Chapter 1. General Information
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Chapter 1. General Information

Table of Contents

1.1. About This Manual
1.2. Typographical and Syntax Conventions
1.3. Overview of the MySQL Database Management System
1.3.1. What is MySQL?
1.3.2. The Main Features of MySQL
1.3.3. History of MySQL
1.4. What Is New in MySQL 5.6
1.5. MySQL Development History
1.6. MySQL Information Sources
1.6.1. MySQL Mailing Lists
1.6.2. MySQL Community Support at the MySQL Forums
1.6.3. MySQL Community Support on Internet Relay Chat (IRC)
1.6.4. MySQL Enterprise
1.7. How to Report Bugs or Problems
1.8. MySQL Standards Compliance
1.8.1. What Standards MySQL Follows
1.8.2. Selecting SQL Modes
1.8.3. Running MySQL in ANSI Mode
1.8.4. MySQL Extensions to Standard SQL
1.8.5. MySQL Differences from Standard SQL
1.8.6. How MySQL Deals with Constraints
1.9. Credits
1.9.1. Contributors to MySQL
1.9.2. Documenters and translators
1.9.3. Packages that support MySQL
1.9.4. Tools that were used to create MySQL
1.9.5. Supporters of MySQL

The MySQL™ software delivers a very fast, multi-threaded, multi-user, and robust SQL (Structured Query Language) database server. MySQL Server is intended for mission-critical, heavy-load production systems as well as for embedding into mass-deployed software. Oracle is a registered trademark of Oracle Corporation and/or its affiliates. MySQL is a trademark of Oracle Corporation and/or its affiliates, and shall not be used by Customer without Oracle's express written authorization. Other names may be trademarks of their respective owners.

The MySQL software is Dual Licensed. Users can choose to use the MySQL software as an Open Source product under the terms of the GNU General Public License (http://www.fsf.org/licenses/) or can purchase a standard commercial license from Oracle. See http://www.mysql.com/company/legal/licensing/ for more information on our licensing policies.

The following list describes some sections of particular interest in this manual:

Important

To report problems or bugs, please use the instructions at Section 1.7, “How to Report Bugs or Problems”. If you have found a sensitive security bug in MySQL Server, please let us know immediately by sending an email message to . Exception: Support customers should report all problems, including security bugs, to Oracle Support.

1.1. About This Manual

This is the Reference Manual for the MySQL Database System, version 5.6, through release 5.6.10. Differences between minor versions of MySQL 5.6 are noted in the present text with reference to release numbers (5.6.x). For license information, see the Legal Notices. This product may contain third-party code. For license information on third-party code, see Appendix A, Licenses for Third-Party Components.

This manual is not intended for use with older versions of the MySQL software due to the many functional and other differences between MySQL 5.6 and previous versions. If you are using an earlier release of the MySQL software, please refer to the appropriate manual. For example, MySQL 5.5 Reference Manual covers the 5.5 series of MySQL software releases.

Because this manual serves as a reference, it does not provide general instruction on SQL or relational database concepts. It also does not teach you how to use your operating system or command-line interpreter.

The MySQL Database Software is under constant development, and the Reference Manual is updated frequently as well. The most recent version of the manual is available online in searchable form at http://dev.mysql.com/doc/. Other formats also are available there, including HTML, PDF, and EPUB versions.

The Reference Manual source files are written in DocBook XML format. The HTML version and other formats are produced automatically, primarily using the DocBook XSL stylesheets. For information about DocBook, see http://docbook.org/

If you have questions about using MySQL, you can ask them using our mailing lists or forums. See Section 1.6.1, “MySQL Mailing Lists”, and Section 1.6.2, “MySQL Community Support at the MySQL Forums”. If you have suggestions concerning additions or corrections to the manual itself, please send them to the http://www.mysql.com/company/contact/.

This manual was originally written by David Axmark and Michael “Monty” Widenius. It is maintained by the MySQL Documentation Team, consisting of Paul DuBois, Stefan Hinz, Philip Olson, John Russell, and Jon Stephens.

1.2. Typographical and Syntax Conventions

This manual uses certain typographical conventions:

  • Text in this style is used for SQL statements; database, table, and column names; program listings and source code; and environment variables. Example: “To reload the grant tables, use the FLUSH PRIVILEGES statement.”

  • Text in this style indicates input that you type in examples.

  • Text in this style indicates the names of executable programs and scripts, examples being mysql (the MySQL command line client program) and mysqld (the MySQL server executable).

  • Text in this style is used for variable input for which you should substitute a value of your own choosing.

  • Text in this style is used for emphasis.

  • Text in this style is used in table headings and to convey especially strong emphasis.

  • Text in this style is used to indicate a program option that affects how the program is executed, or that supplies information that is needed for the program to function in a certain way. Example: “The --host option (short form -h) tells the mysql client program the hostname or IP address of the MySQL server that it should connect to”.

  • File names and directory names are written like this: “The global my.cnf file is located in the /etc directory.”

  • Character sequences are written like this: “To specify a wildcard, use the ‘%’ character.”

When commands are shown that are meant to be executed from within a particular program, the prompt shown preceding the command indicates which command to use. For example, shell> indicates a command that you execute from your login shell, root-shell> is similar but should be executed as root, and mysql> indicates a statement that you execute from the mysql client program: 

shell> type a shell command here
root-shell> type a shell command as root here
mysql> type a mysql statement here

In some areas different systems may be distinguished from each other to show that commands should be executed in two different environments. For example, while working with replication the commands might be prefixed with master and slave: 

master> type a mysql command on the replication master here
slave> type a mysql command on the replication slave here

The “shell” is your command interpreter. On Unix, this is typically a program such as sh, csh, or bash. On Windows, the equivalent program is command.com or cmd.exe, typically run in a console window.

When you enter a command or statement shown in an example, do not type the prompt shown in the example.

Database, table, and column names must often be substituted into statements. To indicate that such substitution is necessary, this manual uses db_name, tbl_name, and col_name. For example, you might see a statement like this: 

mysql> SELECT col_name FROM db_name.tbl_name;

This means that if you were to enter a similar statement, you would supply your own database, table, and column names, perhaps like this: 

mysql> SELECT author_name FROM biblio_db.author_list;

SQL keywords are not case sensitive and may be written in any lettercase. This manual uses uppercase.

In syntax descriptions, square brackets (“[” and “]”) indicate optional words or clauses. For example, in the following statement, IF EXISTS is optional: 

DROP TABLE [IF EXISTS] tbl_name

When a syntax element consists of a number of alternatives, the alternatives are separated by vertical bars (“|”). When one member from a set of choices may be chosen, the alternatives are listed within square brackets (“[” and “]”): 

TRIM([[BOTH | LEADING | TRAILING] [remstr] FROM] str)

When one member from a set of choices must be chosen, the alternatives are listed within braces (“{” and “}”): 

{DESCRIBE | DESC} tbl_name [col_name | wild]

An ellipsis (...) indicates the omission of a section of a statement, typically to provide a shorter version of more complex syntax. For example, SELECT ... INTO OUTFILE is shorthand for the form of SELECT statement that has an INTO OUTFILE clause following other parts of the statement.

An ellipsis can also indicate that the preceding syntax element of a statement may be repeated. In the following example, multiple reset_option values may be given, with each of those after the first preceded by commas: 

RESET reset_option [,reset_option] ...

Commands for setting shell variables are shown using Bourne shell syntax. For example, the sequence to set the CC environment variable and run the configure command looks like this in Bourne shell syntax: 

shell> CC=gcc ./configure

If you are using csh or tcsh, you must issue commands somewhat differently: 

shell> setenv CC gcc
shell> ./configure

1.3. Overview of the MySQL Database Management System

1.3.1. What is MySQL?
1.3.2. The Main Features of MySQL
1.3.3. History of MySQL

1.3.1. What is MySQL?

MySQL, the most popular Open Source SQL database management system, is developed, distributed, and supported by Oracle Corporation.

The MySQL Web site (http://www.mysql.com/) provides the latest information about MySQL software.

  • MySQL is a database management system.

    A database is a structured collection of data. It may be anything from a simple shopping list to a picture gallery or the vast amounts of information in a corporate network. To add, access, and process data stored in a computer database, you need a database management system such as MySQL Server. Since computers are very good at handling large amounts of data, database management systems play a central role in computing, as standalone utilities, or as parts of other applications.

  • MySQL databases are relational.

    A relational database stores data in separate tables rather than putting all the data in one big storeroom. The database structures are organized into physical files optimized for speed. The logical model, with objects such as databases, tables, views, rows, and columns, offers a flexible programming environment. You set up rules governing the relationships between different data fields, such as one-to-one, one-to-many, unique, required or optional, and “pointers” between different tables. The database enforces these rules, so that with a well-designed database, your application never sees inconsistent, duplicate, orphan, out-of-date, or missing data.

    The SQL part of “MySQL” stands for “Structured Query Language”. SQL is the most common standardized language used to access databases. Depending on your programming environment, you might enter SQL directly (for example, to generate reports), embed SQL statements into code written in another language, or use a language-specific API that hides the SQL syntax.

    SQL is defined by the ANSI/ISO SQL Standard. The SQL standard has been evolving since 1986 and several versions exist. In this manual, “SQL-92” refers to the standard released in 1992, “SQL:1999” refers to the standard released in 1999, and “SQL:2003” refers to the current version of the standard. We use the phrase “the SQL standard” to mean the current version of the SQL Standard at any time.

  • MySQL software is Open Source.

    Open Source means that it is possible for anyone to use and modify the software. Anybody can download the MySQL software from the Internet and use it without paying anything. If you wish, you may study the source code and change it to suit your needs. The MySQL software uses the GPL (GNU General Public License), http://www.fsf.org/licenses/, to define what you may and may not do with the software in different situations. If you feel uncomfortable with the GPL or need to embed MySQL code into a commercial application, you can buy a commercially licensed version from us. See the MySQL Licensing Overview for more information (http://www.mysql.com/company/legal/licensing/).

  • The MySQL Database Server is very fast, reliable, scalable, and easy to use.

    If that is what you are looking for, you should give it a try. MySQL Server can run comfortably on a desktop or laptop, alongside your other applications, web servers, and so on, requiring little or no attention. If you dedicate an entire machine to MySQL, you can adjust the settings to take advantage of all the memory, CPU power, and I/O capacity available. MySQL can also scale up to clusters of machines, networked together.

    You can find a performance comparison of MySQL Server with other database managers on our benchmark page. See Section 8.12.2, “The MySQL Benchmark Suite”.

    MySQL Server was originally developed to handle large databases much faster than existing solutions and has been successfully used in highly demanding production environments for several years. Although under constant development, MySQL Server today offers a rich and useful set of functions. Its connectivity, speed, and security make MySQL Server highly suited for accessing databases on the Internet.

  • MySQL Server works in client/server or embedded systems.

    The MySQL Database Software is a client/server system that consists of a multi-threaded SQL server that supports different backends, several different client programs and libraries, administrative tools, and a wide range of application programming interfaces (APIs).

    We also provide MySQL Server as an embedded multi-threaded library that you can link into your application to get a smaller, faster, easier-to-manage standalone product.

  • A large amount of contributed MySQL software is available.

    MySQL Server has a practical set of features developed in close cooperation with our users. It is very likely that your favorite application or language supports the MySQL Database Server.

The official way to pronounce “MySQL” is “My Ess Que Ell” (not “my sequel”), but we do not mind if you pronounce it as “my sequel” or in some other localized way.

1.3.2. The Main Features of MySQL

This section describes some of the important characteristics of the MySQL Database Software. See also Section 1.5, “MySQL Development History”. In most respects, the roadmap applies to all versions of MySQL. For information about features as they are introduced into MySQL on a series-specific basis, see the “In a Nutshell” section of the appropriate Manual:

Internals and Portability:

  • Written in C and C++.

