Appendix B. MySQL 5.6 Frequently Asked Questions

Questions

Questions and Answers

B.1.1: Which version of MySQL is production-ready (GA)?

At the time of writing this (December 2011), MySQL 5.5, MySQL 5.1, and MySQL 5.0 are supported for production use.

MySQL 5.5 achieved General Availability (GA) status with MySQL 5.5.8, which was released for production use on 3 December 2010.

MySQL 5.1 achieved General Availability (GA) status with MySQL 5.1.30, which was released for production use on 14 November 2008.

MySQL 5.0 achieved General Availability (GA) status with MySQL 5.0.15, which was released for production use on 19 October 2005. Note that active development for MySQL 5.0 has ended.

B.1.2: What is the state of development (non-GA) versions?

MySQL follows a milestone release model that introduces pre-production-quality features and stabilizes them to release quality (see http://dev.mysql.com/doc/mysql-development-cycle/en/index.html). This process then repeats, so releases cycle between pre-production and release quality status. Please check the change logs to identify the status of a given release.

MySQL 5.4 was a development series. Work on this series has ceased.

MySQL 5.5 is being actively developed using the milestone release methodology described above.

MySQL 5.6 is being actively developed using the milestone release methodology described above.

MySQL 5.7 is being actively developed using the milestone release methodology described above.

MySQL 6.0 was a development series. Work on this series has ceased.

B.1.3: Can MySQL 5.6 do subqueries?

Yes. See Section 13.2.10, “Subquery Syntax”.

B.1.4: Can MySQL 5.6 perform multiple-table inserts, updates, and deletes?

Yes. For the syntax required to perform multiple-table updates, see Section 13.2.11, “UPDATE Syntax”; for that required to perform multiple-table deletes, see Section 13.2.2, “DELETE Syntax”.

A multiple-table insert can be accomplished using a trigger whose FOR EACH ROW clause contains multiple INSERT statements within a BEGIN ... END block. See Section 18.3, “Using Triggers”.

B.1.5: Does MySQL 5.6 have a Query Cache? Does it work on Server, Instance or Database?

Yes. The query cache operates on the server level, caching complete result sets matched with the original query string. If an exactly identical query is made (which often happens, particularly in web applications), no parsing or execution is necessary; the result is sent directly from the cache. Various tuning options are available. See Section 8.9.3, “The MySQL Query Cache”.

B.1.6: Does MySQL 5.6 have Sequences?

No. However, MySQL has an AUTO_INCREMENT system, which in MySQL 5.6 can also handle inserts in a multi-master replication setup. With the auto_increment_increment and auto_increment_offset system variables, you can set each server to generate auto-increment values that don't conflict with other servers. The auto_increment_increment value should be greater than the number of servers, and each server should have a unique offset.

B.1.7: Does MySQL 5.6 have a NOW() function with fractions of seconds?

No. This is on the MySQL roadmap as a “rolling feature”. This means that it is not a flagship feature, but will be implemented, development time permitting. Specific customer demand may change this scheduling.

However, MySQL does parse time strings with a fractional component. See Section 11.3.2, “The TIME Type”.

B.1.8: Does MySQL 5.6 work with multi-core processors?

Yes. MySQL is fully multi-threaded, and will make use of multiple CPUs, provided that the operating system supports them.

B.1.9: Why do I see multiple processes for mysqld?

When using LinuxThreads, you should see a minimum of three mysqld processes running. These are in fact threads. There is one thread for the LinuxThreads manager, one thread to handle connections, and one thread to handle alarms and signals.

B.1.10: Have there been there any improvements in error reporting when foreign keys fail? Does MySQL now report which column and reference failed?

The foreign key support in InnoDB has seen improvements in each major version of MySQL. Foreign key support generic to all storage engines is scheduled for MySQL 6.x; this should resolve any inadequacies in the current storage engine specific implementation.

