contain the InnoDB table and index data. There can be a one-to-many relationship between data files and tables, as in the case of the system tablespace, which can hold multiple InnoDB tables as well as the data dictionary. There can also be a one-to-one relationship between data files and tables, as when the file-per-table setting is enabled, causing each newly created table to be stored in a separate tablespace.
See also data dictionary, file-per-table, index, system tablespace, table, tablespace.
See DML.
A database system or application that primarily runs large queries. The read-only or read-mostly data might be organized in denormalized form for query efficiency. Can benefit from the optimizations for read-only transactions in MySQL 5.6 and higher. Contrast with OLTP.
See also denormalized, OLTP, query, read-only transaction.
Within the MySQL data directory, each database is represented by a separate directory. The InnoDB system tablespace, which can hold table data from multiple databases within a MySQL instance, is kept in its data files that reside outside the individual database directories. When file-per-table mode is enabled, the .ibd files representing individual InnoDB tables are stored inside the database directories.
For long-time MySQL users, a database is a familiar notion. Users coming from an Oracle Database background will find that the MySQL meaning of a database is closer to what Oracle Database calls a schema.
See also data files, file-per-table, .ibd file, instance, schema, system tablespace.
Data control language, a set of
SQL statements for managing
privileges. In MySQL, consists of the
GRANT and
REVOKE statements. Contrast with
DDL and
DML.
Data definition language, a set of
SQL statements for manipulating
the database itself rather than individual table rows. Includes
all forms of the CREATE,
ALTER, and DROP
statements. Also includes the TRUNCATE
statement, because it works differently than a DELETE
FROM statement,
even though the ultimate effect is similar.
table_name
DDL statements automatically commit the current transaction; they cannot be rolled back.
The InnoDB-related aspects of DDL include speed improvements for
CREATE INDEX and
DROP INDEX statements, and the
way the file-per-table setting
affects the behavior of the TRUNCATE
TABLE statement.
Contrast with DML and DCL.
See also commit, DCL, DML, file-per-table, rollback, SQL, transaction.
A situation where different transactions are unable to proceed, because each holds a lock that the other needs. Because both transactions are waiting for a resource to become available, neither will ever release the locks it holds.
A deadlock can occur when the transactions lock rows in multiple
tables (through statements such as UPDATE or
SELECT ... FOR UPDATE), but in the opposite
order. A deadlock can also occur when such statements lock
ranges of index records and
gaps, with each transaction
acquiring some locks but not others due to a timing issue.
To reduce the possibility of deadlocks, use transactions rather
than LOCK TABLE statements; keep transactions
that insert or update data small enough that they do not stay
open for long periods of time; when different transactions
update multiple tables or large ranges of rows, use the same
order of operations (such as SELECT ... FOR
UPDATE) in each transaction; create indexes on the
columns used in SELECT ... FOR UPDATE and
UPDATE ... WHERE statements. The possibility
of deadlocks is not affected by the
isolation level, because the
isolation level changes the behavior of read operations, while
deadlocks occur because of write operations.
If a deadlock does occur, InnoDB detects the condition and
rolls back one of the
transactions (the victim).
Thus, even if your application logic is perfectly correct, you
must still handle the case where a transaction must be retried.
To see the last deadlock in an InnoDB user transaction, use the
command SHOW ENGINE INNODB STATUS. If
frequent deadlocks highlight a problem with transaction
structure or application error handling, run with the
innodb_print_all_deadlocks
setting enabled to print information about all deadlocks to the
mysqld error log.
For background information on how deadlocks are automatically detected and handled, see Section 14.2.4.9, “Deadlock Detection and Rollback”. For tips on avoiding and recovering from deadlock conditions, see Section 14.2.4.10, “How to Cope with Deadlocks”.
See also concurrency, gap, isolation level, lock, locking, rollback, transaction, victim.
A mechanism that automatically detects when a deadlock occurs, and automatically rolls back one of the transactions involved (the victim).
See also deadlock, rollback, transaction, victim.
When InnoDB processes a DELETE statement, the
rows are immediately marked for deletion and no longer are
returned by queries. The storage is reclaimed sometime later,
during the periodic garbage collection known as the
purge operation, performed by a
separate thread. For removing large quantities of data, related
operations with their own performance characteristics are
truncate and
drop.
The technique of storing index changes due to
DELETE operations in the
insert buffer rather than
writing them immediately, so that the physical writes can be
performed to minimize random I/O. (Because delete operations are
a two-step process, this operation buffers the write that
normally marks an index record for deletion.) It is one of the
types of change buffering; the
others are insert buffering and
purge buffering.
See also change buffer, change buffering, insert buffer, insert buffering, purge buffering.
A data storage strategy that duplicates data across different tables, rather than linking the tables with foreign keys and join queries. Typically used in data warehouse applications, where the data is not updated after loading. In such applications, query performance is more important than making it simple to maintain consistent data during updates. Contrast with normalized.
See also normalized.
A type of index available with some database systems, where
index storage is optimized to process ORDER BY
clauses.
Currently, although MySQL allows the column DESCDESC
keyword in the CREATE TABLE
statement, it does not use any special storage layout for the
resulting index.
See also index.
A page in the InnoDB buffer pool that has been updated in memory, where the changes are not yet written (flushed) to the data files. The opposite of a clean page.
See also buffer pool, clean page, data files, flush, page.
An operation that retrieves unreliable data, data that was updated by another transaction but not yet committed. It is only possible with the isolation level known as read uncommitted.
This kind of operation does not adhere to the ACID principle of database design. It is considered very risky, because the data could be rolled back, or updated further before being committed; then, the transaction doing the dirty read would be using data that was never confirmed as accurate.
Its polar opposite is consistent read, where InnoDB goes to great lengths to ensure that a transaction does not read information updated by another transaction, even if the other transaction commits in the meantime.
See also ACID, commit, consistent read, isolation level, READ COMMITTED, READ UNCOMMITTED, rollback.
A kind of database that primarily organizes data on disk storage (hard drives or equivalent). Data is brought back and forth between disk and memory to be operated upon. It is the opposite of an in-memory database. Although InnoDB is disk-based, it also contains features such as the buffer pool, multiple buffer pool instances, and the adaptive hash index that allow certain kinds of workloads to work primarily from memory.
See also adaptive hash index, buffer pool, in-memory database.
A type of workload where the primary bottleneck is CPU operations in memory. Typically involves read-intensive operations where the results can all be cached in the buffer pool.
See also bottleneck, buffer pool, disk-bound, workload.
A type of workload where the primary bottleneck is disk I/O. Typically involves frequent writes to disk, or random reads of more data than can fit into the buffer pool.
See also bottleneck, buffer pool, disk-bound, workload.
Data manipulation language, a set of
SQL statements for performing
insert, update, and delete operations. The
SELECT statement is sometimes
considered as a DML statement, because the SELECT ...
FOR UPDATE form is subject to the same considerations
for locking as
INSERT,
UPDATE, and
DELETE.
DML statements for an InnoDB table operate in the context of a transaction, so their effects can be committed or rolled back as a single unit.
Contrast with DDL and DCL.
See also commit, DCL, DDL, locking, rollback, SQL, transaction.
In the InnoDB full-text search
feature, a special column in the table containing the
FULLTEXT index, to uniquely
identify the document associated with each
ilist value. Its name is
FTS_DOC_ID (uppercase required). The column
itself must be of BIGINT UNSIGNED NOT NULL
type, with a unique index named
FTS_DOC_ID_INDEX. Preferably, you define this
column when creating the table. If InnoDB must add the column to
the table while creating a FULLTEXT index,
the indexing operation is considerably more expensive.
See also full-text search, FULLTEXT index, ilist.
InnoDB uses a novel file flush technique called doublewrite. Before writing pages to the data files, InnoDB first writes them to a contiguous area called the doublewrite buffer. Only after the write and the flush to the doublewrite buffer have completed, does InnoDB write the pages to their proper positions in the data file. If the operating system crashes in the middle of a page write, InnoDB can later find a good copy of the page from the doublewrite buffer during crash recovery.
Although data is always written twice, the doublewrite buffer
does not require twice as much I/O overhead or twice as many I/O
operations. Data is written to the buffer itself as a large
sequential chunk, with a single fsync() call
to the operating system.
To turn off the doublewrite buffer, specify the option
innodb_doublewrite=0.
See also crash recovery, data files, page, purge.
A kind of DDL operation that
removes a schema object, through a statement such as
DROP TABLE or
DROP INDEX. It maps internally to
an ALTER TABLE statement. From an
InnoDB perspective, the performance considerations of such
operations involve the time that the data
dictionary is locked to ensure that interrelated
objects are all updated, and the time to update memory
structures such as the buffer
pool. For a table,
the drop operation has somewhat different characteristics than a
truncate operation
(TRUNCATE TABLE statement).
See also buffer pool, data dictionary, DDL, table, truncate.
A row format introduced in the InnoDB Plugin, available as part
of the Barracuda
file format. Because
TEXT and BLOB fields are
stored outside of the rest of the page that holds the row data,
it is very efficient for rows that include large objects. Since
the large fields are typically not accessed to evaluate query
conditions, they are not brought into the
buffer pool as often, resulting
in fewer I/O operations and better utilization of cache memory.
See also Barracuda, buffer pool, file format, row format.
E
A stage similar to beta, when a software product is typically evaluated for performance, functionality, and compatibility in a non-mission-critical setting. InnoDB uses the early adopter designation rather than beta, through a succession of point releases leading up to a GA release.
A type of log showing information about MySQL startup and critical runtime errors and crash information. For details, see Section 5.2.2, “The Error Log”.
The process of removing an item from a cache or other temporary storage area, such as the InnoDB buffer pool. Often, but not always, uses the LRU algorithm to determine which item to remove. When a dirty page is evicted, its contents are flushed to disk, and any dirty neighbor pages might be flushed also.
See also buffer pool, dirty page, flush, LRU.
A kind of lock that prevents any other transaction from locking the same row. Depending on the transaction isolation level, this kind of lock might block other transactions from writing to the same row, or might also block other transactions from reading the same row. The default InnoDB isolation level, REPEATABLE READ, enables higher concurrency by allowing transactions to read rows that have exclusive locks, a technique known as consistent read.
See also concurrency, consistent read, isolation level, lock, REPEATABLE READ, shared lock, transaction.
A group of pages within a tablespace totaling 1 megabyte. With the default page size of 16KB, an extent contains 64 pages. In MySQL 5.6, the page size can also be 4KB or 8KB, in which case an extent contains more pages, still adding up to 1MB.
InnoDB features such as segments, read-ahead requests and the doublewrite buffer use I/O operations that read, write, allocate, or free data one extent at a time.
See also doublewrite buffer, neighbor page, page, page size, read-ahead, segment, tablespace.
F
A capability first introduced in the InnoDB Plugin, now part of the MySQL server in 5.5 and higher, that speeds up creation of InnoDB secondary indexes by avoiding the need to completely rewrite the associated table. The speedup applies to dropping secondary indexes also.
Because index maintenance can add performance overhead to many
data transfer operations, consider doing operations such as
ALTER TABLE ... ENGINE=INNODB or
INSERT INTO ... SELECT * FROM ... without any
secondary indexes in place, and creating the indexes afterward.
In MySQL 5.6, this feature becomes more general: you can read
and write to tables while an index is being created, and many
more kinds of ALTER TABLE
operations can be performed without copying the table, without
blocking DML operations, or
both. Thus in MySQL 5.6 and higher, we typically refer to this
set of features as online DDL
rather than Fast Index Creation.
See also DML, index, online DDL, secondary index.
The default shutdown procedure
for InnoDB, based on the configuration setting
innodb_fast_shutdown=1. To save
time, certain flush operations
are skipped. This type of shutdown is safe during normal usage,
because the flush operations are performed during the next
startup, using the same mechanism as in
crash recovery. In cases where
the database is being shut down for an upgrade or downgrade, do
a slow shutdown instead to
ensure that all relevant changes are applied to the
data files during the shutdown.
See also crash recovery, data files, flush, shutdown, slow shutdown.
The format used by InnoDB for each table, typically with the
file-per-table setting enabled
so that each table is stored in a separate
.ibd file.
Currently, the file formats available in InnoDB are known as
Antelope and
Barracuda. Each file format
supports one or more row
formats. The row formats available for Barracuda
tables, COMPRESSED and
DYNAMIC, enable important new
storage features for InnoDB tables.
See also Antelope, Barracuda, file-per-table, .ibd file, ibdata file, row format.
A general name for the setting controlled by the
innodb_file_per_table option.
That is a very important configuration option that affects many
aspects of InnoDB file storage, availability of features, and
I/O characteristics. In MySQL 5.6.7 and higher, it is enabled by
default. Prior to MySQL 5.6.7, it is disabled by default.
For each table created while this setting is in effect, the data
is stored in a separate .ibd
file rather than in the ibdata
files of the system
tablespace. When table data is stored in individual
files, you have more flexibility to choose nondefault
file formats and
row formats, which are required
for features such as data
compression. The
TRUNCATE TABLE operation is also much faster,
and the reclaimed space can be used by the operating system
rather than remaining reserved for InnoDB.
The MySQL Enterprise Backup product is more flexible for tables that are in their own files. For example, tables can be excluded from a backup, but only if they are in separate files. Thus, this setting is suitable for tables that are backed up less frequently or on a different schedule.
See also compressed row format, compression, file format, .ibd file, ibdata file, innodb_file_per_table, row format, system tablespace.
In an InnoDB index, the proportion of a page that is taken up by index data before the page is split. The unused space when index data is first divided between pages allows for rows to be updated with longer string values without requiring expensive index maintenance operations. If the fill factor is too low, the index consumes more space than needed, causing extra I/O overhead when reading the index. If the fill factor is too high, any update that increases the length of column values can cause extra I/O overhead for index maintenance.