  • Tested with a broad range of different compilers.

  • Works on many different platforms. See Section 2.1.1, “Operating Systems Supported by MySQL Community Server”.

  • For portability, uses CMake in MySQL 5.5 and up. Previous series use GNU Automake, Autoconf, and Libtool.

  • Tested with Purify (a commercial memory leakage detector) as well as with Valgrind, a GPL tool (http://developer.kde.org/~sewardj/).

  • Uses multi-layered server design with independent modules.

  • Designed to be fully multi-threaded using kernel threads, to easily use multiple CPUs if they are available.

  • Provides transactional and nontransactional storage engines.

  • Uses very fast B-tree disk tables (MyISAM) with index compression.

  • Designed to make it relatively easy to add other storage engines. This is useful if you want to provide an SQL interface for an in-house database.

  • Uses a very fast thread-based memory allocation system.

  • Executes very fast joins using an optimized nested-loop join.

  • Implements in-memory hash tables, which are used as temporary tables.

  • Implements SQL functions using a highly optimized class library that should be as fast as possible. Usually there is no memory allocation at all after query initialization.

  • Provides the server as a separate program for use in a client/server networked environment, and as a library that can be embedded (linked) into standalone applications. Such applications can be used in isolation or in environments where no network is available.

Data Types:

Statements and Functions:

  • Full operator and function support in the SELECT list and WHERE clause of queries. For example: 

    mysql> SELECT CONCAT(first_name, ' ', last_name)
    -> FROM citizen
    -> WHERE income/dependents > 10000 AND age > 30;

  • Full support for SQL GROUP BY and ORDER BY clauses. Support for group functions (COUNT(), AVG(), STD(), SUM(), MAX(), MIN(), and GROUP_CONCAT()).

  • Support for LEFT OUTER JOIN and RIGHT OUTER JOIN with both standard SQL and ODBC syntax.

  • Support for aliases on tables and columns as required by standard SQL.

  • Support for DELETE, INSERT, REPLACE, and UPDATE to return the number of rows that were changed (affected), or to return the number of rows matched instead by setting a flag when connecting to the server.

  • Support for MySQL-specific SHOW statements that retrieve information about databases, storage engines, tables, and indexes. MySQL 5.0 adds support for the INFORMATION_SCHEMA database, implemented according to standard SQL.

  • An EXPLAIN statement to show how the optimizer resolves a query.

  • Independence of function names from table or column names. For example, ABS is a valid column name. The only restriction is that for a function call, no spaces are permitted between the function name and the “(” that follows it. See Section 9.3, “Reserved Words”.

  • You can refer to tables from different databases in the same statement.

Security:

  • A privilege and password system that is very flexible and secure, and that enables host-based verification.

  • Password security by encryption of all password traffic when you connect to a server.

Scalability and Limits:

  • Support for large databases. We use MySQL Server with databases that contain 50 million records. We also know of users who use MySQL Server with 200,000 tables and about 5,000,000,000 rows.

  • Support for up to 64 indexes per table (32 before MySQL 4.1.2). Each index may consist of 1 to 16 columns or parts of columns. The maximum index width is 767 bytes for InnoDB tables, or 1000 for MyISAM; before MySQL 4.1.2, the limit is 500 bytes. An index may use a prefix of a column for CHAR, VARCHAR, BLOB, or TEXT column types.

Connectivity:

  • Clients can connect to MySQL Server using several protocols:

    • Clients can connect using TCP/IP sockets on any platform.

    • On Windows systems in the NT family (NT, 2000, XP, 2003, or Vista), clients can connect using named pipes if the server is started with the --enable-named-pipe option. In MySQL 4.1 and higher, Windows servers also support shared-memory connections if started with the --shared-memory option. Clients can connect through shared memory by using the --protocol=memory option.

    • On Unix systems, clients can connect using Unix domain socket files.

  • MySQL client programs can be written in many languages. A client library written in C is available for clients written in C or C++, or for any language that provides C bindings.

  • APIs for C, C++, Eiffel, Java, Perl, PHP, Python, Ruby, and Tcl are available, enabling MySQL clients to be written in many languages. See Chapter 21, Connectors and APIs.

  • The Connector/ODBC (MyODBC) interface provides MySQL support for client programs that use ODBC (Open Database Connectivity) connections. For example, you can use MS Access to connect to your MySQL server. Clients can be run on Windows or Unix. Connector/ODBC source is available. All ODBC 2.5 functions are supported, as are many others. See Section 21.1, “MySQL Connector/ODBC”.

  • The Connector/J interface provides MySQL support for Java client programs that use JDBC connections. Clients can be run on Windows or Unix. Connector/J source is available. See Section 21.3, “MySQL Connector/J”.

  • MySQL Connector/Net enables developers to easily create .NET applications that require secure, high-performance data connectivity with MySQL. It implements the required ADO.NET interfaces and integrates into ADO.NET aware tools. Developers can build applications using their choice of .NET languages. MySQL Connector/Net is a fully managed ADO.NET driver written in 100% pure C#. See Section 21.2, “MySQL Connector/Net”.

Localization:

  • The server can provide error messages to clients in many languages. See Section 10.2, “Setting the Error Message Language”.

  • Full support for several different character sets, including latin1 (cp1252), german, big5, ujis, and more. For example, the Scandinavian characters “å”, “ä” and “ö” are permitted in table and column names. Unicode support is available as of MySQL 4.1.

  • All data is saved in the chosen character set.

  • Sorting and comparisons are done according to the chosen character set and collation (using latin1 and Swedish collation by default). It is possible to change this when the MySQL server is started. To see an example of very advanced sorting, look at the Czech sorting code. MySQL Server supports many different character sets that can be specified at compile time and runtime.

  • As of MySQL 4.1, the server time zone can be changed dynamically, and individual clients can specify their own time zone. Section 10.6, “MySQL Server Time Zone Support”.

Clients and Tools:

  • MySQL includes several client and utility programs. These include both command-line programs such as mysqldump and mysqladmin, and graphical programs such as MySQL Workbench.

  • MySQL Server has built-in support for SQL statements to check, optimize, and repair tables. These statements are available from the command line through the mysqlcheck client. MySQL also includes myisamchk, a very fast command-line utility for performing these operations on MyISAM tables. See Chapter 4, MySQL Programs.

  • MySQL programs can be invoked with the --help or -? option to obtain online assistance.

1.3.3. History of MySQL

We started out with the intention of using the mSQL database system to connect to our tables using our own fast low-level (ISAM) routines. However, after some testing, we came to the conclusion that mSQL was not fast enough or flexible enough for our needs. This resulted in a new SQL interface to our database but with almost the same API interface as mSQL. This API was designed to enable third-party code that was written for use with mSQL to be ported easily for use with MySQL.

MySQL is named after co-founder Monty Widenius's daughter, My.

The name of the MySQL Dolphin (our logo) is “Sakila,” which was chosen from a huge list of names suggested by users in our “Name the Dolphin” contest. The winning name was submitted by Ambrose Twebaze, an Open Source software developer from Swaziland, Africa. According to Ambrose, the feminine name Sakila has its roots in SiSwati, the local language of Swaziland. Sakila is also the name of a town in Arusha, Tanzania, near Ambrose's country of origin, Uganda.

1.4. What Is New in MySQL 5.6

This section summarizes what has been added to and removed from MySQL 5.6.

Added Features

The following features have been added to MySQL 5.6:

  • Security improvements.  These security improvements were made:

  • Changes to server defaults.  Beginning with MySQL 5.6.6, several MySQL Server parameter defaults differ from the defaults in previous releases. The motivation for these changes is to provide better out-of-box performance and to reduce the need for database administrators to change settings manually. For more information, see Section 5.1.2.1, “Changes to Server Defaults”.

  • InnoDB enhancements.  These InnoDB enhancements were added:

    • You can create FULLTEXT indexes on InnoDB tables, and query them using the MATCH() ... AGAINST syntax. This feature includes a new proximity search operator (@) and several new configuration options and INFORMATION_SCHEMA tables: See Section 14.2.4.12.3, “FULLTEXT Indexes” for more information.

    • Several ALTER TABLE operations can be performed without copying the table, without blocking inserts, updates, and deletes to the table, or both. These enhancements are known collectively as online DDL. See Section 14.2.2.6, “Online DDL for InnoDB Tables” for details.

    • You have more flexibility to move around the .ibd files files created in file-per-table mode to suit your storage devices and database servers. When creating a table, you can designate a location outside the MySQL data directory to hold the .ibd file, for example to put a busy table on an SSD device or a huge table on a high-capacity HDD device. You can export a table from one MySQL instance and import it in a different instance, without inconsistencies or mismatches caused by buffered data, in-progress transactions, and internal bookkeeping details such as the space ID and LSN. See Section 14.2.5.2.33, “Improved Tablespace Management” for details.

    • You can now set the InnoDB page size for uncompressed tables to 8KB or 4KB, as an alternative to the default 16KB. This setting is controlled by the innodb_page_size configuration option. You specify the size when creating the MySQL instance. All InnoDB tablespaces within an instance share the same page size. Smaller page sizes can help to avoid redundant or inefficient I/O for certain combinations of workload and storage devices, particularly SSD devices with small block sizes.

    • Improvements to the algorithms for adaptive flushing make I/O operations more efficient and consistent under a variety of workloads. The new algorithm and default configuration values are expected to improve performance and concurrency for most users. Advanced users can fine-tune their I/O responsiveness through several configuration options. See Section 14.2.5.2.8, “Improvements to Buffer Pool Flushing” for details.

    • You can code MySQL applications that access InnoDB tables through a NoSQL-style API. This feature uses the popular memcached daemon to relay requests such as ADD, SET, and GET for key-value pairs. These simple operations to store and retrieve data avoid the SQL overhead such as parsing and constructing a query execution plan. You can access the same data through the NoSQL API and SQL. For example, you might use the NoSQL API for fast updates and lookups, and SQL for complex queries and compatibility with existing applications. See Section 14.2.10, “InnoDB Integration with memcached” for details.

    • Optimizer statistics for InnoDB tables are gathered at more predictable intervals and can persist across server restarts, for improved plan stability. You can also control the amount of sampling done for InnoDB indexes, to make the optimizer statistics more accurate and improve the query execution plan. See Section 14.2.5.2.9, “Persistent Optimizer Statistics for InnoDB Tables” for details.

    • New optimizations apply to read-only transactions, improving performance and concurrency for ad-hoc queries and report-generating applications. These optimizations are applied automatically when practical, or you can specify START TRANSACTION READ ONLY to ensure the transaction is read-only. See Section 14.2.5.2.2, “Optimizations for Read-Only Transactions” for details.

    • You can move the InnoDB undo log out of the system tablespace into one or more separate tablespaces. The I/O patterns for the undo log make these new tablespaces good candidates to move to SSD storage, while keeping the system tablespace on hard disk storage. For details, see Section 14.2.5.2.3, “Separate Tablespaces for InnoDB Undo Logs”.

    • The InnoDB redo log files now have a maximum combined size of 512GB, increased from 4GB. You can specify the larger values through the innodb_log_file_size option. The startup behavior now automatically handles the situation where the size of the existing redo log files does not match the size specified by innodb_log_file_size and innodb_log_files_in_group.

    • The --innodb-read-only option lets you run a MySQL server in read-only mode. You can access InnoDB tables on read-only media such as a DVD or CD, or set up a data warehouse with multiple instances all sharing the same data directory. See Section 14.2.6.1, “Support for Read-Only Media” for usage details.

    • Several new InnoDB-related INFORMATION_SCHEMA tables provide information about the InnoDB buffer pool, metadata about tables, indexes, and foreign keys from the InnoDB data dictionary, and low-level information about performance metrics that complements the information from the Performance Schema tables.

    • InnoDB now limits the memory used to hold table information when many tables are opened.