B.1.11: Can MySQL 5.6 perform ACID transactions?

Yes. All current MySQL versions support transactions. The InnoDB storage engine offers full ACID transactions with row-level locking, multi-versioning, nonlocking repeatable reads, and all four SQL standard isolation levels.

The NDB storage engine supports the READ COMMITTED transaction isolation level only.

Questions

Questions and Answers

B.2.1: Where can I obtain complete documentation for MySQL storage engines?

See Chapter 14, Storage Engines. That chapter contains information about all MySQL storage engines except for the NDB storage engine used for MySQL Cluster; NDB is covered in MySQL Cluster NDB 7.2.

B.2.2: Are there any new storage engines in MySQL 5.6?

The features from the optional InnoDB Plugin from MySQL 5.1 are folded into the built-in InnoDB storage engine, so you can take advantage of features such as the Barracuda file format, InnoDB table compression, and the new configuration options for performance. See Section 14.2, “The InnoDB Storage Engine” for details. InnoDB also becomes the default storage engine for new tables. See Section 14.2.1.1, “InnoDB as the Default MySQL Storage Engine” for details.

B.2.3: Have any storage engines been removed in MySQL 5.6?

No.

B.2.4: What are the unique benefits of the ARCHIVE storage engine?

The ARCHIVE storage engine is ideally suited for storing large amounts of data without indexes; it has a very small footprint, and performs selects using table scans. See Section 14.6, “The ARCHIVE Storage Engine”, for details.

B.2.5: Do the new features in MySQL 5.6 apply to all storage engines?

The general new features such as views, stored procedures, triggers, INFORMATION_SCHEMA, precision math (DECIMAL column type), and the BIT column type, apply to all storage engines. There are also additions and changes for specific storage engines.

Questions

Questions and Answers

B.3.1: What are server SQL modes?

Server SQL modes define 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. The MySQL Server apply these modes individually to different clients. For more information, see Section 5.1.7, “Server SQL Modes”.

B.3.2: How many server SQL modes are there?

Each mode can be independently switched on and off. See Section 5.1.7, “Server SQL Modes”, for a complete list of available modes.

B.3.3: How do you determine the server SQL mode?

You can set the default SQL mode (for mysqld startup) with the --sql-mode option. Using the statement SET [GLOBAL|SESSION] sql_mode='modes', you can change the settings from within a connection, either locally to the connection, or to take effect globally. You can retrieve the current mode by issuing a SELECT @@sql_mode statement.

B.3.4: Is the mode dependent on the database or connection?

A mode is not linked to a particular database. Modes can be set locally to the session (connection), or globally for the server. you can change these settings using SET [GLOBAL|SESSION] sql_mode='modes'.

B.3.5: Can the rules for strict mode be extended?

When we refer to strict mode, we mean a mode where at least one of the modes TRADITIONAL, STRICT_TRANS_TABLES, or STRICT_ALL_TABLES is enabled. Options can be combined, so you can add restrictions to a mode. See Section 5.1.7, “Server SQL Modes”, for more information.

B.3.6: Does strict mode impact performance?

The intensive validation of input data that some settings requires more time than if the validation is not done. While the performance impact is not that great, if you do not require such validation (perhaps your application already handles all of this), then MySQL gives you the option of leaving strict mode disabled. However—if you do require it—strict mode can provide such validation.

B.3.7: What is the default server SQL mode when My SQL 5.6 is installed?

By default, no special modes are enabled. See Section 5.1.7, “Server SQL Modes”, for information about all available modes and MySQL's default behavior.

Questions

Questions and Answers

B.4.1: Does MySQL 5.6 support stored procedures and functions?

Yes. MySQL 5.6 supports two types of stored routines—stored procedures and stored functions.

B.4.2: Where can I find documentation for MySQL stored procedures and stored functions?

See Section 18.2, “Using Stored Routines (Procedures and Functions)”.