This row format is used by the MyISAM storage engine, not by
InnoDB. If you create an InnoDB table with the option
row_format=fixed, InnoDB translates this
option to use the compact row
format instead, although the fixed
value might still show up in output such as SHOW TABLE
STATUS reports.
See also compact row format, row format.
To write changes to the database files, that had been buffered in a memory area or a temporary disk storage area. The InnoDB storage structures that are periodically flushed include the redo log, the undo log, and the buffer pool.
Flushing can happen because a memory area becomes full and the
system needs to free some space, because a
commit operation means the
changes from a transaction can be finalized, or because a
slow shutdown operation means
that all outstanding work should be finalized. When it is not
critical to flush all the buffered data at once,
InnoDB can use a technique called
fuzzy checkpointing to flush
small batches of pages to spread out the I/O overhead.
See also buffer pool, commit, fuzzy checkpointing, neighbor page, redo log, slow shutdown, undo log.
An internal InnoDB data structure that tracks dirty pages in the buffer pool: that is, pages that have been changed and need to be written back out to disk. This data structure is updated frequently by InnoDB's internal mini-transactions, and so is protected by its own mutex to allow concurrent access to the buffer pool.
See also buffer pool, dirty page, LRU, mini-transaction, mutex, page, page cleaner.
A type of pointer relationship, between rows in separate InnoDB tables. The foreign key relationship is defined on one column in both the parent table and the child table.
In addition to enabling fast lookup of related information, foreign keys help to enforce referential integrity, by preventing any of these pointers from becoming invalid as data is inserted, updated, and deleted. This enforcement mechanism is a type of constraint. A row that points to another table cannot be inserted if the associated foreign key value does not exist in the other table. If a row is deleted or its foreign key value changed, and rows in another table point to that foreign key value, the foreign key can be set up to prevent the deletion, cause the corresponding column values in the other table to become null, or automatically delete the corresponding rows in the other table.
One of the stages in designing a normalized database is to identify data that is duplicated, separate that data into a new table, and set up a foreign key relationship so that the multiple tables can be queried like a single table, using a join operation.
See also child table, FOREIGN KEY constraint, join, normalized, NULL, parent table, referential integrity, relational.
The type of constraint that
maintains database consistency through a
foreign key relationship. Like
other kinds of constraints, it can prevent data from being
inserted or updated if data would become inconsistent; in this
case, the inconsistency being prevented is between data in
multiple tables. Alternatively, when a
DML operation is performed,
FOREIGN KEY constraints can cause data in
child rows to be deleted,
changed to different values, or set to
null, based on the ON
CASCADE option specified when creating the foreign
key.
See also child table, constraint, DML, foreign key, NULL.
A file containing the metadata, such as the table definition, of a MySQL table.
Although each InnoDB table has a .frm file,
InnoDB maintains its own table metadata (the
data dictionary) in the
system tablespace; the
.frm files are not needed for InnoDB to
operate on InnoDB tables. Problems can occur if a
crash happens while the
.frm file is being written, leading to
inconsistencies between the .frm file and the
data dictionary,
For backups, you must always keep the full set of
.frm files along with the backup data, to be
able to restore tables that are altered or dropped after the
backup. Files with this extension are always included in backups
produced by the mysqlbackup
command of the MySQL Enterprise
Backup product. If you use the
ibbackup command instead, you
must copy the .frm files yourself.
These files are backed up by the MySQL
Enterprise Backup product. These files must not be
modified by an ALTER TABLE operation while
the backup is taking place, which is why backups that include
non-InnoDB tables perform a FLUSH TABLES WITH READ
LOCK operation to freeze such activity while backing
up the .frm files. Restoring a backup can
result in .frm files being created, changed,
or removed to match the state of the database at the time of the
backup.
See also crash, data dictionary, ibbackup command, MySQL Enterprise Backup, mysqlbackup command, system tablespace.
In most contexts, an acronym for full-text search. Sometimes in performance discussions, an acronym for full table scan.
See also full table scan, full-text search.
A backup that includes all the tables in each MySQL database, and all the databases in a MySQL instance. Contrast with partial backup.
See also backup, database, instance, partial backup, table.
An operation that requires reading the entire contents of a table, rather than just selected portions using an index. Typically performed either with small lookup tables, or in data warehousing situations with large tables where all available data is aggregated and analyzed. How frequently these operations occur, and the sizes of the tables relative to available memory, have implications for the algorithms used in query optimization and managing the buffer pool.
The purpose of indexes is to allow lookups for specific values or ranges of values within a large table, thus avoiding full table scans when practical.
See also buffer pool, index, LRU.
The MySQL feature for finding words, phrases, Boolean
combinations of words, and so on within table data, in a faster,
more convenient, and more flexible way than using the SQL
LIKE operator or writing your own
application-level search algorithm. It uses the SQL function
MATCH() and
FULLTEXT indexes.
See also FULLTEXT index.
The special kind of index that
holds the search index in the
MySQL full-text search
mechanism. Represents the words from values of a column,
omitting any that are specified as
stopwords. Originally, only
available for MyISAM tables. Starting in
MySQL 5.6.4, it is also available for
InnoDB tables.
See also full-text search, index, InnoDB, search index, stopword.
A technique that flushes small batches of dirty pages from the buffer pool, rather than flushing all dirty pages at once which would disrupt database processing.
See also buffer pool, dirty page, flush.
G
“Generally available”, the stage when a software product leaves beta and is available for sale, official support, and production use.
See also beta, early adopter.
A place in an InnoDB index data
structure where new values could be inserted. When you lock a
set of rows with a statement such as SELECT ... FOR
UPDATE, InnoDB can create locks that apply to the gaps
as well as the actual values in the index. For example, if you
select all values greater than 10 for update, a gap lock
prevents another transaction from inserting a new value that is
greater than 10. The supremum
record and infimum
record represent the gaps containing all values
greater than or less than all the current index values.
See also concurrency, gap lock, index, infimum record, isolation level, supremum record.
A lock on a
gap between index records, or a
lock on the gap before the first or after the last index record.
For example, SELECT c1 FOR UPDATE FROM t WHERE c1
BETWEEN 10 and 20; prevents other transactions from
inserting a value of 15 into the column t.c1,
whether or not there was already any such value in the column,
because the gaps between all existing values in the range are
locked. Contrast with record
lock and next-key
lock.
Gap locks are part of the tradeoff between performance and concurrency, and are used in some transaction isolation levels and not others.
See also gap, infimum record, lock, next-key lock, record lock, supremum record.
See general query log.
A type of log used for
diagnosis and troubleshooting of SQL statements processed by the
MySQL server. Can be stored in a file or in a database table.
You must enable this feature through the
general_log configuration
option to use it. You can disable it for a specific connection
through the sql_log_off
configuration option.
Records a broader range of queries than the
slow query log. Unlike the
binary log, which is used for
replication, the general query log contains
SELECT statements and does not
maintain strict ordering. For more information, see
Section 5.2.3, “The General Query Log”.
See also binary log, general query log, log.
A type of transaction involved in XA operations. It consists of several actions that are transactional in themselves, but that all must either complete successfully as a group, or all be rolled back as a group. In essence, this extends ACID properties “up a level” so that multiple ACID transactions can be executed in concert as components of a global operation that also has ACID properties. For this type of distributed transaction, you must use the SERIALIZABLE isolation level to achieve ACID properties.
See also ACID, SERIALIZABLE, transaction, XA.
An InnoDB optimization that performs some
low-level I/O operations (log
write) once for a set of
commit operations, rather than
flushing and syncing separately for each commit.
When the binlog is enabled, you typically also set the
configuration option sync_binlog=0, because
group commit for the binary log is only supported if it is set
to 0.
H
A type of index intended for
queries that use equality operators, rather than range operators
such as greater-than or BETWEEN. It is
available for MEMORY tables. Although hash indexes are the
default for MEMORY tables for historic reasons, that storage
engine also supports B-tree
indexes, which are often a better choice for general-purpose
queries.
MySQL includes a variant of this index type, the adaptive hash index, that is constructed automatically for InnoDB tables if needed based on runtime conditions.
See also adaptive hash index, B-tree, index, InnoDB.
Acronym for “hard disk drive”. Refers to storage media using spinning platters, usually when comparing and contrasting with SSD. Its performance characteristics can influence the throughput of a disk-based workload.
See also disk-based, SSD.
A periodic message that is sent to indicate that a system is functioning properly. In a replication context, if the master stops sending such messages, one of the slaves can take its place. Similar techniques can be used between the servers in a cluster environment, to confirm that all of them are operating properly.
See also replication.
A value representing an upper limit, either a hard limit that should not be exceeded at runtime, or a record of the maximum value that was actually reached. Contrast with low-water mark.
See also low-water mark.
A list of transactions with
delete-marked records scheduled to be processed by the
InnoDB purge
operation. Recorded in the undo
log. The length of the history list is reported by
the command SHOW ENGINE INNODB STATUS. If the
history list grows longer than the value of the
innodb_max_purge_lag
configuration option, each DML
operation is delayed slightly to allow the purge operation to
finish flushing the deleted
records.
Also known as purge lag.
See also flush, purge, purge lag, rollback segment, transaction, undo log.
A condition where a row, table, or internal data structure is accessed so frequently, requiring some form of locking or mutual exclusion, that it results in a performance or scalability issue.
Although “hot” typically indicates an undesirable condition, a hot backup is the preferred type of backup.
See also hot backup.
A backup taken while the database and is running and applications are reading and writing to it. The backup involves more than simply copying data files: it must include any data that was inserted or updated while the backup was in process; it must exclude any data that was deleted while the backup was in process; and it must ignore any changes that were not committed.
The Oracle product that performs hot backups, of InnoDB tables especially but also tables from MyISAM and other storage engines, is known as MySQL Enterprise Backup.
The hot backup process consists of two stages. The initial copying of the data files produces a raw backup. The apply step incorporates any changes to the database that happened while the backup was running. Applying the changes produces a prepared backup; these files are ready to be restored whenever necessary.
See also apply, MySQL Enterprise Backup, prepared backup, raw backup.
I
The set of files managed by InnoDB within a MySQL database: the system tablespace, any file-per-table tablespaces, and the (typically 2) redo log files. Used sometimes in detailed discussions of InnoDB file structures and formats, to avoid ambiguity between the meanings of database between different DBMS products, and the non-InnoDB files that may be part of a MySQL database.
See also database, file-per-table, redo log, system tablespace.
A set of files, typically named ib_logfile0
and ib_logfile1, that form the
redo log. Also sometimes
referred to as the log group.
These files record statements that attempt to change data in
InnoDB tables. These statements are replayed automatically to
correct data written by incomplete transactions, on startup
following a crash.
This data cannot be used for manual recovery; for that type of operation, use the binary log.
See also binary log, log group, redo log.
The fundamental command-line tool of the MySQL Enterprise Backup product. It performs a hot backup operation for InnoDB tables. You use this command directly if all your data is in InnoDB tables, if all the important data that you need to back up is in InnoDB tables, or if you are running on the Windows platform. If you also need to back up tables from MyISAM or other storage engines, you use the mysqlbackup command instead (available on UNIX and Linux systems only).
See also hot backup, MySQL Enterprise Backup, mysqlbackup command.
A supplemental backup file created by the
MySQL Enterprise Backup product
during a hot backup operation.
It contains information about any data changes that occurred
while the backup was running. The initial backup files,
including ibbackup_logfile, are known as a
raw backup, because the changes
that occurred during the backup operation are not yet
incorporated. After you perform the
apply step to the raw backup
files, the resulting files do include those final data changes,
and are known as a prepared
backup. At this stage, the
ibbackup_logfile file is no longer necessary.
See also apply, hot backup, MySQL Enterprise Backup, prepared backup, raw backup.
Each InnoDB table created using
the file-per-table mode goes
into its own tablespace file,
with a .ibd extension, inside the
database directory. This file
contains the table data and any
indexes for the table.
File-per-table mode, controlled by the
innodb_file_per_table option,
affects many aspects of InnoDB storage usage and performance,
and is enabled by default in MySQL 5.6.7 and higher.
This extension does not apply to the system tablespace, which consists of the ibdata files.
When a .ibd file is included in a compressed
backup by the MySQL Enterprise
Backup product, the compressed equivalent is a
.ibz file.
See also database, file-per-table, ibdata file, .ibz file, index, innodb_file_per_table, MySQL Enterprise Backup, system tablespace, table, tablespace.
A set of files with names such as ibdata1,
ibdata2, and so on, that make up the InnoDB
system tablespace. These files
contain metadata about InnoDB tables, (the
data dictionary), and the
storage areas for the undo log,
the change buffer, and the
doublewrite buffer. They also
can contain some or all of the table data also (depending on
whether the file-per-table mode
is in effect when each table is created). When the
innodb_file_per_table option is
enabled, data and indexes for newly created tables are stored in
separate .ibd files rather than
in the system tablespace.
The growth of the ibdata files is influenced
by the
innodb_autoextend_increment
configuration option.
See also change buffer, data dictionary, doublewrite buffer, file-per-table, .ibd file, innodb_file_per_table, system tablespace, undo log.
When the MySQL Enterprise
Backup product performs a
compressed backup, it
transforms each tablespace file
that is created using the
file-per-table setting from a
.ibd extension to a .ibz
extension.
The compression applied during backup is distinct from the compressed row format that keeps table data compressed during normal operation. A compressed backup operation skips the compression step for a tablespace that is already in compressed row format, as compressing a second time would slow down the backup but produce little or no space savings.