    • InnoDB has several internal performance enhancements, including reducing contention by splitting the kernel mutex, moving flushing operations from the main thread to a separate thread, enabling multiple purge threads, and reducing contention for the buffer pool on large-memory systems.

    • InnoDB uses a new, faster algorithm to detect deadlocks. Information about all InnoDB deadlocks can be written to the MySQL server error log, to help diagnose application issues.

    • To avoid a lengthy warmup period after restarting the server, particularly for instances with large InnoDB buffer pools, you can reload pages into the buffer pool immediately after a restart. MySQL can dump a compact data file at shutdown, then consult that data file to find the pages to reload on the next restart. You can also manually dump or reload the buffer pool at any time, for example during benchmarking or after complex report-generation queries. See Section 14.2.5.2.7, “Faster Restart by Preloading the InnoDB Buffer Pool” for details.

  • Partitioning.  These table-partitioning enhancements were added:

    • The maximum number of partitions is increased to 8192. This number includes all partitions and all subpartitions of the table.

    • It is now possible to exchange a partition of a partitioned table or a subpartition of a subpartitioned table with a nonpartitioned table that otherwise has the same structure using the ALTER TABLE ... EXCHANGE PARTITION statement. This can be used, for example, to import and export partitions. For more information and examples, see Section 17.3.3, “Exchanging Partitions and Subpartitions with Tables”.

    • Explicit selection of one or more partitions or subpartitions is now supported for queries, as well as for many data modification statements, that act on partitioned tables. For example, assume a table t with some integer column c has 4 partitions named p0, p1, p2, and p3. Then the query SELECT * FROM t PARTITION (p0, p1) WHERE c < 5 returns only those rows from partitions p0 and p1 for which c is less than 5.

      The following statements support explicit partition selection:

      For syntax, see the descriptions of the individual statements. For additional information and examples, see Section 17.5, “Partition Selection”.

    • Partition lock pruning greatly improves performance of many DML and DDL statements acting on tables with many partitions by helping to eliminate locks on partitions that are not affected by these statements. Such statements include many SELECT, SELECT ... PARTITION, UPDATE, REPLACE, INSERT, as well as many other statements. For more information, including a complete listing of the statements whose performance has thus been improved, see Section 17.6.4, “Partitioning and Locking”.

  • Performance Schema.  The Performance Schema includes several new features:

    • Instrumentation for table input and output. Instrumented operations include row-level accesses to persistent base tables or temporary tables. Operations that affect rows are fetch, insert, update, and delete.

    • Event filtering by table, based on schema and/or table names.

    • Event filtering by thread. More information is collected for threads.

    • Summary tables for table and index I/O, and for table locks.

    • Instrumentation for statements and stages within statements.

    • Configuration of instruments and consumers at server startup, which previously was possible only at runtime.

  • Replication and logging.  These replication enhancements were added:

    • MySQL now supports transaction-based replication using global transaction identifiers (also known as “GTIDs”). This makes it possible to identify and track each transaction when it is committed on the originating server and as it is applied by any slaves.

      Enabling of GTIDs in a replication setup is done primarily using the new --gtid-mode and --enforce-gtid-consistency server options. For information about additional options and variables introduced in support of GTIDs, see Section 16.1.4.5, “Global Transaction ID Options and Variables”.

      When using GTIDs it is not necessary to refer to log files or positions within those files when starting a new slave or failing over to a new master, which greatly simplifies these tasks.

      GTID-based replication is completely transaction-based, which makes it simple to check the consistency of masters and slaves. If all transactions committed on a given master are also committed on a given slave, consistency between the two servers is guaranteed.

      For more complete information about the implementation and use of GTIDs in MySQL Replication, see Section 16.1.3, “Replication with Global Transaction Identifiers”.

    • MySQL row-based replication now supports row image control. By logging only those columns required for uniquely identifying and executing changes on each row (as opposed to all columns) for each row change, it is possible to save disk space, network resources, and memory usage. You can determine whether full or minimal rows are logged by setting the binlog_row_image server system variable to one of the values minimal (log required columns only), full (log all columns), or noblob (log all columns except for unneeded BLOB or TEXT columns). See System variables used with the binary log, for more information.

    • Binary logs written and read by the MySQL Server are now crash-safe, because only complete events (or transactions) are logged or read back. By default, the server logs the length of the event as well as the event itself and uses this information to verify that the event was written correctly. You can also cause the server to write checksums for the events using CRC32 checksums by setting the binlog_checksum system variable. To cause the server to read checksums from the binary log, use the master_verify_checksum system variable. The --slave-sql-verify-checksum system variable causes the slave SQL thread to read checksums from the relay log.

    • MySQL now supports logging of master connection information and of slave relay log information to tables as well as files. Use of these tables can be controlled independently, by the --master-info-repository and --relay-log-info-repository server options. Setting --master-info-repository to TABLE causes connection information to be logged in the slave_master_info table; setting --relay-log-info-repository to TABLE causes relay log information to be logged to the slave_relay_log_info table. Both of these tables are created automatically, in the mysql system database.

      Important

      In order for replication to be crash-safe, the slave_master_info and slave_relay_log_info tables must each use a transactional storage engine such as InnoDB. By default, both of these tables use MyISAM; this means that, prior to starting replication, you must change both of these tables to use a transactional storage engine if you wish for replication to be crash-safe. You can do this by means of the appropriate ALTER TABLE ... ENGINE=... statements. You should not attempt to change the storage engine used by either of these tables while replication is actually running.

    • mysqlbinlog now has the capability to back up a binary log in its original binary format. When invoked with the --read-from-remote-server and --raw options, mysqlbinlog connects to a server, requests the log files, and writes output files in the same format as the originals. See Section 4.6.8.3, “Using mysqlbinlog to Back Up Binary Log Files”.

    • MySQL now supports delayed replication such that a slave server deliberately lags behind the master by at least a specified amount of time. The default delay is 0 seconds. Use the new MASTER_DELAY option for CHANGE MASTER TO to set the delay.

      Delayed replication can be used for purposes such as protecting against user mistakes on the master (a DBA can roll back a delayed slave to the time just before the disaster) or testing how the system behaves when there is a lag. See Section 16.3.9, “Delayed Replication”.

    • A replication slave having multiple network interfaces can now be caused to use only one of these (to the exclusion of the others) by using the MASTER_BIND option when issuing a CHANGE MASTER TO statement.

    • The log_bin_basename system variable has been added. This variable contains the complete filename and path to the binary log file. Whereas the log_bin system variable shows only whether or not binary logging is enabled, log_bin_basename reflects the name set with the --log-bin server option.

      Similarly, the relay_log_basename system variable shows the filename and complete path to the relay log file.

    • MySQL Replication now supports parallel execution of transactions with multi-threading on the slave. When parallel execution is enabled, the slave SQL thread acts as the coordinator for a number of slave worker threads as determined by the value of the slave_parallel_workers server system variable. The current implementation of multi-threading on the slave assumes that data and updates are partitioned on a per-database basis, and that updates within a given database occur in the same relative order as they do on the master. However, it is not necessary to coordinate transactions between different databases. Transactions can then also be distributed per database, which means that a worker thread on the slave slave can process successive transactions on a given database without waiting for updates to other databases to complete.

      Since transactions on different databases can occur in a different order on the slave than on the master, simply checking for the most recently executed transaction is not a guarantee that all previous transactions on the master have been executed on the slave. This has implications for logging and recovery when using a multi-threaded slave. For information about how to interpret binary logging information when using multi-threading on the slave, see Section 13.7.5.35, “SHOW SLAVE STATUS Syntax”.

  • Optimizer enhancements.  These query optimizer improvements were implemented:

    • The optimizer now more efficiently handles queries (and subqueries) of the following form: 

      SELECT ... FROM single_table ... ORDER BY non_index_column [DESC] LIMIT [M,]N;

      That type of query is common in web applications that display only a few rows from a larger result set. For example: 

      SELECT col1, ... FROM t1 ... ORDER BY name LIMIT 10;
SELECT col1, ... FROM t1 ... ORDER BY RAND() LIMIT 15;

      The sort buffer has a size of sort_buffer_size. If the sort elements for N rows are small enough to fit in the sort buffer (M+N rows if M was specified), the server can avoid using a merge file and perform the sort entirely in memory. For details, see Section 8.2.1.3, “Optimizing LIMIT Queries”.

    • The optimizer implements Disk-Sweep Multi-Range Read. Reading rows using a range scan on a secondary index can result in many random disk accesses to the base table when the table is large and not stored in the storage engine's cache. With the Disk-Sweep Multi-Range Read (MRR) optimization, MySQL tries to reduce the number of random disk access for range scans by first scanning the index only and collecting the keys for the relevant rows. Then the keys are sorted and finally the rows are retrieved from the base table using the order of the primary key. The motivation for Disk-sweep MRR is to reduce the number of random disk accesses and instead achieve a more sequential scan of the base table data. For more information, see Section 8.13.10, “Multi-Range Read Optimization”.

    • The optimizer implements Index Condition Pushdown (ICP), an optimization for the case where MySQL retrieves rows from a table using an index. Without ICP, the storage engine traverses the index to locate rows in the base table and returns them to the MySQL server which evaluates the WHERE condition for the rows. With ICP enabled, and if parts of the WHERE condition can be evaluated by using only fields from the index, the MySQL server pushes this part of the WHERE condition down to the storage engine. The storage engine then evaluates the pushed index condition by using the index entry and only if this is satisfied is base row be read. ICP can reduce the number of accesses the storage engine has to do against the base table and the number of accesses the MySQL server has to do against the storage engine. For more information, see Section 8.13.4, “Index Condition Pushdown Optimization”.

    • The EXPLAIN statement now provides execution plan information for DELETE, INSERT, REPLACE, and UPDATE statements. Previously, EXPLAIN provided information only for SELECT statements. In addition, the EXPLAIN statement now can produce output in JSON format. See Section 13.8.2, “EXPLAIN Syntax”.

    • The optimizer more efficiently handles subqueries in the FROM clause (that is, derived tables). Materialization of subqueries in the FROM clause is postponed until their contents are needed during query execution, which improves performance. In addition, during query execution, the optimizer may add an index to a derived table to speed up row retrieval from it. For more information, see Section 8.13.15.3, “Optimizing Subqueries in the FROM Clause (Derived Tables)”.

    • The optimizer uses semi-join and materialization strategies to optimize subquery execution. See Section 8.13.15.1, “Optimizing Subqueries with Semi-Join Transformations”, and Section 8.13.15.2, “Optimizing Subqueries with Subquery Materialization”.

    • A Batched Key Access (BKA) join algorithm is now available that uses both index access to the joined table and a join buffer. The BKA algorithm supports inner join, outer join, and semi-join operations, including nested outer joins and nested semi-joins. Benefits of BKA include improved join performance due to more efficient table scanning. For more information, see Section 8.13.11, “Block Nested-Loop and Batched Key Access Joins”.

    • The optimizer now has a tracing capability, primarily for use by developers. The interface is provided by a set of optimizer_trace_xxx system variables and the INFORMATION_SCHEMA.OPTIMIZER_TRACE table. For details, see MySQL Internals: Tracing the Optimizer.

  • Condition handling.  MySQL now supports the GET DIAGNOSTICS statement. GET DIAGNOSTICS provides applications a standardized way to obtain information from the diagnostics area, such as whether the previous SQL statement produced an exception and what it was. For more information, see Section 13.6.7.3, “GET DIAGNOSTICS Syntax”.

    In addition, several deficiencies in condition handler processing rules were corrected so that MySQL behavior is more like standard SQL:

    • Block scope is used in determining which handler to select. Previously, a stored program was treated as having a single scope for handler selection.

    • Condition precedence is more accurately resolved.