See also compressed backup, compressed row format, file-per-table, .ibd file, MySQL Enterprise Backup, tablespace.
Within an InnoDB FULLTEXT index, the data structure consisting of a document ID and positional information for a token (that is, a particular word).
See also FULLTEXT index.
A row lock that InnoDB acquires to ensure consistency, without you specifically requesting it.
See also row lock.
A type of database system that maintains data in memory, to avoid overhead due to disk I/O and translation between disk blocks and memory areas. Some in-memory databases sacrifice durability (the “D” in the ACID design philosophy) and are vulnerable to hardware, power, and other types of failures, making them more suitable for read-only operations. Other in-memory databases do use durability mechanisms such as logging changes to disk or using non-volatile memory.
MySQL features that are address the same kinds of memory-intensive processing include the InnoDB buffer pool, adaptive hash index, and read-only transaction optimization, the MEMORY storage engine, the MyISAM key cache, and the MySQL query cache.
See also ACID, adaptive hash index, buffer pool, disk-based, read-only transaction.
A type of hot backup, performed by the MySQL Enterprise Backup product, that only saves data changed since some point in time. Having a full backup and a succession of incremental backups lets you reconstruct backup data over a long period, without the storage overhead of keeping several full backups on hand. You can restore the full backup and then apply each of the incremental backups in succession, or you can keep the full backup up-to-date by applying each incremental backup to it, then perform a single restore operation.
The granularity of changed data is at the page level. A page might actually cover more than one row. Each changed page is included in the backup.
See also hot backup, MySQL Enterprise Backup, page.
A data structure that provides a fast lookup capability for rows of a table, typically by forming a tree structure (B-tree) representing all the values of a particular column or set of columns.
InnoDB tables always have a clustered index representing the primary key. They can also have one or more secondary indexes defined on one or more columns. Depending on their structure, secondary indexes can be classified as partial, column, or composite indexes.
Indexes are a crucial aspect of query performance. Database architects design tables, queries, and indexes to allow fast lookups for data needed by applications. The ideal database design uses a covering index where practical; the query results are computed entirely from the index, without reading the actual table data. Each foreign key constraint also requires an index, to efficiently check whether values exist in both the parent and child tables.
Although a B-tree index is the most common, a different kind of
data structure is used for hash
indexes, as in the MEMORY storage
engine and the InnoDB adaptive hash
index.
See also adaptive hash index, B-tree, child table, clustered index, column index, composite index, covering index, foreign key, hash index, parent table, partial index, primary key, query, row, secondary index, table.
A memory area that holds the token data for InnoDB
full-text search. It buffers
the data to minimize disk I/O when data is inserted or updated
in columns that are part of a FULLTEXT
index. The token data is written to disk when the
index cache becomes full. Each InnoDB
FULLTEXT index has its own separate index
cache, whose size is controlled by the configuration option
innodb_ft_cache_size.
See also full-text search, FULLTEXT index.
Extended SQL syntax for overriding the
indexes recommended by the
optimizer. For example, the FORCE INDEX,
USE INDEX, and IGNORE
INDEX clauses. Typically used when indexed columns
have unevenly distributed values, resulting in inaccurate
cardinality estimates.
See also cardinality, index.
In an index that applies to multiple columns (known as a composite index), the initial or leading columns of the index. A query that references the first 1, 2, 3, and so on columns of a composite index can use the index, even if the query does not reference all the columns in the index.
See also composite index, index.
See statistics.
A pseudo-record in an
index, representing the
gap below the smallest value in
that index. If a transaction has a statement such as
SELECT ... FOR UPDATE ... WHERE col < 10;,
and the smallest value in the column is 5, it is a lock on the
infimum record that prevents other transactions from inserting
even smaller values such as 0, -10, and so on.
See also gap, index, pseudo-record, supremum record.
The name of the database that
provides a query interface to the MySQL
data dictionary. (This name is
defined by the ANSI SQL standard.) To examine information
(metadata) about the database, you can query tables such as
INFORMATION_SCHEMA.TABLES and
INFORMATION_SCHEMA.COLUMNS, rather than using
SHOW commands that produce unstructured
output.
The information schema contains some tables that are specific to
InnoDB, such as
INNODB_LOCKS and
INNODB_TRX. You use these tables
not to see how the database is structured, but to get real-time
information about the workings of InnoDB tables to help with
performance monitoring, tuning, and troubleshooting. In
particular, these tables provide information about MySQL
features related to
compression, and
transactions and their
associated locks.
See also compression, data dictionary, database, InnoDB, lock, transaction.
A backup command from the
former InnoDB Hot Backup
product. It is superceded by the
mysqlbackup
command in the MySQL Enterprise
Backup product.
See also backup, InnoDB Hot Backup, MySQL Enterprise Backup, mysqlbackup command.
A MySQL component that combines high performance with
transactional capability for
reliability, robustness, and concurrent access. It embodies the
ACID design philosophy.
Represented as a storage
engine; it handles tables created or altered with the
ENGINE=INNODB clause. See
Section 14.2, “The InnoDB Storage Engine” for architectural
details and administration procedures, and
Section 8.5, “Optimizing for InnoDB Tables” for performance advice.
In MySQL 5.5 and higher, InnoDB is the default storage engine
for new tables and the ENGINE=INNODB clause
is not required. In MySQL 5.1 only, many of the advanced InnoDB
features require enabling the component known as the InnoDB
Plugin. See Section 14.2.1.1, “InnoDB as the Default MySQL Storage Engine” for the
considerations involved in transitioning to recent releases
where InnoDB tables are the default.
InnoDB tables are ideally suited for hot backups. See Section 23.2, “MySQL Enterprise Backup” for information about the MySQL Enterprise Backup product for backing up MySQL servers without interrupting normal processing.
See also ACID, hot backup, storage engine, transaction.
A licensed backup product, superceded in MySQL 5.1 and above by MySQL Enterprise Backup.
See also MySQL Enterprise Backup.
The innodb_autoinc_lock_mode
option controls the algorithm used for
auto-increment locking. When
you have an auto-incrementing primary
key, you can use statement-based replication only
with the setting
innodb_autoinc_lock_mode=1.
This setting is known as
consecutive lock mode, because
multi-row inserts within a transaction receive consecutive
auto-increment values. If you have
innodb_autoinc_lock_mode=2, which allows
higher concurrency for insert operations, use row-based
replication rather than statement-based replication. This
setting is known as interleaved
lock mode, because multiple multi-row insert statements running
at the same time can receive autoincrement values that are
interleaved. The setting
innodb_autoinc_lock_mode=0 is the previous
(traditional) default setting and should not be used except for
compatibility purposes.
See also auto-increment locking, mixed-mode insert, primary key.
The innodb_file_format option
determines the file format for
all InnoDB tablespaces created
after you specify a value for this option. To create tablespaces
other than the system
tablespace, you must also use the
file-per-table option.
Currently, you can specify the
Antelope and
Barracuda file formats.
See also Antelope, Barracuda, file format, file-per-table, innodb_file_per_table, system tablespace, tablespace.
A very important configuration option that affects many aspects
of InnoDB file storage, availability of features, and I/O
characteristics. In MySQL 5.6.7 and higher, it is enabled by
default. Prior to MySQL 5.6.7, it is disabled by default. The
innodb_file_per_table option
turns on file-per-table mode,
which stores each newly created InnoDB table and its associated
index in its own .ibd file,
outside the system tablespace.
This option affects the performance and storage considerations
for a number of SQL statements, such as
DROP TABLE and
TRUNCATE TABLE.
This option is needed to take full advantage of many other InnoDB features, such as such as table compression, or backups of named tables in MySQL Enterprise Backup.
This option was once static, but can now be set using the
SET
GLOBAL command.
For reference information, see
innodb_file_per_table. For
usage information, see
Section 14.2.2.1, “Managing InnoDB Tablespaces”.
See also compression, file-per-table, .ibd file, MySQL Enterprise Backup, system tablespace.
The innodb_lock_wait_timeout
option sets the balance between
waiting for shared resources to
become available, or giving up and handling the error, retrying,
or doing alternative processing in your application. Rolls back
any InnoDB transaction that waits more than a specified time to
acquire a lock. Especially
useful if deadlocks are caused
by updates to multiple tables controlled by different storage
engines; such deadlocks are not
detected automatically.
See also deadlock, deadlock detection, lock, wait.
The innodb_strict_mode option
controls whether InnoDB operates in strict
mode, where conditions that are normally treated as
warnings, cause errors instead (and the underlying statements
fail).
This mode is the default setting in MySQL 5.5.5 and higher.
See also strict mode.
One of the primary DML operations in SQL. The performance of inserts is a key factor in data warehouse systems that load millions of rows into tables, and OLTP systems where many concurrent connections might insert rows into the same table, in arbitrary order. If insert performance is important to you, you should learn about InnoDB features such as the insert buffer used in change buffering, and auto-increment columns.
See also auto-increment, change buffering, data warehouse, DML, InnoDB, insert buffer, OLTP, SQL.
Former name for the change buffer. Now that change buffering includes delete and update operations as well as inserts, “change buffer” is the preferred term.
See also change buffer, change buffering.
The technique of storing secondary index changes due to
INSERT operations in the
insert buffer rather than
writing them immediately, so that the physical writes can be
performed to minimize random I/O. It is one of the types of
change buffering; the others
are delete buffering and
purge buffering.
Insert buffering is not used if the secondary index is unique, because the uniqueness of new values cannot be verified before the new entries are written out. Other kinds of change buffering do work for unique indexes.
See also change buffer, change buffering, delete buffering, insert buffer, purge buffering, unique index.
A single mysqld daemon managing a data directory representing one or more databases with a set of tables. It is common in development, testing, and some replication scenarios to have multiple instances on the same server machine, each managing its own data directory and listening on its own port or socket. With one instance running a disk-bound workload, the server might still have extra CPU and memory capacity to run additional instances.
See also data directory, database, disk-bound, mysqld, replication, server.
Modifications at the source code level to collect performance
data for tuning and debugging. In MySQL, data collected by
instrumentation is exposed through a SQL interface using the
INFORMATION_SCHEMA and
PERFORMANCE_SCHEMA databases.
See also INFORMATION_SCHEMA, Performance Schema.
See intention lock.
A kind of lock that applies to
the table level, used to indicate what kind of lock the
transaction intends to acquire on rows in the table. Different
transactions can acquire different kinds of intention locks on
the same table, but the first transaction to acquire an
intention exclusive (IX) lock
on a table prevents other transactions from acquiring any S or X
locks on the table. Conversely, the first transaction to acquire
an intention shared (IS) lock
on a table prevents other transactions from acquiring any X
locks on the table. The two-phase process allows the lock
requests to be resolved in order, without blocking locks and
corresponding operations that are compatible. For more details
on this locking mechanism, see
Section 14.2.4.2, “InnoDB Lock Modes”.
See intention lock.
A data structure optimized for document retrieval systems, used in the implementation of InnoDB full-text search. The InnoDB FULLTEXT index, implemented as an inverted index, records the position of each word within a document, rather than the location of a table row. A single column value (a document stored as a text string) is represented by many entries in the inverted index.
See also full-text search, FULLTEXT index, ilist.
One of the foundations of database processing. Isolation is the I in the acronym ACID; the isolation level is the setting that fine-tunes the balance between performance and reliability, consistency, and reproducibility of results when multiple transactions are making changes and performing queries at the same time.
From highest amount of consistency and protection to the least, the isolation levels supported by InnoDB are: SERIALIZABLE, REPEATABLE READ, READ COMMITTED, and READ UNCOMMITTED.
With InnoDB tables, many users can keep the default isolation level (REPEATABLE READ) for all operations. Expert users might choose the read committed level as they push the boundaries of scalability with OLTP processing, or during data warehousing operations where minor inconsistencies do not affect the aggregate results of large amounts of data. The levels on the edges (SERIALIZABLE and READ UNCOMMITTED) change the processing behavior to such an extent that they are rarely used.
See also ACID, READ COMMITTED, READ UNCOMMITTED, REPEATABLE READ, SERIALIZABLE, transaction.
J
A query that retrieves data from more than one table, by referencing columns in the tables that hold identical values. Ideally, these columns are part of an InnoDB foreign key relationship, which ensures referential integrity and that the join columns are indexed. Often used to save space and improve query performance by replacing repeated strings with numeric IDs, in a normalized data design.
See also foreign key, index, normalized, query, referential integrity.
K
An option to specify the size of data pages within an InnoDB table that uses compressed row format. The default is 8 kilobytes. Lower values risk hitting internal limits that depend on the combination of row size and compression percentage.
See also compressed row format.
L
A lightweight structure used by InnoDB to implement a lock for its own internal memory structures, typically held for a brief time measured in milliseconds or microseconds. A general term that includes both mutexes (for exclusive access) and rw-locks (for shared access). Certain latches are the focus of InnoDB performance tuning, such as the data dictionary mutex. Statistics about latch use and contention are available through the Performance Schema interface.
See also data dictionary, lock, locking, mutex, Performance Schema, rw-lock.
The InnoDB buffer pool is represented as a list of memory pages. The list is reordered as new pages are accessed and enter the buffer pool, as pages within the buffer pool are accessed again and are considered newer, and as pages that are not accessed for a long time are evicted from the buffer pool. The buffer pool is actually divided into sublists, and the replacement policy is a variation of the familiar LRU technique.
See also buffer pool, eviction, LRU, sublist.
The high-level notion of an object that controls access to a resource, such as a table, row, or internal data structure, as part of a locking strategy. For intensive performance tuning, you might delve into the actual structures that implement locks, such as mutexes and latches.
An operation used in some database systems that converts many row locks into a single table lock, saving memory space but reducing concurrent access to the table. InnoDB uses a space-efficient representation for row locks, so that lock escalation is not needed.
See also locking, row lock, table lock.
A shared (S) lock allows a transaction to read a row. Multiple transactions can acquire an S lock on that same row at the same time.
An exclusive (X) lock allows a transaction to update or delete a row. No other transaction can acquire any kind of lock on that same row at the same time.
Intention locks apply to the table level, and are used to indicate what kind of lock the transaction intends to acquire on rows in the table. Different transactions can acquire different kinds of intention locks on the same table, but the first transaction to acquire an intention exclusive (IX) lock on a table prevents other transactions from acquiring any S or X locks on the table. Conversely, the first transaction to acquire an intention shared (IS) lock on a table prevents other transactions from acquiring any X locks on the table. The two-phase process allows the lock requests to be resolved in order, without blocking locks and corresponding operations that are compatible.
See also intention lock, lock, locking.
The system of protecting a transaction from seeing or changing data that is being queried or changed by other transactions. The locking strategy must balance reliability and consistency of database operations (the principles of the ACID philosophy) against the performance needed for good concurrency. Fine-tuning the locking strategy often involves choosing an isolation level and ensuring all your database operations are safe and reliable for that isolation level.
See also ACID, concurrency, isolation level, latch, lock, mutex, transaction.
A SELECT statement that also
performs a locking operation on
an InnoDB table. Either
SELECT ... FOR
UPDATE or SELECT ... LOCK
IN SHARE MODE. It has the potential to produce a
deadlock, depending on the
isolation level of the
transaction. The opposite of a non-locking
read. Not allowed for global tables in a
read-only transaction.
See also deadlock, isolation level, locking, non-locking read, read-only transaction.
In the InnoDB context, “log” or “log files” typically refers to the redo log represented by the ib_logfile* files. Another log area, which is physically part of the system tablespace, is the undo log.
Other kinds of logs that are important in MySQL are the error log (for diagnosing startup and runtime problems), binary log (for working with replication and performing point-in-time restores), the general query log (for diagnosing application problems), and the slow query log (for diagnosing performance problems).
See also binary log, error log, general query log, ib_logfile, redo log, slow query log, system tablespace, undo log.
The memory area that holds data to be written to the
log files that make up the
redo log. It is controlled by
the innodb_log_buffer_size
configuration option.
One of the
ib_logfile files
that make up the redo log. Data
is written to these files from the log
buffer memory area.
N
See also ib_logfile, log buffer, redo log.
The set of files that make up the redo
log, typically named ib_logfile0
and ib_logfile1. (For that reason, sometimes
referred to collectively as
ib_logfile.)
See also ib_logfile, redo log.
A type of operation that involves high-level, abstract aspects such as tables, queries, indexes, and other SQL concepts. Typically, logical aspects are important to make database administration and application development convenient and usable. Contrast with physical.
See also logical backup, physical.
A backup that reproduces table
structure and data, without copying the actual data files. For
example, the
mysqldump
command produces a logical backup, because its output contains
statements such as CREATE TABLE and
INSERT that can re-create the data. Contrast
with physical backup. A logical
backup offers flexibility (for example, you could edit table
definitions or insert statements before restoring), but can take
substantially longer to restore
than a physical backup.
See also backup, mysqldump, physical backup, restore.
In MySQL 5.1, a prefix added to InnoDB configuration options when installing the InnoDB Plugin after server startup, so any new configuration options not recognized by the current level of MySQL do not cause a startup failure. MySQL processes configuration options that start with this prefix, but gives a warning rather than a failure if the part after the prefix is not a recognized option.
See also plugin.
A value representing a lower limit, typically a threshold value at which some corrective action begins or becomes more aggressive. Contrast with high-water mark.
See also high-water mark.
An acronym for “least recently used”, a common
method for managing storage areas. The items that have not been
used recently are evicted when
space is needed to cache newer items. InnoDB uses the LRU
mechanism by default to manage the
pages within the
buffer pool, but makes
exceptions in cases where a page might be read only a single
time, such as during a full table
scan. This variation of the LRU algorithm is called
the midpoint insertion
strategy. The ways in which the buffer pool
management differs from the traditional LRU algorithm is
fine-tuned by the options
innodb_old_blocks_pct,
innodb_old_blocks_time, and the
new MySQL 5.6 options
innodb_lru_scan_depth and
innodb_flush_neighbors.
See also buffer pool, eviction, full table scan, midpoint insertion strategy.
Acronym for “log sequence number”. This arbitrary, ever-increasing value represents a point in time corresponding to operations recorded in the redo log. (This point in time is regardless of transaction boundaries; it can fall in the middle of one or more transactions.) It is used internally by InnoDB during crash recovery and for managing the buffer pool.
In the MySQL Enterprise Backup
product, you can specify an LSN to represent the point in time
from which to take an incremental
backup. The relevant LSN is displayed by the output
of the ibbackup command. Once you have the
LSN corresponding to the time of a full backup, you can specify
that value to take a subsequent incremental backup, whose output
contains another LSN for the next incremental backup.
See also crash recovery, incremental backup, MySQL Enterprise Backup, redo log, transaction.
M
Frequently shortened to “master”. A database server machine in a replication scenario that processes the initial insert, update, and delete requests for data. These changes are propagated to, and repeated on, other servers known as slave servers.
See also replication, slave server.
An InnoDB thread that performs various tasks in the background. Most of these tasks are I/O related, such as writing changes from the insert buffer to the appropriate secondary indexes.
To improve concurrency, sometimes actions are moved from the master thread to separate background threads. For example, in MySQL 5.6 and higher, dirty pages are flushed from the buffer pool by the page cleaner thread rather than the master thread.
See also buffer pool, dirty page, flush, insert buffer, page cleaner, thread.
See also metadata lock.
A popular component of many MySQL and NoSQL software stacks, allowing fast reads and writes for single values and caching the results entirely in memory. Traditionally, applications required extra logic to write the same data to a MySQL database for permanent storage, or to read data from a MySQL database when it was not cached yet in memory. Now, applications can use the simple memcached protocol, supported by client libraries for many languages, to communicate directly with MySQL servers using InnoDB or MySQL Cluster tables. These NoSQL interfaces to MySQL tables allow applications to achieve higher read and write performance than by issuing SQL commands directly, and can simplify application logic and deployment configurations for systems that already incorporated memcached for in-memory caching.
The memcached interface to InnoDB tables is
available in MySQL 5.6 and higher; see
Section 14.2.10, “InnoDB Integration with memcached” for details. The
memcached interface to MySQL Cluster tables
is available in MySQL Cluster 7.2; see
http://dev.mysql.com/doc/ndbapi/en/ndbmemcache.html
for details.
To apply changes to data cached in memory, such as when a page is brought into the buffer pool, and any applicable changes recorded in the change buffer are incorporated into the page in the buffer pool. The updated data is eventually written to the tablespace by the flush mechanism.
See also buffer pool, change buffer, flush, tablespace.
A type of lock that prevents DDL operations on a table that is being used at the same time by another transaction. For details, see Section 8.10.4, “Metadata Locking Within Transactions”.
Enhancements to online
operations, particularly in MySQL 5.6 and higher, are focused on
reducing the amount of metadata locking. The objective is for
DDL operations that do not change the table structure (such as
CREATE INDEX and
DROP INDEX for
InnoDB tables) to proceed while the table is
being queried, updated, and so on by other transactions.
See also DDL, lock, online, transaction.
A feature implemented by the
innodb_metrics table in the
information_schema, in MySQL
5.6 and higher. You can query
counts and totals for low-level
InnoDB operations, and use the results for performance tuning in
combination with data from the
performance_schema.
See also counter, INFORMATION_SCHEMA, Performance Schema.
The technique of initially bringing
pages into the InnoDB
buffer pool not at the
“newest” end of the list, but instead somewhere in
the middle. The exact location of this point can vary, based on
the setting of the
innodb_old_blocks_pct option.
The intent is that blocks that are only read once, such as
during a full table scan, can
be aged out of the buffer pool sooner than with a strict
LRU algorithm.
See also buffer pool, full table scan, LRU, page.
An internal phase of InnoDB processing, when making changes to the data dictionary during a DDL operation. The changes to the data dictionary are made without affecting the transaction that is being processed by MySQL.
See also data dictionary, DDL, transaction.
An INSERT statement where
auto-increment values are
specified for some but not all of the new rows. For example, a
multi-value INSERT could specify a value for
the auto-increment column in some cases and
NULL in other cases.
InnoDB generates auto-increment values for
the rows where the column value was specified as
NULL. Another example is an
INSERT ...
ON DUPLICATE KEY UPDATE statement, where
auto-increment values might be generated but not used, for any
duplicate rows that are processed as UPDATE
rather than INSERT statements.
Can cause consistency issues between master and slave servers in a replication configuration. Can require adjusting the value of the innodb_autoinc_lock_mode configuration option.
See also auto-increment, innodb_autoinc_lock_mode, master server, replication, slave server.
A file containing references to other tables, used by the
MERGE storage engine. Files with this
extension are always included in backups produced by the
mysqlbackup command of the
MySQL Enterprise Backup
product.
See also MySQL Enterprise Backup, mysqlbackup command.
A type of processor that can take advantage of multi-threaded programs, such as the MySQL server.
See MVCC.
Informal abbreviation for “mutex variable”. (Mutex itself is short for “mutual exclusion”.) The low-level object that InnoDB uses to represent and enforce exclusive-access locks to internal in-memory data structures. Once the lock is acquired, any other process, thread, and so on is prevented from acquiring the same lock. Contrast with rw-locks, which allow shared access. Mutexes and rw-locks are known collectively as latches.
See also latch, lock, Performance Schema, Pthreads, rw-lock.
Acronym for “multiversion concurrency control”. This technique lets InnoDB transactions with certain isolation levels to perform consistent read operations; that is, to query rows that are being updated by other transactions, and see the values from before those updates occurred. This is a powerful technique to increase concurrency, by allowing queries to proceed without waiting due to locks held by the other transactions.
This technique is not universal in the database world. Some other database products, and some other MySQL storage engines, do not support it.
See also ACID, concurrency, consistent read, isolation level, lock, transaction.
The name, on UNIX or Linux systems, of the MySQL option file.
See also my.ini, option file.
The name, on Windows systems, of the MySQL option file.
See also my.cnf, option file.
The file that MySQL uses to store data for a MyISAM table. Files
with this extension are always included in backups produced by
the mysqlbackup command of the
MySQL Enterprise Backup
product.
See also .MYI file, MySQL Enterprise Backup, mysqlbackup command.
A file that MySQL uses to store indexes for a MyISAM table.
Files with this extension are always included in backups
produced by the mysqlbackup command of the
MySQL Enterprise Backup
product.
See also .MYD file, MySQL Enterprise Backup, mysqlbackup command.
The mysql program is the command-line
interpreter for the MySQL database. It processes
SQL statements, and also
MySQL-specific commands such as SHOW TABLES,
by passing requests to the
mysqld
daemon.
A licensed product, superceding InnoDB Hot Backup, that performs hot backups of MySQL databases. It offers the most efficiency and flexibility when backing up InnoDB tables, but can also back up MyISAM and other kinds of tables.
See also hot backup, InnoDB.
A command-line tool of the MySQL Enterprise Backup product. It performs a hot backup operation for InnoDB tables, and a warm backup for MyISAM and other kinds of tables. See Section 23.2, “MySQL Enterprise Backup” for more information about this command.
See also hot backup, ibbackup command, MySQL Enterprise Backup, warm backup.
The mysqld program is the database engine for
the MySQL database. It runs as a UNIX daemon or Windows service,
constantly waiting for requests and performing maintenance work
in the background.
See also mysql.
A command that performs a logical backup of some combination of databases, tables, and table data. The results are SQL statements that reproduce the original schema objects, data, or both. For substantial amounts of data, a physical backup solution such as MySQL Enterprise Backup is faster, particularly for the restore operation.
See also logical backup, MySQL Enterprise Backup, physical backup, restore.
N
A indexed column, typically a primary key, where the values have some real-world significance. Usually advised against because:
If the value should ever change, there is potentially a lot of index maintenance to re-sort the clustered index and update the copies of the primary key value that are repeated in each secondary index.
Even seemingly stable values can change in unpredictable ways that are difficult to represent correctly in the database. For example, one country can change into two or several, making the original country code obsolete. Or, rules about unique values might have exceptions. For example, even if taxpayer IDs are intended to be unique to a single person, a database might have to handle records that violate that rule, such as in cases of identity theft. Taxpayer IDs and other sensitive ID numbers also make poor primary keys, because they may need to be secured, encrypted, and otherwise treated differently than other columns.
Thus, it is typically better to use arbitrary numeric values to form a synthetic key, for example using an auto-increment column.
See also auto-increment, primary key, secondary index, synthetic key.
Any page in the same
extent as a particular page.
When a page is selected to be
flushed, any neighbor pages
that are dirty are typically
flushed as well, as an I/O optimization for traditional hard
disks. In MySQL 5.6 and up, this behavior can be controlled by
the configuration variable
innodb_flush_neighbors; you
might turn that setting off for SSD drives, which do not have
the same overhead for writing smaller batches of data at random
locations.
See also dirty page, extent, flush, page.
A combination of a record lock on the index record and a gap lock on the gap before the index record.
See also gap lock, locking, record lock.
An industry term that means the same as asynchronous I/O.
See also asynchronous I/O.
A query that does not use the
SELECT ... FOR UPDATE or SELECT ...
LOCK IN SHARE MODE clauses. The only kind of query
allowed for global tables in a read-only
transaction. The opposite of a
locking read.
See also locking read, query, read-only transaction.