    • Diagnostics area clearing has changed. Bug #55843 caused handled conditions to be cleared from the diagnostics area before activating the handler. This made condition information unavailable within the handler. Now condition information is available to the handler, which can inspect it with the GET DIAGNOSTICS statement. The condition information is cleared when the handler exits, if it has not already been cleared during handler execution.

    • Previously, handlers were activated as soon as a condition occurred. Now they are not activated until the statement in which the condition occurred finishes execution, at which point the most appropriate handler is chosen. This can make a difference for statements that raise multiple conditions, if a condition raised later during statement execution has higher precedence than an earlier condition and there are handlers in the same scope for both conditions. Previously, the handler for the first condition raised would be chosen, even if it had a lower precedence than other handlers. Now the handler for the condition with highest precedence is chosen, even if it is not the first condition raised by the statement.

    For more information, see Section 13.6.7.6, “Scope Rules for Handlers”.

  • Data types.  These data type changes have been implemented:

  • Host cache.  MySQL now provides more information about the causes of errors that occur when clients connect to the server, as well as improved access to the host cache, which contains client IP address and host name information and is used to avoid DNS lookups. These changes have been implemented:

    • New Connection_errors_xxx status variables provide information about connection errors that do not apply to specific client IP addresses.

    • Counters have been added to the host cache to track errors that do apply to specific IP addresses, and a new host_cache Performance Schema table exposes the contents of the host cache so that it can be examined using SELECT statements. Access to host cache contents makes it possible to answer questions such as how many hosts are cached, what kinds of connection errors are occurring for which hosts, or how close host error counts are to reaching the max_connect_errors system variable limit.

    • The host cache size now is configurable using the host_cache_size system variable.

    For more information, see Section 8.11.5.2, “DNS Lookup Optimization and the Host Cache”, and Section 20.9.9.1, “The host_cache Table”.

  • OpenGIS.  The OpenGIS specification defines functions that test the relationship between two geometry values. MySQL originally implemented these functions such that they used object bounding rectangles and returned the same result as the corresponding MBR-based functions. Corresponding versions are now available that use precise object shapes. These versions are named with an ST_ prefix. For example, Contains() uses object bounding rectangles, whereas ST_Contains() uses object shapes. For more information, see Section 12.17.5.4.2, “Functions That Test Spatial Relationships Between Geometries”.

Removed Features

The following constructs are obsolete and have been removed in MySQL 5.6. Where alternatives are shown, applications should be updated to use them.

  • The --log server option and the log system variable. Instead, use the --general_log option to enable the general query log and the --general_log_file=file_name option to set the general query log file name.

  • The --log-slow-queries server option and the log_slow_queries system variable. Instead, use the --slow_query_log option to enable the slow query log and the --slow_query_log_file=file_name option to set the slow query log file name.

  • The --one-thread server option. Use --thread_handling=no-threads instead.

  • The --safe-mode server option.

  • The --skip-thread-priority server option.

  • The --table-cache server option. Use the table_open_cache system variable instead.

  • The --init-rpl-role and --rpl-recovery-rank options, the rpl_recovery_rank system variable, and the Rpl_status status variable.

  • The engine_condition_pushdown system variable. Use the engine_condition_pushdown flag of the optimizer_switch variable instead.

  • The have_csv, have_innodb, have_ndbcluster, and have_partitioning system variables. Use SHOW ENGINES instead.

  • The sql_big_tables system variable. Use big_tables instead.

  • The sql_low_priority_updates system variable. Use low_priority_updates instead.

  • The sql_max_join_size system variable. Use max_join_size instead.

  • The max_long_data_size system variable. Use max_allowed_packet instead.

  • The FLUSH MASTER and FLUSH SLAVE statements. Use the RESET MASTER and RESET SLAVE statements instead.

  • The SLAVE START and SLAVE STOP statements. Use The START SLAVE and STOP SLAVE statements.

  • The SHOW AUTHORS and SHOW CONTRIBUTORS statements.

  • The OPTION and ONE_SHOT modifiers for the SET statement.

  • It is explicitly disallowed to assign the value DEFAULT to stored procedure or function parameters or stored program local variables (for example with a SET var_name = DEFAULT statement). It remains permissible to assign DEFAULT to system variables, as before.

1.5. MySQL Development History

This section describes the general MySQL development history, provides an overview about features that have been implemented in previous series and that are new in MySQL 5.6, the release series covered in this manual. The maturity level this release series is rc. Information about maturity levels can be found in Section 2.1.2.1, “Choosing Which Version of MySQL to Install”.

Before upgrading from one release series to the next, please see the notes in Section 2.11.1, “Upgrading MySQL”.

The most requested features and the versions in which they were implemented are summarized in the following table.

FeatureMySQL Series
Unions4.0
Subqueries4.1
R-trees4.1 (for the MyISAM storage engine)
Stored procedures and functions5.0
Views5.0
Cursors5.0
XA transactions5.0
Triggers5.0 and 5.1
Event scheduler5.1
Partitioning5.1
Pluggable storage engine API5.1
Plugin API5.1
Row-based replication5.1
Server log tables5.1
Scalability and performance improvements5.1 (with InnoDB Plugin)
DTrace support5.5
Semisynchronous replication5.5
SIGNAL/RESIGNAL support in stored routines5.5
Performance Schema5.5
Supplementary Unicode characters5.5

1.6. MySQL Information Sources

1.6.1. MySQL Mailing Lists
1.6.2. MySQL Community Support at the MySQL Forums
1.6.3. MySQL Community Support on Internet Relay Chat (IRC)
1.6.4. MySQL Enterprise

This section lists sources of additional information that you may find helpful, such as the MySQL mailing lists and user forums, and Internet Relay Chat.

1.6.1. MySQL Mailing Lists

1.6.1.1. Guidelines for Using the Mailing Lists

This section introduces the MySQL mailing lists and provides guidelines as to how the lists should be used. When you subscribe to a mailing list, you receive all postings to the list as email messages. You can also send your own questions and answers to the list.

To subscribe to or unsubscribe from any of the mailing lists described in this section, visit http://lists.mysql.com/. For most of them, you can select the regular version of the list where you get individual messages, or a digest version where you get one large message per day.

Please do not send messages about subscribing or unsubscribing to any of the mailing lists, because such messages are distributed automatically to thousands of other users.

Your local site may have many subscribers to a MySQL mailing list. If so, the site may have a local mailing list, so that messages sent from lists.mysql.com to your site are propagated to the local list. In such cases, please contact your system administrator to be added to or dropped from the local MySQL list.

To have traffic for a mailing list go to a separate mailbox in your mail program, set up a filter based on the message headers. You can use either the List-ID: or Delivered-To: headers to identify list messages.

The MySQL mailing lists are as follows:

  • announce

    The list for announcements of new versions of MySQL and related programs. This is a low-volume list to which all MySQL users should subscribe.

  • mysql

    The main list for general MySQL discussion. Please note that some topics are better discussed on the more-specialized lists. If you post to the wrong list, you may not get an answer.

  • bugs

    The list for people who want to stay informed about issues reported since the last release of MySQL or who want to be actively involved in the process of bug hunting and fixing. See Section 1.7, “How to Report Bugs or Problems”.

  • internals

    The list for people who work on the MySQL code. This is also the forum for discussions on MySQL development and for posting patches.

  • mysqldoc

    The list for people who work on the MySQL documentation.

  • benchmarks

    The list for anyone interested in performance issues. Discussions concentrate on database performance (not limited to MySQL), but also include broader categories such as performance of the kernel, file system, disk system, and so on.

  • packagers

    The list for discussions on packaging and distributing MySQL. This is the forum used by distribution maintainers to exchange ideas on packaging MySQL and on ensuring that MySQL looks and feels as similar as possible on all supported platforms and operating systems.

  • java

    The list for discussions about the MySQL server and Java. It is mostly used to discuss JDBC drivers such as MySQL Connector/J.

  • win32

    The list for all topics concerning the MySQL software on Microsoft operating systems, such as Windows 9x, Me, NT, 2000, XP, and 2003.

  • myodbc

    The list for all topics concerning connecting to the MySQL server with ODBC.

  • gui-tools

    The list for all topics concerning MySQL graphical user interface tools such as MySQL Workbench.

  • cluster

    The list for discussion of MySQL Cluster.

  • dotnet

    The list for discussion of the MySQL server and the .NET platform. It is mostly related to MySQL Connector/Net.

  • plusplus

    The list for all topics concerning programming with the C++ API for MySQL.

  • perl

    The list for all topics concerning Perl support for MySQL with DBD::mysql.

If you're unable to get an answer to your questions from a MySQL mailing list or forum, one option is to purchase support from Oracle. This puts you in direct contact with MySQL developers.

The following MySQL mailing lists are in languages other than English. These lists are not operated by Oracle.

1.6.1.1. Guidelines for Using the Mailing Lists

Please do not post mail messages from your browser with HTML mode turned on. Many users do not read mail with a browser.

When you answer a question sent to a mailing list, if you consider your answer to have broad interest, you may want to post it to the list instead of replying directly to the individual who asked. Try to make your answer general enough that people other than the original poster may benefit from it. When you post to the list, please make sure that your answer is not a duplication of a previous answer.

Try to summarize the essential part of the question in your reply. Do not feel obliged to quote the entire original message.

When answers are sent to you individually and not to the mailing list, it is considered good etiquette to summarize the answers and send the summary to the mailing list so that others may have the benefit of responses you received that helped you solve your problem.

1.6.2. MySQL Community Support at the MySQL Forums

The forums at http://forums.mysql.com are an important community resource. Many forums are available, grouped into these general categories:

  • Migration

  • MySQL Usage

  • MySQL Connectors

  • Programming Languages

  • Tools

  • 3rd-Party Applications

  • Storage Engines

  • MySQL Technology

  • SQL Standards

  • Business

1.6.3. MySQL Community Support on Internet Relay Chat (IRC)

In addition to the various MySQL mailing lists and forums, you can find experienced community people on Internet Relay Chat (IRC). These are the best networks/channels currently known to us:

freenode (see http://www.freenode.net/ for servers)

  • #mysql is primarily for MySQL questions, but other database and general SQL questions are welcome. Questions about PHP, Perl, or C in combination with MySQL are also common.

If you are looking for IRC client software to connect to an IRC network, take a look at xChat (http://www.xchat.org/). X-Chat (GPL licensed) is available for Unix as well as for Windows platforms (a free Windows build of X-Chat is available at http://www.silverex.org/download/).

1.6.4. MySQL Enterprise

Oracle offers technical support in the form of MySQL Enterprise. For organizations that rely on the MySQL DBMS for business-critical production applications, MySQL Enterprise is a commercial subscription offering which includes:

  • MySQL Enterprise Server

  • MySQL Enterprise Monitor

  • Monthly Rapid Updates and Quarterly Service Packs

  • MySQL Knowledge Base

  • 24x7 Technical and Consultative Support

MySQL Enterprise is available in multiple tiers, giving you the flexibility to choose the level of service that best matches your needs. For more information, see MySQL Enterprise.

1.7. How to Report Bugs or Problems

Before posting a bug report about a problem, please try to verify that it is a bug and that it has not been reported already:

  • Start by searching the MySQL online manual at http://dev.mysql.com/doc/. We try to keep the manual up to date by updating it frequently with solutions to newly found problems. The change history (http://dev.mysql.com/doc/mysql/en/news.html) can be particularly useful since it is quite possible that a newer version contains a solution to your problem.

  • If you get a parse error for an SQL statement, please check your syntax closely. If you cannot find something wrong with it, it is extremely likely that your current version of MySQL Server doesn't support the syntax you are using. If you are using the current version and the manual doesn't cover the syntax that you are using, MySQL Server doesn't support your statement.

    If the manual covers the syntax you are using, but you have an older version of MySQL Server, you should check the MySQL change history to see when the syntax was implemented. In this case, you have the option of upgrading to a newer version of MySQL Server.