The situation when a query retrieves data, and a later query within the same transaction retrieves what should be the same data, but the queries return different results (changed by another transaction committing in the meantime).
This kind of operation goes against the ACID principle of database design. Within a transaction, data should be consistent, with predictable and stable relationships.
Among different isolation levels, non-repeatable reads are prevented by the serializable read and repeatable read levels, and allowed by the consistent read, and read uncommitted levels.
See also ACID, consistent read, isolation level, READ UNCOMMITTED, REPEATABLE READ, SERIALIZABLE, transaction.
A database design strategy where data is split into multiple tables, and duplicate values condensed into single rows represented by an ID, to avoid storing, querying, and updating redundant or lengthy values. It is typically used in OLTP applications.
For example, an address might be given a unique ID, so that a census database could represent the relationship lives at this address by associating that ID with each member of a family, rather than storing multiple copies of a complex value such as 123 Main Street, Anytown, USA.
For another example, although a simple address book application might store each phone number in the same table as a person's name and address, a phone company database might give each phone number a special ID, and store the numbers and IDs in a separate table. This normalized representation could simplify large-scale updates when area codes split apart.
Normalization is not always recommended. Data that is primarily queried, and only updated by deleting entirely and reloading, is often kept in fewer, larger tables with redundant copies of duplicate values. This data representation is referred to as denormalized, and is frequently found in data warehousing applications.
See also denormalized, foreign key, OLTP, relational.
A broad term for a set of data access technologies that do not
use the SQL language as their
primary mechanism for reading and writing data. Some NoSQL
technologies act as key-value stores, only accepting
single-value reads and writes; some relax the restrictions of
the ACID methodology; still
others do not require a pre-planned
schema. MySQL users can combine
NoSQL-style processing for speed and simplicity with SQL
operations for flexibility and convenience, by using the
memcached API to directly
access some kinds of MySQL tables. The
memcached interface to InnoDB tables is
available in MySQL 5.6 and higher; see
Section 14.2.10, “InnoDB Integration with memcached” for details. The
memcached interface to MySQL Cluster tables
is available in MySQL Cluster 7.2; see
http://dev.mysql.com/doc/ndbapi/en/ndbmemcache.html
for details.
A type of constraint that specifies that a column cannot contain any NULL values. It helps to preserve referential integrity, as the database server can identify data with erroneous missing values. It also helps in the arithmetic involved in query optimization, allowing the optimizer to predict the number of entries in an index on that column.
See also column, constraint, NULL, primary key, referential integrity.
A special value in SQL,
indicating the absence of data. Any arithmetic operation or
equality test involving a NULL value, in turn
produces a NULL result. (Thus it is similar
to the IEEE floating-point concept of NaN, “not a
number”.) Any aggregate calculation such as
AVG() ignores rows with
NULL values, when determining how many rows
to divide by. The only test that works with
NULL values uses the SQL idioms IS
NULL or IS NOT NULL.
NULL values play a part in index operations,
because for performance a database must minimize the overhead of
keeping track of missing data values. Typically,
NULL values are not stored in an index,
because a query that tests an indexed column using a standard
comparison operator could never match a row with a
NULL value for that column. For the same
reason, unique indexes do not prevent NULL
values; those values simply are not represented in the index.
Declaring a NOT NULL constraint on a column
provides reassurance that there are no rows left out of the
index, allowing for better query optimization (accurate counting
of rows and estimation of whether to use the index).
Because the primary key must be
able to uniquely identify every row in the table, a
single-column primary key cannot contain any
NULL values, and a multi-column primary key
cannot contain any rows with NULL values in
all columns.
Although the Oracle database allows a NULL
value to be concatenated with a string, InnoDB treats the result
of such an operation as NULL.
See also index, primary key, SQL.
O
A column containing variable-length data (such as
BLOB and VARCHAR) that is
too long to fit on a B-tree
page. The data is stored in overflow
pages. The DYNAMIC row format in
the InnoDB Barracuda file
format is more efficient for such storage than the older
COMPACT row format.
See also B-tree, Barracuda, overflow page.
Acronym for “Online Transaction Processing”. A database system, or a database application, that runs a workload with many transactions, with frequent writes as well as reads, typically affecting small amounts of data at a time. For example, an airline reservation system or an application that processes bank deposits. The data might be organized in normalized form for a balance between DML (insert/update/delete) efficiency and query efficiency. Contrast with data warehouse.
With its row-level locking and transactional capability, InnoDB is the ideal storage engine for MySQL tables used in OLTP applications.
See also data warehouse, DML, InnoDB, query, row lock, transaction.
A type of operation that involves no downtime, blocking, or restricted operation for the database. Typically applied to DDL. Operations that shorten the periods of restricted operation, such as fast index creation, have evolved into a wider set of online DDL operations in MySQL 5.6.
In the context of backups, a hot backup is an online operation and a warm backup is partially an online operation.
See also DDL, Fast Index Creation, hot backup, online DDL, warm backup.
A feature that improves the performance, concurrency, and
availability of InnoDB tables during
DDL (primarily
ALTER TABLE) operations. See
Section 14.2.2.6, “Online DDL for InnoDB Tables” for details.
The details vary according to the type of operation. In some
cases, the table can be modified concurrently while the
ALTER TABLE is in progress. The operation
might be able to be performed without doing a table copy, or
using a specially optimized type of table copy. Space usage is
controlled by the
innodb_online_alter_log_max_size
configuration option.
This feature is an enhancement of the Fast Index Creation feature in MySQL 5.5 and the InnoDB Plugin for MySQL 5.1.
See also DDL, Fast Index Creation, online.
A file containing database configuration information. Files with
this extension are always included in backups produced by the
mysqlbackup command of the
MySQL Enterprise Backup
product.
See also MySQL Enterprise Backup, mysqlbackup command.
A methodology that guides low-level implementation decisions for a relational database system. The requirements of performance and concurrency in a relational database mean that operations must be started or dispatched quickly. The requirements of consistency and referential integrity mean that any operation could fail: a transaction might be rolled back, a DML operation could violate a constraint, a request for a lock could cause a deadlock, a network error could cause a timeout. An optimistic strategy is one that assumes most requests or attempts will succeed, so that relatively little work is done to prepare for the failure case. When this assumption is true, the database does little unnecessary work; when requests do fail, extra work must be done to clean up and undo changes.
InnoDB uses optimistic strategies for operations such as locking and commits. For example, data changed by a transaction can be written to the data files before the commit occurs, making the commit itself very fast, but requiring more work to undo the changes if the transaction is rolled back.
The opposite of an optimistic strategy is a pessimistic one, where a system is optimized to deal with operations that are unreliable and frequently unsuccessful. This methodology is rare in a database system, because so much care goes into choosing reliable hardware, networks, and algorithms.
See also commit, concurrency, DML, locking, pessimistic.
The MySQL component that determines the best indexes and join order to use for a query, based on characteristics and data distribution of the relevant tables.
A configuration parameter for MySQL, either stored in the option file or passed on the command line.
For the options that apply to
InnoDB tables, each option name
starts with the prefix innodb_.
See also InnoDB, option file.
The file that holds the configuration
options for the MySQL instance.
Traditionally, on Linux and UNIX this file is named
my.cnf, and on Windows it is named
my.ini.
See also configuration file, my.cnf, option.
Separately allocated disk pages
that hold variable-length columns (such as
BLOB and VARCHAR) that are
too long to fit on a B-tree
page. The associated columns are known as
off-page columns.
See also B-tree, off-page column, page.
P
A unit representing how much data InnoDB transfers at any one time between disk (the data files) and memory (the buffer pool). A page can contain one or more rows, depending on how much data is in each row. If a row does not fit entirely into a single page, InnoDB sets up additional pointer-style data structures so that the information about the row can be stored in one page.
One way to fit more data in each page is to use compressed row format. For tables that use BLOBs or large text fields, compact row format allows those large columns to be stored separately from the rest of the row, reducing I/O overhead and memory usage for queries that do not reference those columns.
When InnoDB reads or writes sets of pages as a batch to increase I/O throughput, it reads or writes an extent at a time.
All the InnoDB disk data structures within a MySQL instance share the same page size.
See also buffer pool, compact row format, compressed row format, data files, extent, page size, row.
An InnoDB background thread that flushes dirty pages from the buffer pool. Prior to MySQL 5.6, this activity was performed by the master thread
See also buffer pool, dirty page, flush, master thread, thread.
For releases up to and including MySQL 5.5, the size of each InnoDB page is fixed at 16 kilobytes. This value represents a balance: large enough to hold the data for most rows, yet small enough to minimize the performance overhead of transferring unneeded data to memory. Other values are not tested or supported.
Starting in MySQL 5.6, the page size for an InnoDB
instance can be either 4KB,
8KB, or 16KB, controlled by the
innodb_page_size configuration
option. You set the size when creating the MySQL instance, and
it remains constant afterwards. The same page size applies to
all InnoDB tablespaces, both
the system tablespace and any
separate tablespaces created in
file-per-table mode.
Smaller page sizes can help performance with storage devices that use small block sizes, particularly for SSD devices in disk-bound workloads, such as for OLTP applications. As individual rows are updated, less data is copied into memory, written to disk, reorganized, locked, and so on.
See also disk-bound, file-per-table, instance, OLTP, page, SSD, system tablespace, tablespace.
A file containing partition definitions. Files with this
extension are always included in backups produced by the
mysqlbackup command of the
MySQL Enterprise Backup
product.
See also MySQL Enterprise Backup, mysqlbackup command.
The table in a foreign key
relationship that holds the initial column values pointed to
from the child table. The
consequences of deleting, or updating rows in the parent table
depend on the ON UPDATE and ON
DELETE clauses in the foreign key definition. Rows
with corresponding values in the child table could be
automatically deleted or updated in turn, or those columns could
be set to NULL, or the operation could be
prevented.
See also child table, foreign key.
A backup that contains some of the tables in a MySQL database, or some of the databases in a MySQL instance. Contrast with full backup.
See also backup, full backup, table.
An index that represents only
part of a column value, typically the first N characters (the
prefix) of a long
VARCHAR value.
See also index, index prefix.
The performance_schema schema, in MySQL 5.5
and up, presents a set of tables that you can query to get
detailed information about the performance characteristics of
many internal parts of the MySQL server.
A feature in MySQL 5.6 that stores
index statistics for InnoDB
tables on disk, providing
better plan stability for
queries. The statistics are
recalculated only by running the ALTER
TABLE statement.
See also index, optimizer, plan stability, query, table.
A methodology that sacrifices performance or concurrency in favor of safety. It is appropriate if a high proportion of requests or attempts might fail, or if the consequences of a failed request are severe. InnoDB uses what is known as a pessimistic locking strategy, to minimize the chance of deadlocks. At the application level, you might avoid deadlocks by using a pessimistic strategy of acquiring all locks needed by a transaction at the very beginning.
Many built-in database mechanisms use the opposite optimistic methodology.
See also deadlock, locking, optimistic.
A row that appears in the result set of a query, but not in the
result set of an earlier query. For example, if a query is run
twice within a transaction, and
in the meantime, another transaction commits after inserting a
new row or updating a row so that it matches the
WHERE clause of the query.
This occurrence is known as a phantom read. It is harder to guard against than a non-repeatable read, because locking all the rows from the first query result set does not prevent the changes that cause the phantom to appear.
Among different isolation levels, phantom reads are prevented by the serializable read level, and allowed by the repeatable read, consistent read, and read uncommitted levels.
See also consistent read, isolation level, non-repeatable read, READ UNCOMMITTED, REPEATABLE READ, SERIALIZABLE, transaction.
A type of operation that involves hardware-related aspects such as disk blocks, memory pages, files, bits, disk reads, and so on. Typically, physical aspects are important during expert-level performance tuning and problem diagnosis. Contrast with logical.
See also logical, physical backup.
A backup that copies the actual
data files. For example, the
mysqlbackup
command of the MySQL Enterprise
Backup product produces a physical backup, because
its output contains data files that can be used directly by the
mysqld server, resulting in a faster
restore operation. Contrast
with logical backup.
See also backup, logical backup, MySQL Enterprise Backup, restore.
Acronym for point-in-time recovery.
See also point-in-time recovery.
A property of a query execution plan, where the optimizer makes the same choices each time for a given query, so that performance is consistent and predictable.
See also query, query execution plan.
In MySQL 5.1 and earlier, a separately installable form of the InnoDB storage engine that includes features and performance enhancements not included in the built-in InnoDB for those releases.
For MySQL 5.5 and higher, the MySQL distribution includes the very latest InnoDB features and performance enhancements, known as InnoDB 1.1, and there is no longer a separate InnoDB Plugin.
This distinction is important mainly in MySQL 5.1, where a feature or bug fix might apply to the InnoDB Plugin but not the built-in InnoDB, or vice versa.
The process of restoring a backup to recreate the state of the database at a specific date and time. Commonly abbreviated PITR. Because it is unlikely that the specified time corresponds exactly to the time of a backup, this technique usually requires a combination of a physical backup and a logical backup. For example, with the MySQL Enterprise Backup product, you restore the last backup that you took before the specified point in time, then replay changes from the binary log between the time of the backup and the PITR time.
See also backup, logical backup, MySQL Enterprise Backup, physical backup, PITR.
See index prefix.
A set of backup files, produced by the MySQL Enterprise Backup product, after all the stages of applying binary logs and incremental backups are finished. The resulting files are ready to be restored. Prior to the apply steps, the files are known as a raw backup.
See also binary log, hot backup, incremental backup, MySQL Enterprise Backup, raw backup, restore.
A set of columns -- and by implication, the index based on this
set of columns -- that can uniquely identify every row in a
table. As such, it must be a unique index that does not contain
any NULL values.
InnoDB requires that every table has such an index (also called the clustered index or cluster index), and organizes the table storage based on the column values of the primary key.
When choosing primary key values, consider using arbitrary values (a synthetic key) rather than relying on values derived from some other source (a natural key).
See also clustered index, index, natural key, synthetic key.
An instance of an executing program. The operating switches between multiple running processes, allowing for a certain degree of concurrency. On most operating systems, processes can contain multiple threads of execution that share resources. Context-switching between threads is faster than the equivalent switching between processes.
See also concurrency, thread.
An artificial record in an index, used for locking key values or ranges that do not currently exist.
See also infimum record, locking, supremum record.
The POSIX threads standard, which defines an API for threading and locking operations on UNIX and Linux systems. On UNIX and Linux systems, InnoDB uses this implementation for mutexes.
See also mutex.
A type of garbage collection performed by a separate thread,
running on a periodic schedule. The purge includes these
actions: removing obsolete values from indexes; physically
removing rows that were marked for deletion by previous
DELETE statements.
See also crash recovery, delete, doublewrite buffer.
The technique of storing index changes due to
DELETE operations in the
insert buffer rather than
writing them immediately, so that the physical writes can be
performed to minimize random I/O. (Because delete operations are
a two-step process, this operation buffers the write that
normally purges an index record that was previously marked for
deletion.) It is one of the types of
change buffering; the others
are insert buffering. and
delete buffering
See also change buffer, change buffering, delete buffering, insert buffer, insert buffering.
Another name for the InnoDB
history list. Related to the
innodb_max_purge_lag
configuration option.
See also history list, purge.
A thread within the InnoDB
process that is dedicated to performing the periodic
purge operation. In MySQL 5.6
and higher, multiple purge threads are enabled by the
innodb_purge_threads
configuration option.
Q
In SQL, an operation that reads information from one or more tables. Depending on the organization of data and the parameters of the query, the lookup might be optimized by consulting an index. If multiple tables are involved, the query is known as a join.
For historical reasons, sometimes discussions of internal processing for statements use “query” in a broader sense, including other types of MySQL statements such as DDL and DML statements.
The set of decisions made by the optimizer about how to perform a query most efficiently, including which index or indexes to use, and the order in which to join tables. Plan stability involves the same choices being made consistently for a given query.
See also index, join, plan stability, query.
See general query log.
To reduce the amount of database activity, often in preparation
for an operation such as an ALTER
TABLE, a backup, or a
shutdown. Might or might not
involve doing as much flushing
as possible, so that InnoDB
does not continue doing background I/O.
In MySQL 5.6 and higher, the syntax FLUSH TABLES ...
FOR EXPORT writes some data to disk for
InnoDB tables that make it simpler to back up
those tables by copying the data files.
R
Acronym for “Redundant Array of Inexpensive Drives”. Spreading I/O operations across multiple drives enables greater concurrency at the hardware level, and improves the efficiency of low-level write operations that otherwise would be performed in sequence.
See also concurrency.
A technique for quickly estimating the number of different values in a column (the column's cardinality). InnoDB samples pages at random from the index and uses that data to estimate the number of different values. This operation occurs when each table is first opened.
Originally, the number of sampled pages was fixed at 8; now, it
is determined by the setting of the
innodb_stats_sample_pages
parameter.
The way the random pages are picked depends on the setting of the innodb_use_legacy_cardinality_algorithm parameter. The default setting (OFF) has better randomness than in older releases.
See also cardinality.
The initial set of backup files produced by the MySQL Enterprise Backup product, before the changes reflected in the binary log and any incremental backups are applied. At this stage, the files are not ready to restore. After these changes are applied, the files are known as a prepared backup.
See also binary log, hot backup, ibbackup_logfile, incremental backup, MySQL Enterprise Backup, prepared backup, restore.
An isolation level that uses a locking strategy that relaxes some of the protection between transactions, in the interest of performance. Transactions cannot see uncommitted data from other transactions, but they can see data that is committed by another transaction after the current transaction started. Thus, a transaction never sees any bad data, but the data that it does see may depend to some extent on the timing of other transactions.
When a transaction with this isolation level performs
UPDATE ... WHERE or DELETE ...
WHERE operations, other transactions might have to
wait. The transaction can perform SELECT ... FOR
UPDATE, and LOCK IN SHARE MODE
operations without making other transactions wait.
See also ACID, isolation level, locking, REPEATABLE READ, SERIALIZABLE, transaction.
The isolation level that provides the least amount of protection between transactions. Queries employ a locking strategy that allows them to proceed in situations where they would normally wait for another transaction. However, this extra performance comes at the cost of less reliable results, including data that has been changed by other transactions and not committed yet (known as dirty read). Use this isolation level only with great caution, and be aware that the results might not be consistent or reproducible, depending on what other transactions are doing at the same time. Typically, transactions with this isolation level do only queries, not insert, update, or delete operations.
See also ACID, dirty read, isolation level, locking, transaction.
An internal snapshot used by the MVCC mechanism of InnoDB. Certain transactions, depending on their isolation level, see the data values as they were at the time the transaction (or in some cases, the statement) started. Isolation levels that use a read view are REPEATABLE READ, READ COMMITTED, and READ UNCOMMITTED.
See also isolation level, MVCC, READ COMMITTED, READ UNCOMMITTED, REPEATABLE READ, transaction.
A type of I/O request that prefetches a group of
pages (an entire
extent) into the
buffer pool asynchronously, in
anticipation that these pages will be needed soon. The linear
read-ahead technique prefetches all the pages of one extent
based on access patterns for pages in the preceding extent, and
is part of all MySQL versions starting with the InnoDB Plugin
for MySQL 5.1. The random read-ahead technique prefetches all
the pages for an extent once a certain number of pages from the
same extent are in the buffer pool. Random read-ahead is not
part of MySQL 5.5, but is re-introduced in MySQL 5.6 under the
control of the innodb_random_read_ahead
configuration option.
See also buffer pool, extent, page.
A type of transaction that can be optimized for
InnoDB tables by eliminating some of the
bookkeeping involved with creating a read
view for each transaction. Can only perform
non-locking read queries. It
can be started explicitly with the syntax
START TRANSACTION READ
ONLY, or automatically under certain conditions. See
Section 14.2.5.2.2, “Optimizations for Read-Only Transactions” for details.
See also non-locking read, read view, transaction.
A lock on an index record. For
example, SELECT c1 FOR UPDATE FROM t WHERE c1 =
10; prevents any other transaction from inserting,
updating, or deleting rows where the value of
t.c1 is 10. Contrast with
gap lock and
next-key lock.
See also gap lock, lock, next-key lock.
The data, in units of records, recorded in the redo log when DML statements make changes to InnoDB tables. It is used during crash recovery to correct data written by incomplete transactions. The ever-increasing LSN value represents the cumulative amount of redo data that has passed through the redo log.
See also crash recovery, DML, LSN, redo log, transaction.
A disk-based data structure used during crash recovery, to correct data written by incomplete transactions. During normal operation, it encodes requests to change InnoDB table data, which result from SQL statements or low-level API calls through NoSQL interfaces. Modifications that did not finish updating the data files before an unexpected shutdown are replayed automatically.
The redo log is physically represented as a set of files,
typically named ib_logfile0 and
ib_logfile1. The data in the redo log is
encoded in terms of records affected; this data is collectively
referred to as redo. The
passage of data through the redo logs is represented by the
ever-increasing LSN value. The
4GB limit on maximum size for the redo log is raised in MySQL
5.6.
The disk layout of the redo log is influenced by the
configuration options
innodb_log_file_size,
innodb_log_group_home_dir, and
(rarely)
innodb_log_files_in_group. The
performance of redo log operations is also affected by the
log buffer, which is controlled
by the innodb_log_buffer_size
configuration option.
See also crash recovery, data files, ib_logfile, log buffer, LSN, redo, shutdown, transaction.
The oldest InnoDB row format, available for tables using the Antelope file format. Prior to MySQL 5.0.3, it was the only row format available in InnoDB. In My SQL 5.0.3 and later, the default is compact row format. You can still specify redundant row format for compatibility with older InnoDB tables.
See also Antelope, compact row format, file format, row format.
The technique of maintaining data always in a consistent format, part of the ACID philosophy. In particular, data in different tables is kept consistent through the use of foreign key constraints, which can prevent changes from happening or automatically propagate those changes to all related tables. Related mechanisms include the unique constraint, which prevents duplicate values from being inserted by mistake, and the NOT NULL constraint, which prevents blank values from being inserted by mistake.
See also ACID, FOREIGN KEY constraint, NOT NULL constraint, unique constraint.
An important aspect of modern database systems. The database server encodes and enforces relationships such as one-to-one, one-to-many, many-to-one, and uniqueness. For example, a person might have zero, one, or many phone numbers in an address database; a single phone number might be associated with several family members. In a financial database, a person might be required to have exactly one taxpayer ID, and any taxpayer ID could only be associated with one person.
The database server can use these relationships to prevent bad data from being inserted, and to find efficient ways to look up information. For example, if a value is declared to be unique, the server can stop searching as soon as the first match is found, and it can reject attempts to insert a second copy of the same value.
At the database level, these relationships are expressed through
SQL features such as columns
within a table, unique and NOT NULL
constraints,
foreign keys, and different
kinds of join operations. Complex relationships typically
involve data split between more than one table. Often, the data
is normalized, so that
duplicate values in one-to-many relationships are stored only
once.
In a mathematical context, the relations within a database are
derived from set theory. For example, the OR
and AND operators of a
WHERE clause represent the notions of union
and intersection.
See also ACID, constraint, foreign key, normalized.
In the full-text search feature, a number signifying the similarity between the search string and the data in the FULLTEXT index. For example, when you search for a single word, that word is typically more relevant for a row where if it occurs several times in the text than a row where it appears only once.
See also full-text search, FULLTEXT index.
The default isolation level for InnoDB. It prevents any rows that are queried from being changed by other transactions, thus blocking non-repeatable reads but not phantom reads. It uses a moderately strict locking strategy so that all queries within a transaction see data from the same snapshot, that is, the data as it was at the time the transaction started.
When a transaction with this isolation level performs
UPDATE ... WHERE, DELETE ...
WHERE, SELECT ... FOR UPDATE, and
LOCK IN SHARE MODE operations, other
transactions might have to wait.
See also ACID, consistent read, isolation level, locking, phantom, SERIALIZABLE, transaction.
The practice of sending changes from a master database, to one or more slave databases, so that all databases have the same data. This technique has a wide range of uses, such as load-balancing for better scalability, disaster recovery, and testing software upgrades and configuration changes. The changes can be sent between the database by methods called row-based replication and statement-based replication.
See also row-based replication, statement-based replication.
The process of putting a set of backup files from the
MySQL Enterprise Backup product
in place for use by MySQL. This operation can be performed to
fix a corrupted database, to return to some earlier point in
time, or (in a replication
context) to set up a new slave
database. In the MySQL
Enterprise Backup product, this operation is
performed by the copy-back option of the
mysqlbackup command.
See also hot backup, MySQL Enterprise Backup, mysqlbackup command, prepared backup, replication.
A SQL statement that ends a transaction, undoing any changes made by the transaction. It is the opposite of commit, which makes permanent any changes made in the transaction.
By default, MySQL uses the autocommit setting, which automatically issues a commit following each SQL statement. You must change this setting before you can use the rollback technique.
See also ACID, commit, transaction.
The storage area containing the undo log, part of the system tablespace.
See also system tablespace, undo log.
The logical data structure defined by a set of columns. A set of rows makes up a table. Within InnoDB data files, each page can contain one or more rows.
Although InnoDB uses the term row format for consistency with MySQL syntax, the row format is a property of each table and applies to all rows in that table.
See also column, data files, page, row format, table.
The disk storage format for a row from an InnoDB table. As InnoDB gains new capabilities such as compression, new row formats are introduced to support the resulting improvements in storage efficiency and performance.
Each table has its own row format, specified through the
ROW_FORMAT option. To see the row format for
each InnoDB table, issue the command SHOW TABLE
STATUS. Because all the tables in the system
tablespace share the same row format, to take advantage of other
row formats typically requires setting the
innodb_file_per_table option,
so that each table is stored in a separate tablespace.
See also compact row format, compressed row format, dynamic row format, fixed row format, redundant row format, row, table.
A lock that prevents a row from being accessed in an incompatible way by another transaction. Other rows in the same table can be freely written to by other transactions. This is the type of locking done by DML operations on InnoDB tables.
Contrast with table locks used by MyISAM, or during DDL operations on InnoDB tables that cannot be done with online DDL; those locks block concurrent access to the table.
See also DDL, DML, InnoDB, lock, locking, online DDL, table lock, transaction.
A form of replication where
events are propagated from the
master server specifying how to
change individual rows on the
slave server. It is safe to use
for all settings of the
innodb_autoinc_lock_mode
option.
See also auto-increment locking, innodb_autoinc_lock_mode, replication, statement-based replication.
The locking mechanism used for InnoDB tables, relying on row locks rather than table locks. Multiple transactions can modify the same table concurrently. Only if two transactions try to modify the same row does one of the transactions wait for the other to complete (and release its row locks).
See also InnoDB, locking, row lock, table lock, transaction.
The low-level object that InnoDB uses to represent and enforce shared-access locks to internal in-memory data structures. Once the lock is acquired, any other process, thread, and so on can read the data structure, but no one else can write to it. Contrast with mutexes, which enforce exclusive access. Mutexes and rw-locks are known collectively as latches.
See also latch, lock, mutex, Performance Schema.
S
Savepoints help to implement nested transactions. They can be used to provide scope to operations on tables that are part of a larger transaction. For example, scheduling a trip in a reservation system might involve booking several different flights; if a desired flight is unavailable, you might roll back the changes involved in booking that one leg, without rolling back the earlier flights that were successfully booked.