  • For solutions to some common problems, see Section C.5, “Problems and Common Errors”.

  • Search the bugs database at http://bugs.mysql.com/ to see whether the bug has been reported and fixed.

  • Search the MySQL mailing list archives at http://lists.mysql.com/. See Section 1.6.1, “MySQL Mailing Lists”.

  • You can also use http://www.mysql.com/search/ to search all the Web pages (including the manual) that are located at the MySQL Web site.

If you cannot find an answer in the manual, the bugs database, or the mailing list archives, check with your local MySQL expert. If you still cannot find an answer to your question, please use the following guidelines for reporting the bug.

The normal way to report bugs is to visit http://bugs.mysql.com/, which is the address for our bugs database. This database is public and can be browsed and searched by anyone. If you log in to the system, you can enter new reports.

Bugs posted in the bugs database at http://bugs.mysql.com/ that are corrected for a given release are noted in the change history.

If you have found a sensitive security bug in MySQL, you can send email to .

To discuss problems with other users, you can use one of the MySQL mailing lists. Section 1.6.1, “MySQL Mailing Lists”.

Writing a good bug report takes patience, but doing it right the first time saves time both for us and for yourself. A good bug report, containing a full test case for the bug, makes it very likely that we will fix the bug in the next release. This section helps you write your report correctly so that you do not waste your time doing things that may not help us much or at all. Please read this section carefully and make sure that all the information described here is included in your report.

Preferably, you should test the problem using the latest production or development version of MySQL Server before posting. Anyone should be able to repeat the bug by just using mysql test < script_file on your test case or by running the shell or Perl script that you include in the bug report. Any bug that we are able to repeat has a high chance of being fixed in the next MySQL release.

It is most helpful when a good description of the problem is included in the bug report. That is, give a good example of everything you did that led to the problem and describe, in exact detail, the problem itself. The best reports are those that include a full example showing how to reproduce the bug or problem. See MySQL Internals: Porting to Other Systems.

Remember that it is possible for us to respond to a report containing too much information, but not to one containing too little. People often omit facts because they think they know the cause of a problem and assume that some details do not matter. A good principle to follow is that if you are in doubt about stating something, state it. It is faster and less troublesome to write a couple more lines in your report than to wait longer for the answer if we must ask you to provide information that was missing from the initial report.

The most common errors made in bug reports are (a) not including the version number of the MySQL distribution that you use, and (b) not fully describing the platform on which the MySQL server is installed (including the platform type and version number). These are highly relevant pieces of information, and in 99 cases out of 100, the bug report is useless without them. Very often we get questions like, “Why doesn't this work for me?” Then we find that the feature requested wasn't implemented in that MySQL version, or that a bug described in a report has been fixed in newer MySQL versions. Errors often are platform-dependent. In such cases, it is next to impossible for us to fix anything without knowing the operating system and the version number of the platform.

If you compiled MySQL from source, remember also to provide information about your compiler if it is related to the problem. Often people find bugs in compilers and think the problem is MySQL-related. Most compilers are under development all the time and become better version by version. To determine whether your problem depends on your compiler, we need to know what compiler you used. Note that every compiling problem should be regarded as a bug and reported accordingly.

If a program produces an error message, it is very important to include the message in your report. If we try to search for something from the archives, it is better that the error message reported exactly matches the one that the program produces. (Even the lettercase should be observed.) It is best to copy and paste the entire error message into your report. You should never try to reproduce the message from memory.

If you have a problem with Connector/ODBC (MyODBC), please try to generate a trace file and send it with your report. See Section 21.1.8.2, “How to Report Connector/ODBC Problems or Bugs”.

If your report includes long query output lines from test cases that you run with the mysql command-line tool, you can make the output more readable by using the --vertical option or the \G statement terminator. The EXPLAIN SELECT example later in this section demonstrates the use of \G.

Please include the following information in your report:

  • The version number of the MySQL distribution you are using (for example, MySQL 5.0.19). You can find out which version you are running by executing mysqladmin version. The mysqladmin program can be found in the bin directory under your MySQL installation directory.

  • The manufacturer and model of the machine on which you experience the problem.

  • The operating system name and version. If you work with Windows, you can usually get the name and version number by double-clicking your My Computer icon and pulling down the “Help/About Windows” menu. For most Unix-like operating systems, you can get this information by executing the command uname -a.

  • Sometimes the amount of memory (real and virtual) is relevant. If in doubt, include these values.

  • If you are using a source distribution of the MySQL software, include the name and version number of the compiler that you used. If you have a binary distribution, include the distribution name.

  • If the problem occurs during compilation, include the exact error messages and also a few lines of context around the offending code in the file where the error occurs.

  • If mysqld died, you should also report the statement that crashed mysqld. You can usually get this information by running mysqld with query logging enabled, and then looking in the log after mysqld crashes. See MySQL Internals: Porting to Other Systems.

  • If a database table is related to the problem, include the output from the SHOW CREATE TABLE db_name.tbl_name statement in the bug report. This is a very easy way to get the definition of any table in a database. The information helps us create a situation matching the one that you have experienced.

  • The SQL mode in effect when the problem occurred can be significant, so please report the value of the sql_mode system variable. For stored procedure, stored function, and trigger objects, the relevant sql_mode value is the one in effect when the object was created. For a stored procedure or function, the SHOW CREATE PROCEDURE or SHOW CREATE FUNCTION statement shows the relevant SQL mode, or you can query INFORMATION_SCHEMA for the information: 

    SELECT ROUTINE_SCHEMA, ROUTINE_NAME, SQL_MODE
FROM INFORMATION_SCHEMA.ROUTINES;

    For triggers, you can use this statement: 

    SELECT EVENT_OBJECT_SCHEMA, EVENT_OBJECT_TABLE, TRIGGER_NAME, SQL_MODE
FROM INFORMATION_SCHEMA.TRIGGERS;

  • For performance-related bugs or problems with SELECT statements, you should always include the output of EXPLAIN SELECT ..., and at least the number of rows that the SELECT statement produces. You should also include the output from SHOW CREATE TABLE tbl_name for each table that is involved. The more information you provide about your situation, the more likely it is that someone can help you.

    The following is an example of a very good bug report. The statements are run using the mysql command-line tool. Note the use of the \G statement terminator for statements that would otherwise provide very long output lines that are difficult to read. 

    mysql> SHOW VARIABLES;
mysql> SHOW COLUMNS FROM ...\G
       <output from SHOW COLUMNS>
mysql> EXPLAIN SELECT ...\G
       <output from EXPLAIN>
mysql> FLUSH STATUS;
mysql> SELECT ...;
       <A short version of the output from SELECT,
       including the time taken to run the query>
mysql> SHOW STATUS;
       <output from SHOW STATUS>

  • If a bug or problem occurs while running mysqld, try to provide an input script that reproduces the anomaly. This script should include any necessary source files. The more closely the script can reproduce your situation, the better. If you can make a reproducible test case, you should upload it to be attached to the bug report.

    If you cannot provide a script, you should at least include the output from mysqladmin variables extended-status processlist in your report to provide some information on how your system is performing.

  • If you cannot produce a test case with only a few rows, or if the test table is too big to be included in the bug report (more than 10 rows), you should dump your tables using mysqldump and create a README file that describes your problem. Create a compressed archive of your files using tar and gzip or zip, and use FTP to transfer the archive to ftp://ftp.mysql.com/pub/mysql/upload/. Then enter the problem into our bugs database at http://bugs.mysql.com/.

  • If you believe that the MySQL server produces a strange result from a statement, include not only the result, but also your opinion of what the result should be, and an explanation describing the basis for your opinion.

  • When you provide an example of the problem, it is better to use the table names, variable names, and so forth that exist in your actual situation than to come up with new names. The problem could be related to the name of a table or variable. These cases are rare, perhaps, but it is better to be safe than sorry. After all, it should be easier for you to provide an example that uses your actual situation, and it is by all means better for us. If you have data that you do not want to be visible to others in the bug report, you can use FTP to transfer it to ftp://ftp.mysql.com/pub/mysql/upload/. If the information is really top secret and you do not want to show it even to us, go ahead and provide an example using other names, but please regard this as the last choice.

  • Include all the options given to the relevant programs, if possible. For example, indicate the options that you use when you start the mysqld server, as well as the options that you use to run any MySQL client programs. The options to programs such as mysqld and mysql, and to the configure script, are often key to resolving problems and are very relevant. It is never a bad idea to include them. If your problem involves a program written in a language such as Perl or PHP, please include the language processor's version number, as well as the version for any modules that the program uses. For example, if you have a Perl script that uses the DBI and DBD::mysql modules, include the version numbers for Perl, DBI, and DBD::mysql.

  • If your question is related to the privilege system, please include the output of mysqlaccess, the output of mysqladmin reload, and all the error messages you get when trying to connect. When you test your privileges, you should first run mysqlaccess. After this, execute mysqladmin reload version and try to connect with the program that gives you trouble. mysqlaccess can be found in the bin directory under your MySQL installation directory.

  • If you have a patch for a bug, do include it. But do not assume that the patch is all we need, or that we can use it, if you do not provide some necessary information such as test cases showing the bug that your patch fixes. We might find problems with your patch or we might not understand it at all. If so, we cannot use it.

    If we cannot verify the exact purpose of the patch, we will not use it. Test cases help us here. Show that the patch handles all the situations that may occur. If we find a borderline case (even a rare one) where the patch will not work, it may be useless.

  • Guesses about what the bug is, why it occurs, or what it depends on are usually wrong. Even the MySQL team cannot guess such things without first using a debugger to determine the real cause of a bug.

  • Indicate in your bug report that you have checked the reference manual and mail archive so that others know you have tried to solve the problem yourself.

  • If your data appears corrupt or you get errors when you access a particular table, first check your tables with CHECK TABLE. If that statement reports any errors:

    • The InnoDB crash recovery mechanism handles cleanup when the server is restarted after being killed, so in typical operation there is no need to “repair” tables. If you encounter an error with InnoDB tables, restart the server and see whether the problem persists, or whether the error affected only cached data in memory. If data is corrupted on disk, consider restarting with the innodb_force_recovery option enabled so that you can dump the affected tables.

    • For non-transactional tables, try to repair them with REPAIR TABLE or with myisamchk. See Chapter 5, MySQL Server Administration.

    If you are running Windows, please verify the value of lower_case_table_names using the SHOW VARIABLES LIKE 'lower_case_table_names' statement. This variable affects how the server handles lettercase of database and table names. Its effect for a given value should be as described in Section 9.2.2, “Identifier Case Sensitivity”.

  • If you often get corrupted tables, you should try to find out when and why this happens. In this case, the error log in the MySQL data directory may contain some information about what happened. (This is the file with the .err suffix in the name.) See Section 5.2.2, “The Error Log”. Please include any relevant information from this file in your bug report. Normally mysqld should never crash a table if nothing killed it in the middle of an update. If you can find the cause of mysqld dying, it is much easier for us to provide you with a fix for the problem. See Section C.5.1, “How to Determine What Is Causing a Problem”.

  • If possible, download and install the most recent version of MySQL Server and check whether it solves your problem. All versions of the MySQL software thoroughly tested and should work without problems. We believe in making everything as backward-compatible as possible, and you should be able to switch MySQL versions without difficulty. See Section 2.1.2, “Choosing Which MySQL Distribution to Install”.

1.8. MySQL Standards Compliance

1.8.1. What Standards MySQL Follows
1.8.2. Selecting SQL Modes
1.8.3. Running MySQL in ANSI Mode
1.8.4. MySQL Extensions to Standard SQL
1.8.5. MySQL Differences from Standard SQL
1.8.6. How MySQL Deals with Constraints

This section describes how MySQL relates to the ANSI/ISO SQL standards. MySQL Server has many extensions to the SQL standard, and here you can find out what they are and how to use them. You can also find information about functionality missing from MySQL Server, and how to work around some of the differences.