See also rollback, transaction.
The ability to add more work and issue more simultaneous requests to a system, without a sudden drop in performance due to exceeding the limits of system capacity. Software architecture, hardware configuration, application coding, and type of workload all play a part in scalability. When the system reaches its maximum capacity, popular techniques for increasing scalability are scale up (increasing the capacity of existing hardware or software) and scale out (adding new hardware or more instances of server software).
Conceptually, a schema is a set of interrelated database objects, such as tables, table columns, data types of the columns, indexes, foreign keys, and so on. These objects are connected through SQL syntax, because the columns make up the tables, the foreign keys refer to tables and columns, and so on. Ideally, they are also connected logically, working together as part of a unified application or flexible framework. For example, the information_schema and performance_schema databases use “schema” in their names to emphasize the close relationships between the tables and columns they contain.
In MySQL, physically, a schema
is synonymous with a database.
You can substitute the keyword SCHEMA instead
of DATABASE in MySQL SQL syntax, for example
using CREATE SCHEMA instead of
CREATE DATABASE.
Some other database products draw a distinction. For example, in the Oracle Database product, a schema represents only a part of a database: the tables and other objects owned by a single user.
See also database, ib-file set, INFORMATION_SCHEMA, Performance Schema.
In MySQL, full-text search
queries use a special kind of index, the
FULLTEXT index. In MySQL 5.6.4
and up, InnoDB and MyISAM
tables both support FULLTEXT indexes;
formerly, these indexes were only available for
MyISAM tables.
See also full-text search, FULLTEXT index.
A type of InnoDB index that represents a subset of table columns. An InnoDB table can have zero, one, or many secondary indexes. (Contrast with the clustered index, which is required for each InnoDB table, and stores the data for all the table columns.)
A secondary index can be used to satisfy queries that only require values from the indexed columns. For more complex queries, it can be used to identify the relevant rows in the table, which are then retrieved through lookups using the clustered index.
Creating and dropping secondary indexes has traditionally
involved significant overhead from copying all the data in the
InnoDB table. The fast index
creation feature of the InnoDB Plugin makes both
CREATE INDEX and DROP
INDEX statements much faster for InnoDB secondary
indexes.
See also clustered index, Fast Index Creation, index.
A division within an InnoDB tablespace. If a tablespace is analogous to a directory, the segments are analogous to files within that directory. A segment can grow. New segments can be created.
For example, within a file-per-table tablespace, the table data is in one segment and each associated index is in its own segment. The system tablespace contains many different segments, because it can hold many tables and their associated indexes. The system tablespace also includes up to 128 rollback segments making up the undo log.
Segments grow and shrink as data is inserted and deleted. When a segment needs more room, it is extended by one extent (1 megabyte) at a time. Similarly, a segment releases one extent's worth of space when all the data in that extent is no longer needed.
See also extent, file-per-table, rollback segment, system tablespace, tablespace, undo log.
A property of data distribution, the number of distinct values
in a column (its cardinality)
divided by the number of records in the table. High selectivity
means that the column values are relatively unique, and can
retrieved efficiently through an index. If you (or the query
optimizer) can predict that a test in a WHERE
clause only matches a small number (or proportion) of rows in a
table, the overall query tends
to be efficient if it evaluates that test first, using an index.
See also cardinality, query.
A type of read operation used for UPDATE
statements, that is a combination of read
committed and consistent
read. When an UPDATE statement
examines a row that is already locked, InnoDB returns the latest
committed version to MySQL so that MySQL can determine whether
the row matches the WHERE condition of the
UPDATE. If the row matches (must be updated),
MySQL reads the row again, and this time InnoDB either locks it
or waits for a lock on it. This type of read operation can only
happen when the transaction has the read committed
isolation level, or when the
innodb_locks_unsafe_for_binlog
option is enabled.
See also consistent read, isolation level, READ COMMITTED.
The isolation level that uses the most conservative locking strategy, to prevent any other transactions from inserting or changing data that was read by this transaction, until it is finished. This way, the same query can be run over and over within a transaction, and be certain to retrieve the same set of results each time. Any attempt to change data that was committed by another transaction since the start of the current transaction, cause the current transaction to wait.
This is the default isolation level specified by the SQL standard. In practice, this degree of strictness is rarely needed, so the default isolation level for InnoDB is the next most strict, repeatable read.
See also ACID, consistent read, isolation level, locking, REPEATABLE READ, transaction.
A type of program that runs continuously, waiting to receive and act upon requests from another program (the client). Because often an entire computer is dedicated to running one or more server programs (such as a database server, a web server, an application server, or some combination of these), the term server can also refer to the computer that runs the server software.
A kind of lock that allows other transactions to read the locked object, and to also acquire other shared locks on it, but not to write to it. The opposite of exclusive lock.
See also exclusive lock, lock, transaction.
Another way of referring to the system tablespace.
See also system tablespace.
The process of flushing to disk all dirty buffer pool pages whose redo entries are contained in certain portion of the redo log. Occurs before InnoDB reuses a portion of a log file; the log files are used in a circular fashion. Typically occurs with write-intensive workloads.
See also dirty page, flush, redo log, workload.
The process of stopping the MySQL server. By default, this process does cleanup operations for InnoDB tables, so it can slow to shut down, but fast to start up later. If you skip the cleanup operations, it is fast to shut down but must do the cleanup during the next restart.
The shutdown mode is controlled by the
innodb_fast_shutdown option.
See also fast shutdown, InnoDB, slow shutdown, startup.
Frequently shortened to “slave”. A database server machine in a replication scenario that receives changes from another server (the master) and applies those same changes. Thus it maintains the same contents as the master, although it might lag somewhat behind.
In MySQL, slave servers are commonly used in disaster recovery, to take the place of a master servers that fails. They are also commonly used for testing software upgrades and new settings, to ensure that database configuration changes do not cause problems with performance or reliability.
Slave servers typically have high workloads, because they process all the DML (write) operations relayed from the master, as well as user queries. To ensure that slave servers can apply changes from the master fast enough, they frequently have fast I/O devices and sufficient CPU and memory to run multiple database instances on the same slave server. For example, the master server might use hard drive storage while the slave servers use SSDs.
See also DML, replication, server, SSD.
A type of log used for performance tuning of SQL statements processed by the MySQL server. The log information is stored in a file. You must enable this feature to use it. You control which categories of “slow” SQL statements are logged. For more information, see Section 5.2.5, “The Slow Query Log”.
See also general query log, log.
A type of shutdown that does additional
InnoDB flushing operations before completing.
Also known as a clean shutdown.
Specified by the configuration parameter
innodb_fast_shutdown=0 or the
command SET GLOBAL innodb_fast_shutdown=0;.
Although the shutdown itself can take longer, that time will be
saved on the subsequent startup.
See also clean shutdown, fast shutdown, shutdown.
A representation of data at a particular time, which remains the same even as changes are committed by other transactions. Used by certain isolation levels to allow consistent reads.
See also commit, consistent read, isolation level, transaction.
An identifier used to uniquely identify an
InnoDB
tablespace within a MySQL
instance. The space ID for the system
tablespace is always zero; this same ID applies to
all tables within the system tablespace. Each tablespace file
created in file-per-table mode
also has its own space ID.
Prior to MySQL 5.6, this hardcoded value presented difficulties
in moving InnoDB tablespace files between
MySQL instances. Starting in MySQL 5.6, you can copy tablespace
files between instances by using the
transportable tablespace
feature involving the statements FLUSH TABLES ... FOR
EXPORT, ALTER TABLE ... DISCARD
TABLESPACE, and ALTER TABLE ... IMPORT
TABLESPACE. The information needed to adjust the space
ID is conveyed in the .cfg file
which you copy along with the tablespace. See
Section 14.2.5.2.33, “Improved Tablespace Management” for details.
See also .cfg file, file-per-table, .ibd file, system tablespace, tablespace, transportable tablespace.
A type of wait operation that continuously tests whether a resource becomes available. This technique is used for resources that are typically held only for brief periods, where it is more efficient to wait in a “busy loop” than to put the thread to sleep and perform a context switch. If the resource does not become available within a short time, the spin loop ceases and another wait technique is used.
The Structured Query Language that is standard for performing database operations. Often divided into the categories DDL, DML, and queries.
Acronym for “solid-state drive”. A type of storage device with different performance characteristics than a traditional hard disk drive (HDD): smaller storage capacity, faster for random reads, no moving parts, and with a number of considerations affecting write performance. Its performance characteristics can influence the throughput of a disk-bound workload.
See also disk-bound, SSD.
The process of starting the MySQL server. Typically done by one of the programs listed in Section 4.3, “MySQL Server and Server-Startup Programs”. The opposite of shutdown.
See also shutdown.
A form of replication where SQL
statements are sent from the
master server and replayed on
the slave server. It requires
some care with the setting for the
innodb_autoinc_lock_mode
option, to avoid potential timing problems with
auto-increment locking.
See also auto-increment locking, innodb_autoinc_lock_mode, replication, row-based replication.
Estimated values relating to each InnoDB
table and
index, used to construct an
efficient query execution plan.
The main values are the
cardinality (number of distinct
values) and the total number of table rows or index entries. The
statistics for the table represent the data in its
primary key index. The
statistics for a secondary
index represent the rows covered by that index.
The values are estimated rather than counted precisely because
at any moment, different
transactions can be inserting
and deleting rows from the same table. To keep the values from
being recalculated frequently, you can enable
persistent statistics, where
the values are stored in InnoDB system
tables, and refreshed only when you issue an
ANALYZE TABLE statement.
You can control how NULL values
are treated when calculating statistics through the
innodb_stats_method
configuration option.
Other types of statistics are available for database objects and database activity through the INFORMATION_SCHEMA and PERFORMANCE_SCHEMA tables.
See also cardinality, index, INFORMATION_SCHEMA, NULL, Performance Schema, persistent statistics, primary key, query execution plan, secondary index, table, transaction.
The ability to search for different variations of a word based on a common root word, such as singular and plural, or past, present, and future verb tense. This feature is currently supported in MyISAM full-text search feature but not in FULLTEXT indexes for InnoDB tables.
See also full-text search, FULLTEXT index.
In a FULLTEXT index, a word
that is considered common or trivial enough that it is omitted
from the search index and
ignored in search queries. Different configuration settings
control stopword processing for InnoDB and
MyISAM tables. See
Section 12.9.4, “Full-Text Stopwords” for details.
See also FULLTEXT index, search index.
A component of the MySQL database that performs the low-level
work of storing, updating, and querying data. In MySQL 5.5 and
higher, InnoDB is the default
storage engine for new tables, superceding MyISAM. Different
storage engines are designed with different tradeoffs between
factors such as memory usage versus disk usage, read speed
versus write speed, and speed versus robustness. Each storage
engine manages specific tables, so we refer to
InnoDB tables,
MyISAM tables, and so on.
The MySQL Enterprise Backup product is optimized for backing up InnoDB tables. It can also back up tables handled by MyISAM and other storage engines.
See also InnoDB, MySQL Enterprise Backup, table type.
The general name for the setting controlled by the
innodb_strict_mode option.
Turning on this setting causes certain conditions that are
normally treated as warnings, to be considered errors. For
example, certain invalid combinations of options related to
file format and
row format, that normally
produce a warning and continue with default values, now cause
the CREATE TABLE operation to fail.
MySQL also has something called strict mode.
See also file format, innodb_strict_mode, row format.
Within the list structure that represents the buffer pool, pages that are relatively old and relatively new are represented by different portions of the list. A set of parameters control the size of these portions and the dividing point between the new and old pages.
See also buffer pool, eviction, list, LRU.
A pseudo-record in an index,
representing the gap above the
largest value in that index. If a transaction has a statement
such as SELECT ... FOR UPDATE ... WHERE col >
10;, and the largest value in the column is 20, it is
a lock on the supremum record that prevents other transactions
from inserting even larger values such as 50, 100, and so on.
See also gap, infimum record, pseudo-record.
Synonym name for synthetic key.
See also synthetic key.
A indexed column, typically a primary key, where the values are assigned arbitrarily. Often done using an auto-increment column. By treating the value as completely arbitrary, you can avoid overly restrictive rules and faulty application assumptions. For example, a numeric sequence representing employee numbers might have a gap if an employee was approved for hiring but never actually joined. Or employee number 100 might have a later hiring date than employee number 500, if they left the company and later rejoined. Numeric values also produce shorter values of predictable length. For example, storing numeric codes meaning “Road”, “Boulevard”, “Expressway”, and so on is more space-efficient than repeating those strings over and over.
Also known as a surrogate key. Contrast with natural key.
See also auto-increment, natural key, primary key, surrogate key.
A small set of data files (the
ibdata files) containing the
metadata for InnoDB-related objects (the
data dictionary), and the
storage areas for the undo log,
the change buffer, and the
doublewrite buffer. Depending
on the setting of the
innodb_file_per_table, when
tables are created, it might also contain table and index data
for some or all InnoDB tables. The data and metadata in the
system tablespace apply to all the
databases in a MySQL
instance.
Prior to MySQL 5.6.7, the default was to keep all InnoDB tables and indexes inside the system tablespace, often causing this file to become very large. Because the system tablespace never shrinks, storage problems could arise if large amounts of temporary data were loaded and then deleted. In MySQL 5.6.7 and higher, the default is file-per-table mode, where each table and its associated indexes are stored in a separate .ibd file. This new default makes it easier to use InnoDB features that rely on the Barracuda file format, such as table compression and the DYNAMIC row format.
In MySQL 5.6 and higher, setting a value for the
innodb_undo_tablespaces option
splits the undo log into one or
more separate tablespace files. These files are still considered
part of the system tablespace.
Keeping all table data in the system tablespace or in separate
.ibd files has implications for storage
management in general. The MySQL
Enterprise Backup product might back up a small set
of large files, or many smaller files. On systems with thousands
of tables, the filesystem operations to process thousands of
.ibd files can cause bottlenecks.
See also Barracuda, change buffer, compression, data dictionary, database, doublewrite buffer, dynamic row format, file-per-table, .ibd file, ibdata file, innodb_file_per_table, instance, MySQL Enterprise Backup, tablespace, undo log.
T
Each MySQL table is associated with a particular storage engine. InnoDB tables have particular physical and logical characteristics that affect performance, scalability, backup, administration, and application development.
In terms of file storage, each InnoDB table is either part of
the single big InnoDB system
tablespace, or in a separate
.ibd file if
the table is created in
file-per-table mode. The
.ibd file
holds data for the table and all its
indexes, and is known as a
tablespace.
InnoDB tables created in file-per-table mode can use the Barracuda file format. Barracuda tables can use the DYNAMIC row format or the COMPRESSED row format. These relatively new settings enable a number of InnoDB features, such as compression, fast index creation, and off-page columns
For backward compatibility with MySQL 5.1 and earlier, InnoDB tables inside the system tablespace must use the Antelope file format, which supports the compact row format and the redundant row format.
The rows of an InnoDB table are organized into an index structure known as the clustered index, with entries sorted based on the primary key columns of the table. Data access is optimized for queries that filter and sort on the primary key columns, and each index contains a copy of the associated primary key columns for each entry. Modifying values for any of the primary key columns is an expensive operation. Thus an important aspect of InnoDB table design is choosing a primary key with columns that are used in the most important queries, and keeping the primary key short, with rarely changing values.
See also Antelope, backup, Barracuda, clustered index, compact row format, compressed row format, compression, dynamic row format, Fast Index Creation, file-per-table, .ibd file, index, off-page column, primary key, redundant row format, row, system tablespace, tablespace.
A lock that prevents any other
transaction from accessing a
table. InnoDB makes considerable effort to make such locks
unnecessary, by using techniques such as
online DDL,
row locks and
consistent reads for processing
DML statements and
queries. You can create such a
lock through SQL using the LOCK TABLE
statement; one of the steps in migrating from other database
systems or MySQL storage engines is to remove such statements
wherever practical.
See also consistent read, DML, lock, locking, online DDL, query, row lock, table, transaction.
See full table scan.
See statistics.
Obsolete synonym for storage
engine. We refer to
InnoDB tables,
MyISAM tables, and so on.
See also InnoDB, storage engine.
A data file that can hold data for one or more InnoDB
tables and associated
indexes. The
system tablespace contains the
tables that make up the data
dictionary, and prior to MySQL 5.6 holds all the
other InnoDB tables by default. Turning on the
innodb_file_per_table option,
the default in MySQL 5.6 and higher, allows newly created tables
to each have their own tablespace, with a separate
data file for each table.
Using multiple tablespaces, by turning on the
innodb_file_per_table option,
is vital to using many MySQL features such as table compression
and transportable tablespaces, and managing disk usage. See
Section 14.2.2.1, “Managing InnoDB Tablespaces” and
Section 14.2.5.2.33, “Improved Tablespace Management” for details.
Tablespaces created by the built-in InnoDB storage engine are upward compatible with the InnoDB Plugin. Tablespaces created by the InnoDB Plugin are downward compatible with the built-in InnoDB storage engine, if they use the Antelope file format.
MySQL Cluster also groups its tables into tablespaces. See MySQL Cluster Disk Data Objects for details.
See also Antelope, Barracuda, compressed row format, data dictionary, data files, file-per-table, index, innodb_file_per_table, system tablespace, table.
A representation of the data
dictionary metadata for a table, within the InnoDB
tablespace. This metadata can
be checked against the .frm
file for consistency when the table is opened, to
diagnose errors resulting from out-of-date
.frm files. This information is present for
InnoDB tables that are part of the system
tablespace, as well as for tables that have their own
.ibd file because of the
file-per-table option.
See also data dictionary, file-per-table, .frm file, .ibd file, system tablespace, tablespace.
A table whose data does not need to be truly permanent. For example, temporary tables might be used as storage areas for intermediate results in complicated calculations or transformations; this intermediate data would not need to be recovered after a crash. Database products can take various shortcuts to improve the performance of operations on temporary tables, by being less scrupulous about writing data to disk and other measures to protect the data across restarts.
Sometimes, the data itself is removed automatically at a set time, such as when the transaction ends or when the session ends. With some database products, the table itself is removed automatically too.
See also table.
The set of columns included in a FULLTEXT index.
See also FULLTEXT index.
A unit of processing that is typically more lightweight than a process, allowing for greater concurrency.
See also concurrency, master thread, process, Pthreads.
An error condition that can occur due to a combination of I/O device configuration and hardware failure. If data is written out in chunks smaller than the InnoDB page size (by default, 16KB), a hardware failure while writing could result in only part of a page being stored to disk. The InnoDB doublewrite buffer guards against this possibility.
See also doublewrite buffer.
Acronym for “transactions per second”, a unit of measurement sometimes used in benchmarks. Its value depends on the workload represented by a particular benchmark test, combined with factors that you control such as the hardware capacity and database configuration.
See also transaction, workload.
Transactions are atomic units of work that can be committed or rolled back. When a transaction makes multiple changes to the database, either all the changes succeed when the transaction is committed, or all the changes are undone when the transaction is rolled back.
Database transactions, as implemented by InnoDB, have properties that are collectively known by the acronym ACID, for atomicity, consistency, isolation, and durability.
See also ACID, commit, isolation level, lock, rollback.
An internal field associated with each row. This field is physically changed by INSERT, UPDATE, and DELETE operations to record which transaction has locked the row.
See also implicit row lock.
A feature that allows a
tablespace to be moved from one
instance to another. Traditionally, this has not been possible
for InnoDB tablespaces because all table data was part of the
system tablespace. In MySQL 5.6
and higher, the FLUSH
TABLES ... FOR EXPORT syntax prepares an InnoDB table
for copying to another server; running
ALTER TABLE ...
DISCARD TABLESPACE and
ALTER TABLE ...
IMPORT TABLESPACE on the other server brings the
copied data file into the other instance. A separate
.cfg file, copied along with the
.ibd file, is used to update
the table metadata (for example the space
ID) as the tablespace is imported. See
Section 14.2.5.2.33, “Improved Tablespace Management” for usage
information.
See also .ibd file, space ID, system tablespace, tablespace.
A file containing trigger
parameters. Files with this extension are always included in
backups produced by the mysqlbackup command
of the MySQL Enterprise Backup
product.
See also MySQL Enterprise Backup, mysqlbackup command, .TRN file.
A file containing trigger namespace information. Files with this
extension are always included in backups produced by the
mysqlbackup command of the
MySQL Enterprise Backup
product.
See also MySQL Enterprise Backup, mysqlbackup command, .TRG file.
Resources for troubleshooting InnoDB reliability and performance issues include: the Information Schema tables.
A DDL operation that removes
the entire contents of a table, while leaving the table and
related indexes intact. Contrast with
drop. Although conceptually it
has the same result as a DELETE statement
with no WHERE clause, it operates differently
behind the scenes: InnoDB creates a new empty table, drops the
old table, then renames the new table to take the place of the
old one. Because this is a DDL operation, it cannot be
rolled back.
If the table being truncated contains foreign keys that
reference another table, the truncation operation uses a slower
method of operation, deleting one row at a time so that
corresponding rows in the referenced table can be deleted as
needed by any ON DELETE CASCADE clause.
(MySQL 5.5 and higher do not allow this slower form of truncate,
and return an error instead if foreign keys are involved. In
this case, use a DELETE statement instead.
See also DDL, drop, foreign key, rollback.
A technical term designating an ordered set of elements. It is an abstract notion, used in formal discussions of database theory. In the database field, tuples are usually represented by the columns of a table row. They could also be represented by the result sets of queries, for example, queries that retrieved only some columns of a table, or columns from joined tables.
See also cursor.
An operation that is part of a distributed transaction, under the XA specification. (Sometimes abbreviated as 2PC.) When multiple databases participate in the transaction, either all databases commit the changes, or all databases roll back the changes.
See also commit, rollback, transaction, XA.
U
Data that is maintained throughout the life of a transaction, recording all changes so that they can be undone in case of a rollback operation. It is stored in the undo log, also known as the rollback segment, either within the system tablespace or in separate undo tablespaces.
See also rollback, rollback segment, system tablespace, transaction, undo log, undo tablespace.
See undo log.
A storage area that holds copies of data modified by active transactions. If another transaction needs to see the original data (as part of a consistent read operation), the unmodified data is retrieved from this storage area.
By default, this area is physically part of the
system tablespace. In MySQL 5.6
and higher, you can use the
innodb_undo_tablespaces and
innodb_undo_directory
configuration options to split it into one or more separate
tablespace files, the
undo tablespaces, optionally
stored on another storage device such as an
SSD.
The undo log is split into separate portions, the insert undo buffer and the update undo buffer. Collectively, these parts are also known as the rollback segment, a familiar term for Oracle DBAs.
See also consistent read, rollback segment, SSD, system tablespace, transaction, undo tablespace.
One of a set of files containing the undo
log, when the undo log is separated from the
system tablespace by the
innodb_undo_tablespaces and
innodb_undo_directory
configuration options. Only applies to MySQL 5.6 and higher.
See also system tablespace, undo log.
A kind of constraint that asserts that a column cannot contain any duplicate values. In terms of relational algebra, it is used to specify 1-to-1 relationships. For efficiency in checking whether a value can be inserted (that is, the value does not already exist in the column), a unique constraint is supported by an underlying unique index.
See also constraint, relational, unique index.
An index on a column or set of columns that have a unique constraint. Because the index is known not to contain any duplicate values, certain kinds of lookups and count operations are more efficient than in the normal kind of index. Most of the lookups against this type of index are simply to determine if a certain value exists or not. The number of values in the index is the same as the number of rows in the table, or at least the number of rows with non-null values for the associated columns.
The insert buffering
optimization does not apply to unique indexes. As a workaround,
you can temporarily set unique_checks=0 while
doing a bulk data load into an InnoDB table.
See also cardinality, insert buffering, unique constraint, unique key.
The set of columns (one or more) comprising a
unique index. When you can
define a WHERE condition that matches exactly
one row, and the query can use an associated unique index, the
lookup and error handling can be performed very efficiently.
See also cardinality, unique constraint, unique index.
V
The transaction that is automatically chosen to be rolled back when a deadlock is detected. InnoDB rolls back the transaction that has updated the fewest rows.
See also deadlock, deadlock detection, innodb_lock_wait_timeout.
W
When an operation, such as acquiring a
lock,
mutex, or
latch, cannot be completed
immediately, InnoDB pauses and tries again. The mechanism for
pausing is elaborate enough that this operation has its own
name, the wait. Individual
threads are paused using a combination of internal InnoDB
scheduling, operating system wait() calls,
and short-duration spin loops.
On systems with heavy load and many transactions, you might use
the output from the SHOW INNODB STATUS
command to determine whether threads are spending too much time
waiting, and if so, how you can improve
concurrency.
See also concurrency, latch, lock, mutex, spin.
A backup taken while the database is running, but that restricts some database operations during the backup process. For example, tables might become read-only. For busy applications and web sites, you might prefer a hot backup.
See also backup, cold backup, hot backup.
To run a system under a typical workload for some time after startup, so that the buffer pool and other memory regions are filled as they would be under normal conditions.
This process happens naturally over time when a MySQL server is
restarted or subjected to a new workload. Starting in MySQL 5.6,
you can speed up the warmup process by setting the configuration
variables
innodb_buffer_pool_dump_at_shutdown=ON
and
innodb_buffer_pool_load_at_startup=ON,
to bring the contents of the buffer pool back into memory after
a restart. Typically, you run a workload for some time to warm
up the buffer pool before running performance tests, to ensure
consistent results across multiple runs; otherwise, performance
might be artificially low during the first run.
See also buffer pool, workload.
The built-in InnoDB storage engine and the InnoDB Plugin are supported on all the same Microsoft Windows versions as the MySQL server. The MySQL Enterprise Backup product has more comprehensive support for Windows systems than the InnoDB Hot Backup product that it supercedes.
See also InnoDB, InnoDB Hot Backup, MySQL Enterprise Backup, plugin.
The combination and volume of SQL and other database operations, performed by a database application during typical or peak usage. You can subject the database to a particular workload during performance testing to identify bottlenecks, or during capacity planning.
See also bottleneck, disk-bound, disk-bound, SQL.
An optimization technique that reduces write operations when dirty pages are flushed from the InnoDB buffer pool. If a row in a page is updated multiple times, or multiple rows on the same page are updated, all of those changes are stored to the data files in a single write operation rather than one write for each change.
See also buffer pool, dirty page, flush.
X
A standard interface for coordinating distributed transactions, allowing multiple databases to participate in a transaction while maintaining ACID compliance. For full details, see Section 13.3.7, “XA Transactions”.
XA Distributed Transaction support is turned on by default. If you are not using this feature, note that it adds an extra fsync for each transaction, which may adversely affect performance.
See also commit, transaction, two-phase commit.
Y
A characteristic of a page in
the InnoDB buffer
pool meaning it has been accessed recently, and so is
moved within the buffer pool data structure so that it will not
be flushed soon by the
LRU algorithm. This term is
used in some information schema
column names of tables related to the buffer pool.
See also buffer pool, flush, INFORMATION_SCHEMA, LRU, page.