The SQL standard has been evolving since 1986 and several versions exist. In this manual, “SQL-92” refers to the standard released in 1992, “SQL:1999” refers to the standard released in 1999, “SQL:2003” refers to the standard released in 2003, and “SQL:2008” refers to the most recent version of the standard, released in 2008. We use the phrase “the SQL standard” or “standard SQL” to mean the current version of the SQL Standard at any time.

One of our main goals with the product is to continue to work toward compliance with the SQL standard, but without sacrificing speed or reliability. We are not afraid to add extensions to SQL or support for non-SQL features if this greatly increases the usability of MySQL Server for a large segment of our user base. The HANDLER interface is an example of this strategy. See Section 13.2.4, “HANDLER Syntax”.

We continue to support transactional and nontransactional databases to satisfy both mission-critical 24/7 usage and heavy Web or logging usage.

MySQL Server was originally designed to work with medium-sized databases (10-100 million rows, or about 100MB per table) on small computer systems. Today MySQL Server handles terabyte-sized databases, but the code can also be compiled in a reduced version suitable for hand-held and embedded devices. The compact design of the MySQL server makes development in both directions possible without any conflicts in the source tree.

Currently, we are not targeting real-time support, although MySQL replication capabilities offer significant functionality.

MySQL supports high-availability database clustering using the NDBCLUSTER storage engine. See MySQL Cluster NDB 7.2.

We are implementing XML functionality beginning in MySQL 5.1, which supports most of the W3C XPath standard. We plan to increase support for XML as part of future MySQL development. See Section 12.11, “XML Functions”.

1.8.1. What Standards MySQL Follows

Our aim is to support the full ANSI/ISO SQL standard, but without making concessions to speed and quality of the code.

ODBC levels 0 to 3.51.

1.8.2. Selecting SQL Modes

The MySQL server can operate in different SQL modes, and can apply these modes differentially for different clients. This capability enables each application to tailor the server's operating mode to its own requirements.

SQL modes control aspects of server operation such as what SQL syntax MySQL should support and what kind of data validation checks it should perform. This makes it easier to use MySQL in different environments and to use MySQL together with other database servers.

You can set the default SQL mode by starting mysqld with the --sql-mode="mode_value" option. You can also change the mode at runtime by setting the sql_mode system variable with a SET [GLOBAL|SESSION] sql_mode='mode_value' statement.

For more information on setting the SQL mode, see Section 5.1.7, “Server SQL Modes”.

1.8.3. Running MySQL in ANSI Mode

You can tell mysqld to run in ANSI mode with the --ansi startup option. Running the server in ANSI mode is the same as starting it with the following options: 

--transaction-isolation=SERIALIZABLE --sql-mode=ANSI

You can achieve the same effect at runtime by executing these two statements: 

SET GLOBAL TRANSACTION ISOLATION LEVEL SERIALIZABLE;
SET GLOBAL sql_mode = 'ANSI';

You can see that setting the sql_mode system variable to 'ANSI' enables all SQL mode options that are relevant for ANSI mode as follows: 

mysql> SET GLOBAL sql_mode='ANSI';
mysql> SELECT @@global.sql_mode;
        -> 'REAL_AS_FLOAT,PIPES_AS_CONCAT,ANSI_QUOTES,IGNORE_SPACE,ANSI'

Running the server in ANSI mode with --ansi is not quite the same as setting the SQL mode to 'ANSI'. The --ansi option affects the SQL mode and also sets the transaction isolation level. Setting the SQL mode to 'ANSI' has no effect on the isolation level.

See Section 5.1.3, “Server Command Options”, and Section 1.8.2, “Selecting SQL Modes”.

1.8.4. MySQL Extensions to Standard SQL

MySQL Server supports some extensions that you probably won't find in other SQL DBMSs. Be warned that if you use them, your code won't be portable to other SQL servers. In some cases, you can write code that includes MySQL extensions, but is still portable, by using comments of the following form: 

/*! MySQL-specific code */

In this case, MySQL Server parses and executes the code within the comment as it would any other SQL statement, but other SQL servers will ignore the extensions. For example, MySQL Server recognizes the STRAIGHT_JOIN keyword in the following statement, but other servers will not: 

SELECT /*! STRAIGHT_JOIN */ col1 FROM table1,table2 WHERE ...

If you add a version number after the “!” character, the syntax within the comment is executed only if the MySQL version is greater than or equal to the specified version number. The TEMPORARY keyword in the following comment is executed only by servers from MySQL 3.23.02 or higher: 

CREATE /*!32302 TEMPORARY */ TABLE t (a INT);

The following descriptions list MySQL extensions, organized by category.

1.8.5. MySQL Differences from Standard SQL

1.8.5.1. SELECT INTO TABLE Differences
1.8.5.2. UPDATE Differences
1.8.5.3. Transaction and Atomic Operation Differences
1.8.5.4. Foreign Key Differences
1.8.5.5. '--' as the Start of a Comment

We try to make MySQL Server follow the ANSI SQL standard and the ODBC SQL standard, but MySQL Server performs operations differently in some cases:

1.8.5.1. SELECT INTO TABLE Differences

MySQL Server doesn't support the SELECT ... INTO TABLE Sybase SQL extension. Instead, MySQL Server supports the INSERT INTO ... SELECT standard SQL syntax, which is basically the same thing. See Section 13.2.5.1, “INSERT ... SELECT Syntax”. For example: 

INSERT INTO tbl_temp2 (fld_id)
    SELECT tbl_temp1.fld_order_id
    FROM tbl_temp1 WHERE tbl_temp1.fld_order_id > 100;

Alternatively, you can use SELECT ... INTO OUTFILE or CREATE TABLE ... SELECT.

You can use SELECT ... INTO with user-defined variables. The same syntax can also be used inside stored routines using cursors and local variables. See Section 13.2.9.1, “SELECT ... INTO Syntax”.

1.8.5.2. UPDATE Differences

If you access a column from the table to be updated in an expression, UPDATE uses the current value of the column. The second assignment in the following statement sets col2 to the current (updated) col1 value, not the original col1 value. The result is that col1 and col2 have the same value. This behavior differs from standard SQL. 

UPDATE t1 SET col1 = col1 + 1, col2 = col1;

1.8.5.3. Transaction and Atomic Operation Differences

MySQL Server (version 3.23-max and all versions 4.0 and above) supports transactions with the InnoDB transactional storage engine. In MySQL 5.5 and up, newly created tables use InnoDB by default, as explained in Section 14.2.1.1, “InnoDB as the Default MySQL Storage Engine”. By default, InnoDB provides full ACID compliance; see Section 14.2.9, “MySQL and the ACID Model” for ways that you can adjust settings to balance ACID compliance with raw performance. For information about InnoDB differences from standard SQL with regard to treatment of transaction errors, see Section 14.2.4.14, “InnoDB Error Handling”.

The nontransactional storage engines in MySQL Server (such as MyISAM) follow a different paradigm for data integrity called “atomic operations”. MyISAM tables effectively always operate in autocommit = 1 mode. Because changed data is written to disk one statement at a time, it is harder to guarantee the consistency of a sequence of related DML operations, which could be interrupted partway through. Thus, this mode is suitable for read-mostly workloads. In transactional terms, while each specific update is running, no other user can interfere with it, there can never be an automatic rollback, and there are no dirty reads. However, these features apply to single operations, not related updates that succeed or fail as a unit. Workarounds such as the LOCK TABLES statement limit concurrent write access to nontransactional tables.

You can choose which paradigm to use, even for different tables within the same application: transactional features for reliability combined with high performance, or atomic operations for non-critical, read-mostly data (for example, on replication slave servers).

Transactional storage engines such as InnoDB offer many significant features to support high reliability for heavy read/write workloads. As a result, transactional tables can have higher memory and disk space requirements, and more CPU overhead. MySQL Server's modular design enables the concurrent use of different storage engines to suit different requirements and deliver optimum performance in all situations.

Workarounds for Reliability with Non-Transactional Tables

But how do you use the features of MySQL Server to maintain integrity even with the nontransactional MyISAM tables, and how do these features compare with the transactional storage engines?

  • If your applications are written in a way that is dependent on being able to call ROLLBACK rather than COMMIT in critical situations, transactions are more convenient. Transactions also ensure that unfinished updates or corrupting activities are not committed to the database; the server is given the opportunity to do an automatic rollback and your database is saved.

    If you use nontransactional tables, you must resolve potential problems at the application level by including checks before updates and by running scripts that check the databases for inconsistencies and automatically repair or warn if such an inconsistency occurs. You can normally fix tables with no data integrity loss by using the MySQL log or even adding one extra log.

  • Sometimes, critical transactional updates can be rewritten to be atomic. Multiple DML operations can be done with LOCK TABLES or atomic updates, ensuring that there are no deadlocks by limiting concurrent write access. If you obtain a READ LOCAL lock (as opposed to a write lock) for a table that enables concurrent inserts at the end of the table, reads are permitted, as are inserts by other clients. The newly inserted records are not be seen by the client that has the read lock until it releases the lock. With INSERT DELAYED, you can write inserts that go into a local queue until the locks are released, without having the client wait for the insert to complete. See Section 8.10.3, “Concurrent Inserts”, and Section 13.2.5.2, “INSERT DELAYED Syntax”.

  • To be safe with MySQL Server, regardless of what kinds of tables you use, make regular backups and have binary logging turned on. It is always good to have backups, regardless of which database system you use.

Following are some techniques for working with nontransactional tables:

  • Loops that need transactions normally can be coded with the help of LOCK TABLES, and you don't need cursors to update records on the fly.

  • To avoid using ROLLBACK, you can employ the following strategy:

    1. Use LOCK TABLES to lock all the tables you want to access.

    2. Test the conditions that must be true before performing the update.

    3. Update if the conditions are satisfied.

    4. Use UNLOCK TABLES to release your locks.

    Note

    This solution does not handle the situation when someone kills the threads in the middle of an update. In that case, all locks are released but some of the updates may not have been executed.

  • You can also use functions to update records in a single operation, using the following techniques:

    • Modify columns relative to their current value. This makes the update correct even if another client has changed the column values in the meantime.

    • Update only those columns that actually have changed. This is a good database practice in general.

  • When managing unique identifiers, you can avoid statements such as LOCK TABLES or ROLLBACK by using an AUTO_INCREMENT column and either the LAST_INSERT_ID() SQL function or the mysql_insert_id() C API function. See Section 12.14, “Information Functions”, and Section 21.9.3.37, “mysql_insert_id()”.

    For situations that require row-level locking, use InnoDB tables. Otherwise, with MyISAM tables, you can use a flag column in the table and do something like the following: 

    UPDATE tbl_name SET row_flag=1 WHERE id=ID;

    MySQL returns 1 for the number of affected rows if the row was found and row_flag wasn't 1 in the original row. You can think of this as though MySQL Server changed the preceding statement to: 

    UPDATE tbl_name SET row_flag=1 WHERE id=ID AND row_flag <> 1;

1.8.5.4. Foreign Key Differences

The InnoDB storage engine supports checking of foreign key constraints, including CASCADE, ON DELETE, and ON UPDATE. See Section 14.2.2.5, “FOREIGN KEY Constraints”.

For storage engines other than InnoDB, MySQL Server parses the FOREIGN KEY syntax in CREATE TABLE statements, but does not use or store it. In the future, the implementation will be extended to store this information in the table specification file so that it may be retrieved by mysqldump and ODBC. At a later stage, foreign key constraints will be implemented for MyISAM tables as well.

Foreign key enforcement offers several benefits to database developers:

  • Assuming proper design of the relationships, foreign key constraints make it more difficult for a programmer to introduce an inconsistency into the database.

  • Centralized checking of constraints by the database server makes it unnecessary to perform these checks on the application side. This eliminates the possibility that different applications may not all check the constraints in the same way.

  • Using cascading updates and deletes can simplify the application code.

  • Properly designed foreign key rules aid in documenting relationships between tables.

Do keep in mind that these benefits come at the cost of additional overhead for the database server to perform the necessary checks. Additional checking by the server affects performance, which for some applications may be sufficiently undesirable as to be avoided if possible. (Some major commercial applications have coded the foreign key logic at the application level for this reason.)

MySQL gives database developers the choice of which approach to use. If you don't need foreign keys and want to avoid the overhead associated with enforcing referential integrity, you can choose another storage engine instead, such as MyISAM. (For example, the MyISAM storage engine offers very fast performance for applications that perform only INSERT and SELECT operations. In this case, the table has no holes in the middle and the inserts can be performed concurrently with retrievals. See Section 8.10.3, “Concurrent Inserts”.)

If you choose not to take advantage of referential integrity checks, keep the following considerations in mind:

  • In the absence of server-side foreign key relationship checking, the application itself must handle relationship issues. For example, it must take care to insert rows into tables in the proper order, and to avoid creating orphaned child records. It must also be able to recover from errors that occur in the middle of multiple-record insert operations.

  • If ON DELETE is the only referential integrity capability an application needs, you can achieve a similar effect as of MySQL Server 4.0 by using multiple-table DELETE statements to delete rows from many tables with a single statement. See Section 13.2.2, “DELETE Syntax”.

  • A workaround for the lack of ON DELETE is to add the appropriate DELETE statements to your application when you delete records from a table that has a foreign key. In practice, this is often as quick as using foreign keys and is more portable.

Be aware that the use of foreign keys can sometimes lead to problems:

  • Foreign key support addresses many referential integrity issues, but it is still necessary to design key relationships carefully to avoid circular rules or incorrect combinations of cascading deletes.

  • It is not uncommon for a DBA to create a topology of relationships that makes it difficult to restore individual tables from a backup. (MySQL alleviates this difficulty by enabling you to temporarily disable foreign key checks when reloading a table that depends on other tables. See Section 14.2.2.5, “FOREIGN KEY Constraints”. As of MySQL 4.1.1, mysqldump generates dump files that take advantage of this capability automatically when they are reloaded.)

Foreign keys in SQL are used to check and enforce referential integrity, not to join tables. If you want to get results from multiple tables from a SELECT statement, you do this by performing a join between them: 

SELECT * FROM t1 INNER JOIN t2 ON t1.id = t2.id;

See Section 13.2.9.2, “JOIN Syntax”, and Section 3.6.6, “Using Foreign Keys”.

The FOREIGN KEY syntax without ON DELETE ... is often used by ODBC applications to produce automatic WHERE clauses.

1.8.5.5. '--' as the Start of a Comment

Standard SQL uses the C syntax /* this is a comment */ for comments, and MySQL Server supports this syntax as well. MySQL also support extensions to this syntax that enable MySQL-specific SQL to be embedded in the comment, as described in Section 9.6, “Comment Syntax”.

Standard SQL uses “--” as a start-comment sequence. MySQL Server uses “#” as the start comment character. MySQL Server 3.23.3 and up also supports a variant of the “--” comment style. That is, the “--” start-comment sequence must be followed by a space (or by a control character such as a newline). The space is required to prevent problems with automatically generated SQL queries that use constructs such as the following, where we automatically insert the value of the payment for payment: 

UPDATE account SET credit=credit-payment

Consider about what happens if payment has a negative value such as -1: 

UPDATE account SET credit=credit--1

credit--1 is a valid expression in SQL, but “--” is interpreted as the start of a comment, part of the expression is discarded. The result is a statement that has a completely different meaning than intended: 

UPDATE account SET credit=credit

The statement produces no change in value at all. This illustrates that permitting comments to start with “--” can have serious consequences.

Using our implementation requires a space following the “--” for it to be recognized as a start-comment sequence in MySQL Server 3.23.3 and newer. Therefore, credit--1 is safe to use.

Another safe feature is that the mysql command-line client ignores lines that start with “--”.

The following information is relevant only if you are running a MySQL version earlier than 3.23.3:

If you have an SQL script in a text file that contains “--” comments, you should use the replace utility as follows to convert the comments to use “#” characters before executing the script: 

shell> replace " --" " #" < text-file-with-funny-comments.sql \
         | mysql db_name

That is safer than executing the script in the usual way: 

shell> mysql db_name < text-file-with-funny-comments.sql

You can also edit the script file “in place” to change the “--” comments to “#” comments: 

shell> replace " --" " #" -- text-file-with-funny-comments.sql

Change them back with this command: 

shell> replace " #" " --" -- text-file-with-funny-comments.sql

See Section 4.8.2, “replace — A String-Replacement Utility”.

1.8.6. How MySQL Deals with Constraints

1.8.6.1. PRIMARY KEY and UNIQUE Index Constraints
1.8.6.2. Constraints on Invalid Data
1.8.6.3. ENUM and SET Constraints

MySQL enables you to work both with transactional tables that permit rollback and with nontransactional tables that do not. Because of this, constraint handling is a bit different in MySQL than in other DBMSs. We must handle the case when you have inserted or updated a lot of rows in a nontransactional table for which changes cannot be rolled back when an error occurs.

The basic philosophy is that MySQL Server tries to produce an error for anything that it can detect while parsing a statement to be executed, and tries to recover from any errors that occur while executing the statement. We do this in most cases, but not yet for all.

The options MySQL has when an error occurs are to stop the statement in the middle or to recover as well as possible from the problem and continue. By default, the server follows the latter course. This means, for example, that the server may coerce invalid values to the closest valid values.

Several SQL mode options are available to provide greater control over handling of bad data values and whether to continue statement execution or abort when errors occur. Using these options, you can configure MySQL Server to act in a more traditional fashion that is like other DBMSs that reject improper input. The SQL mode can be set globally at server startup to affect all clients. Individual clients can set the SQL mode at runtime, which enables each client to select the behavior most appropriate for its requirements. See Section 5.1.7, “Server SQL Modes”.

The following sections describe how MySQL Server handles different types of constraints.

1.8.6.1. PRIMARY KEY and UNIQUE Index Constraints

Normally, errors occurs for data-change statements (such as INSERT or UPDATE) that would violate primary-key, unique-key, or foreign-key constraints. If you are using a transactional storage engine such as InnoDB, MySQL automatically rolls back the statement. If you are using a nontransactional storage engine, MySQL stops processing the statement at the row for which the error occurred and leaves any remaining rows unprocessed.

MySQL supports an IGNORE keyword for INSERT, UPDATE, and so forth. If you use it, MySQL ignores primary-key or unique-key violations and continues processing with the next row. See the section for the statement that you are using (Section 13.2.5, “INSERT Syntax”, Section 13.2.11, “UPDATE Syntax”, and so forth).

You can get information about the number of rows actually inserted or updated with the mysql_info() C API function. You can also use the SHOW WARNINGS statement. See Section 21.9.3.35, “mysql_info()”, and Section 13.7.5.41, “SHOW WARNINGS Syntax”.

Currently, only InnoDB tables support foreign keys. See Section 14.2.2.5, “FOREIGN KEY Constraints”.

1.8.6.2. Constraints on Invalid Data

By default, MySQL is forgiving of invalid or improper data values and coerces them to valid values for data entry. However, you can change the server SQL mode to select more traditional treatment of bad values such that the server rejects them and aborts the statement in which they occur. See Section 5.1.7, “Server SQL Modes”.

This section describes the default (forgiving) behavior of MySQL, as well as the strict SQL mode and how it differs.

If you are not using strict mode, then whenever you insert an “incorrect” value into a column, such as a NULL into a NOT NULL column or a too-large numeric value into a numeric column, MySQL sets the column to the “best possible value” instead of producing an error: The following rules describe in more detail how this works:

  • If you try to store an out of range value into a numeric column, MySQL Server instead stores zero, the smallest possible value, or the largest possible value, whichever is closest to the invalid value.

  • For strings, MySQL stores either the empty string or as much of the string as can be stored in the column.

  • If you try to store a string that doesn't start with a number into a numeric column, MySQL Server stores 0.

  • Invalid values for ENUM and SET columns are handled as described in Section 1.8.6.3, “ENUM and SET Constraints”.

  • MySQL enables you to store certain incorrect date values into DATE and DATETIME columns (such as '2000-02-31' or '2000-02-00'). The idea is that it is not the job of the SQL server to validate dates. If MySQL can store a date value and retrieve exactly the same value, MySQL stores it as given. If the date is totally wrong (outside the server's ability to store it), the special “zero” date value '0000-00-00' is stored in the column instead.

  • If you try to store NULL into a column that doesn't take NULL values, an error occurs for single-row INSERT statements. For multiple-row INSERT statements or for INSERT INTO ... SELECT statements, MySQL Server stores the implicit default value for the column data type. In general, this is 0 for numeric types, the empty string ('') for string types, and the “zero” value for date and time types. Implicit default values are discussed in Section 11.5, “Data Type Default Values”.

  • If an INSERT statement specifies no value for a column, MySQL inserts its default value if the column definition includes an explicit DEFAULT clause. If the definition has no such DEFAULT clause, MySQL inserts the implicit default value for the column data type.

The reason for using the preceding rules in nonstrict mode is that we can't check these conditions until the statement has begun executing. We can't just roll back if we encounter a problem after updating a few rows, because the storage engine may not support rollback. The option of terminating the statement is not that good; in this case, the update would be “half done,” which is probably the worst possible scenario. In this case, it is better to “do the best you can” and then continue as if nothing happened.

In MySQL 5.0.2 and up, you can select stricter treatment of input values by using the STRICT_TRANS_TABLES or STRICT_ALL_TABLES SQL modes: 

SET sql_mode = 'STRICT_TRANS_TABLES';
SET sql_mode = 'STRICT_ALL_TABLES';

STRICT_TRANS_TABLES enables strict mode for transactional storage engines, and also to some extent for nontransactional engines. It works like this:

  • For transactional storage engines, bad data values occurring anywhere in a statement cause the statement to abort and roll back.

  • For nontransactional storage engines, a statement aborts if the error occurs in the first row to be inserted or updated. (When the error occurs in the first row, the statement can be aborted to leave the table unchanged, just as for a transactional table.) Errors in rows after the first do not abort the statement, because the table has already been changed by the first row. Instead, bad data values are adjusted and result in warnings rather than errors. In other words, with STRICT_TRANS_TABLES, a wrong value causes MySQL to roll back all updates done so far, if that can be done without changing the table. But once the table has been changed, further errors result in adjustments and warnings.

For even stricter checking, enable STRICT_ALL_TABLES. This is the same as STRICT_TRANS_TABLES except that for nontransactional storage engines, errors abort the statement even for bad data in rows following the first row. This means that if an error occurs partway through a multiple-row insert or update for a nontransactional table, a partial update results. Earlier rows are inserted or updated, but those from the point of the error on are not. To avoid this for nontransactional tables, either use single-row statements or else use STRICT_TRANS_TABLES if conversion warnings rather than errors are acceptable. To avoid problems in the first place, do not use MySQL to check column content. It is safest (and often faster) to let the application ensure that it passes only valid values to the database.

With either of the strict mode options, you can cause errors to be treated as warnings by using INSERT IGNORE or UPDATE IGNORE rather than INSERT or UPDATE without IGNORE.

1.8.6.3. ENUM and SET Constraints

ENUM and SET columns provide an efficient way to define columns that can contain only a given set of values. See Section 11.4.4, “The ENUM Type”, and Section 11.4.5, “The SET Type”. However, before MySQL 5.0.2, ENUM and SET columns do not provide true constraints on entry of invalid data:

  • ENUM columns always have a default value. If you specify no default value, then it is NULL for columns that can have NULL, otherwise it is the first enumeration value in the column definition.

  • If you insert an incorrect value into an ENUM column or if you force a value into an ENUM column with IGNORE, it is set to the reserved enumeration value of 0, which is displayed as an empty string in string context.

  • If you insert an incorrect value into a SET column, the incorrect value is ignored. For example, if the column can contain the values 'a', 'b', and 'c', an attempt to assign 'a,x,b,y' results in a value of 'a,b'.

As of MySQL 5.0.2, you can configure the server to use strict SQL mode. See Section 5.1.7, “Server SQL Modes”. With strict mode enabled, the definition of a ENUM or SET column does act as a constraint on values entered into the column. An error occurs for values that do not satisfy these conditions:

  • An ENUM value must be one of those listed in the column definition, or the internal numeric equivalent thereof. The value cannot be the error value (that is, 0 or the empty string). For a column defined as ENUM('a','b','c'), values such as '', 'd', or 'ax' are invalid and are rejected.

  • A SET value must be the empty string or a value consisting only of the values listed in the column definition separated by commas. For a column defined as SET('a','b','c'), values such as 'd' or 'a,b,c,d' are invalid and are rejected.

Errors for invalid values can be suppressed in strict mode if you use INSERT IGNORE or UPDATE IGNORE. In this case, a warning is generated rather than an error. For ENUM, the value is inserted as the error member (0). For SET, the value is inserted as given except that any invalid substrings are deleted. For example, 'a,x,b,y' results in a value of 'a,b'.

1.9. Credits

1.9.1. Contributors to MySQL
1.9.2. Documenters and translators
1.9.3. Packages that support MySQL
1.9.4. Tools that were used to create MySQL
1.9.5. Supporters of MySQL

The following sections list developers, contributors, and supporters that have helped to make MySQL what it is today.

1.9.1. Contributors to MySQL

Although Oracle Corporation and/or its affiliates own all copyrights in the MySQL server and the MySQL manual, we wish to recognize those who have made contributions of one kind or another to the MySQL distribution. Contributors are listed here, in somewhat random order:

  • Gianmassimo Vigazzola or

    The initial port to Win32/NT.

  • Per Eric Olsson

    For constructive criticism and real testing of the dynamic record format.

  • Irena Pancirov

    Win32 port with Borland compiler. mysqlshutdown.exe and mysqlwatch.exe.

  • David J. Hughes

    For the effort to make a shareware SQL database. At TcX, the predecessor of MySQL AB, we started with mSQL, but found that it couldn't satisfy our purposes so instead we wrote an SQL interface to our application builder Unireg. mysqladmin and mysql client are programs that were largely influenced by their mSQL counterparts. We have put a lot of effort into making the MySQL syntax a superset of mSQL. Many of the API's ideas are borrowed from mSQL to make it easy to port free mSQL programs to the MySQL API. The MySQL software doesn't contain any code from mSQL. Two files in the distribution (client/insert_test.c and client/select_test.c) are based on the corresponding (noncopyrighted) files in the mSQL distribution, but are modified as examples showing the changes necessary to convert code from mSQL to MySQL Server. (mSQL is copyrighted David J. Hughes.)

  • Patrick Lynch

    For helping us acquire http://www.mysql.com/.

  • Fred Lindberg

    For setting up qmail to handle the MySQL mailing list and for the incredible help we got in managing the MySQL mailing lists.

  • Igor Romanenko

    mysqldump (previously msqldump, but ported and enhanced by Monty).

  • Yuri Dario

    For keeping up and extending the MySQL OS/2 port.

  • Tim Bunce

    Author of mysqlhotcopy.

  • Zarko Mocnik

    Sorting for Slovenian language.

  • "TAMITO"

    The _MB character set macros and the ujis and sjis character sets.

  • Joshua Chamas

    Base for concurrent insert, extended date syntax, debugging on NT, and answering on the MySQL mailing list.

  • Yves Carlier

    mysqlaccess, a program to show the access rights for a user.

  • Rhys Jones (And GWE Technologies Limited)

    For one of the early JDBC drivers.

  • Dr Xiaokun Kelvin ZHU

    Further development of one of the early JDBC drivers and other MySQL-related Java tools.

  • James Cooper

    For setting up a searchable mailing list archive at his site.

  • Rick Mehalick

    For xmysql, a graphical X client for MySQL Server.

  • Doug Sisk

    For providing RPM packages of MySQL for Red Hat Linux.

  • Diemand Alexander V.

    For providing RPM packages of MySQL for Red Hat Linux-Alpha.

  • Antoni Pamies Olive

    For providing RPM versions of a lot of MySQL clients for Intel and SPARC.

  • Jay Bloodworth

    For providing RPM versions for MySQL 3.21.

  • David Sacerdote

    Ideas for secure checking of DNS host names.

  • Wei-Jou Chen

    Some support for Chinese(BIG5) characters.

  • Wei He

    A lot of functionality for the Chinese(GBK) character set.

  • Jan Pazdziora

    Czech sorting order.

  • Zeev Suraski

    FROM_UNIXTIME() time formatting, ENCRYPT() functions, and bison advisor. Active mailing list member.

  • Luuk de Boer

    Ported (and extended) the benchmark suite to DBI/DBD. Have been of great help with crash-me and running benchmarks. Some new date functions. The mysql_setpermission script.

  • Alexis Mikhailov

    User-defined functions (UDFs); CREATE FUNCTION and DROP FUNCTION.

  • Andreas F. Bobak

    The AGGREGATE extension to user-defined functions.

  • Ross Wakelin

    Help to set up InstallShield for MySQL-Win32.

  • Jethro Wright III

    The libmysql.dll library.

  • James Pereria

    Mysqlmanager, a Win32 GUI tool for administering MySQL Servers.

  • Curt Sampson

    Porting of MIT-pthreads to NetBSD/Alpha and NetBSD 1.3/i386.

  • Martin Ramsch

    Examples in the MySQL Tutorial.

  • Steve Harvey

    For making mysqlaccess more secure.

  • Konark IA-64 Centre of Persistent Systems Private Limited

    http://www.pspl.co.in/konark/. Help with the Win64 port of the MySQL server.

  • Albert Chin-A-Young.

    Configure updates for Tru64, large file support and better TCP wrappers support.

  • John Birrell

    Emulation of pthread_mutex() for OS/2.

  • Benjamin Pflugmann

    Extended MERGE tables to handle INSERTS. Active member on the MySQL mailing lists.

  • Jocelyn Fournier

    Excellent spotting and reporting innumerable bugs (especially in the MySQL 4.1 subquery code).

  • Marc Liyanage

    Maintaining the Mac OS X packages and providing invaluable feedback on how to create Mac OS X packages.

  • Robert Rutherford

    Providing invaluable information and feedback about the QNX port.

  • Previous developers of NDB Cluster

    Lots of people were involved in various ways summer students, master thesis students, employees. In total more than 100 people so too many to mention here. Notable name is Ataullah Dabaghi who up until 1999 contributed around a third of the code base. A special thanks also to developers of the AXE system which provided much of the architectural foundations for NDB Cluster with blocks, signals and crash tracing functionality. Also credit should be given to those who believed in the ideas enough to allocate of their budgets for its development from 1992 to present time.

  • Google Inc.

    We wish to recognize Google Inc. for contributions to the MySQL distribution: Mark Callaghan's SMP Performance patches and other patches.

Other contributors, bugfinders, and testers: James H. Thompson, Maurizio Menghini, Wojciech Tryc, Luca Berra, Zarko Mocnik, Wim Bonis, Elmar Haneke, , , , Ted Deppner , Mike Simons, Jaakko Hyvatti.

And lots of bug report/patches from the folks on the mailing list.

A big tribute goes to those that help us answer questions on the MySQL mailing lists:

1.9.2. Documenters and translators

The following people have helped us with writing the MySQL documentation and translating the documentation or error messages in MySQL.

  • Paul DuBois

    Ongoing help with making this manual correct and understandable. That includes rewriting Monty's and David's attempts at English into English as other people know it.

  • Kim Aldale

    Helped to rewrite Monty's and David's early attempts at English into English.

  • Michael J. Miller Jr.

    For the first MySQL manual. And a lot of spelling/language fixes for the FAQ (that turned into the MySQL manual a long time ago).

  • Yan Cailin

    First translator of the MySQL Reference Manual into simplified Chinese in early 2000 on which the Big5 and HK coded (http://mysql.hitstar.com/) versions were based. Personal home page at linuxdb.yeah.net.

  • Jay Flaherty

    Big parts of the Perl DBI/DBD section in the manual.

  • Paul Southworth , Ray Loyzaga

    Proof-reading of the Reference Manual.

  • Therrien Gilbert , Jean-Marc Pouyot

    French error messages.

  • Petr Snajdr,

    Czech error messages.

  • Jaroslaw Lewandowski

    Polish error messages.

  • Miguel Angel Fernandez Roiz

    Spanish error messages.

  • Roy-Magne Mo

    Norwegian error messages and testing of MySQL 3.21.xx.

  • Timur I. Bakeyev

    Russian error messages.

  • & Filippo Grassilli

    Italian error messages.

  • Dirk Munzinger

    German error messages.

  • Billik Stefan

    Slovak error messages.

  • Stefan Saroiu

    Romanian error messages.

  • Peter Feher

    Hungarian error messages.

  • Roberto M. Serqueira

    Portuguese error messages.

  • Carsten H. Pedersen

    Danish error messages.

  • Arjen Lentz

    Dutch error messages, completing earlier partial translation (also work on consistency and spelling).

1.9.3. Packages that support MySQL

The following is a list of creators/maintainers of some of the most important API/packages/applications that a lot of people use with MySQL.

We cannot list every possible package here because the list would then be way to hard to maintain. For other packages, please refer to the software portal at http://solutions.mysql.com/software/.

  • Tim Bunce, Alligator Descartes

    For the DBD (Perl) interface.

  • Andreas Koenig

    For the Perl interface for MySQL Server.

  • Jochen Wiedmann

    For maintaining the Perl DBD::mysql module.

  • Eugene Chan

    For porting PHP for MySQL Server.

  • Georg Richter

    MySQL 4.1 testing and bug hunting. New PHP 5.0 mysqli extension (API) for use with MySQL 4.1 and up.

  • Giovanni Maruzzelli

    For porting iODBC (Unix ODBC).

  • Xavier Leroy

    The author of LinuxThreads (used by the MySQL Server on Linux).

1.9.4. Tools that were used to create MySQL

The following is a list of some of the tools we have used to create MySQL. We use this to express our thanks to those that has created them as without these we could not have made MySQL what it is today.

  • Free Software Foundation

    From whom we got an excellent compiler (gcc), an excellent debugger (gdb and the libc library (from which we have borrowed strto.c to get some code working in Linux).

  • Free Software Foundation & The XEmacs development team

    For a really great editor/environment.

  • Julian Seward

    Author of valgrind, an excellent memory checker tool that has helped us find a lot of otherwise hard to find bugs in MySQL.

  • Dorothea Lütkehaus and Andreas Zeller

    For DDD (The Data Display Debugger) which is an excellent graphical front end to gdb).

1.9.5. Supporters of MySQL

Although Oracle Corporation and/or its affiliates own all copyrights in the MySQL server and the MySQL manual, we wish to recognize the following companies, which helped us finance the development of the MySQL server, such as by paying us for developing a new feature or giving us hardware for development of the MySQL server.

  • VA Linux / Andover.net

    Funded replication.

  • NuSphere

    Editing of the MySQL manual.

  • Stork Design studio

    The MySQL Web site in use between 1998-2000.

  • Intel

    Contributed to development on Windows and Linux platforms.

  • Compaq

    Contributed to Development on Linux/Alpha.

  • SWSoft

    Development on the embedded mysqld version.

  • FutureQuest

    The --skip-show-database option.

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