B.4.3: Is there a discussion forum for MySQL stored procedures?

Yes. See http://forums.mysql.com/list.php?98.

B.4.4: Where can I find the ANSI SQL 2003 specification for stored procedures?

Unfortunately, the official specifications are not freely available (ANSI makes them available for purchase). However, there are books—such as SQL-99 Complete, Really by Peter Gulutzan and Trudy Pelzer—which give a comprehensive overview of the standard, including coverage of stored procedures.

B.4.5: How do you manage stored routines?

It is always good practice to use a clear naming scheme for your stored routines. You can manage stored procedures with CREATE [FUNCTION|PROCEDURE], ALTER [FUNCTION|PROCEDURE], DROP [FUNCTION|PROCEDURE], and SHOW CREATE [FUNCTION|PROCEDURE]. You can obtain information about existing stored procedures using the ROUTINES table in the INFORMATION_SCHEMA database (see Section 19.19, “The INFORMATION_SCHEMA ROUTINES Table”).

B.4.6: Is there a way to view all stored procedures and stored functions in a given database?

Yes. For a database named dbname, use this query on the INFORMATION_SCHEMA.ROUTINES table:

SELECT ROUTINE_TYPE, ROUTINE_NAME
    FROM INFORMATION_SCHEMA.ROUTINES
    WHERE ROUTINE_SCHEMA='dbname';

For more information, see Section 19.19, “The INFORMATION_SCHEMA ROUTINES Table”.

The body of a stored routine can be viewed using SHOW CREATE FUNCTION (for a stored function) or SHOW CREATE PROCEDURE (for a stored procedure). See Section 13.7.5.11, “SHOW CREATE PROCEDURE Syntax”, for more information.

B.4.7: Where are stored procedures stored?

In the proc table of the mysql system database. However, you should not access the tables in the system database directly. Instead, use SHOW CREATE FUNCTION to obtain information about stored functions, and SHOW CREATE PROCEDURE to obtain information about stored procedures. See Section 13.7.5.11, “SHOW CREATE PROCEDURE Syntax”, for more information about these statements.

You can also query the ROUTINES table in the INFORMATION_SCHEMA database—see Section 19.19, “The INFORMATION_SCHEMA ROUTINES Table”, for information about this table.

B.4.8: Is it possible to group stored procedures or stored functions into packages?

No. This is not supported in MySQL 5.6.

B.4.9: Can a stored procedure call another stored procedure?

Yes.

B.4.10: Can a stored procedure call a trigger?

A stored procedure can execute an SQL statement, such as an UPDATE, that causes a trigger to activate.

B.4.11: Can a stored procedure access tables?

Yes. A stored procedure can access one or more tables as required.

B.4.12: Do stored procedures have a statement for raising application errors?

Yes. MySQL 5.6 implements the SQL standard SIGNAL and RESIGNAL statements. See Section 13.6.7, “Condition Handling”.

B.4.13: Do stored procedures provide exception handling?

MySQL implements HANDLER definitions according to the SQL standard. See Section 13.6.7.2, “DECLARE ... HANDLER Syntax”, for details.

B.4.14: Can MySQL 5.6 stored routines return result sets?

Stored procedures can, but stored functions cannot. If you perform an ordinary SELECT inside a stored procedure, the result set is returned directly to the client. You need to use the MySQL 4.1 (or above) client/server protocol for this to work. This means that—for instance—in PHP, you need to use the mysqli extension rather than the old mysql extension.

B.4.15: Is WITH RECOMPILE supported for stored procedures?

Not in MySQL 5.6.

B.4.16: Is there a MySQL equivalent to using mod_plsql as a gateway on Apache to talk directly to a stored procedure in the database?

There is no equivalent in MySQL 5.6.

B.4.17: Can I pass an array as input to a stored procedure?

Not in MySQL 5.6.

B.4.18: Can I pass a cursor as an IN parameter to a stored procedure?

In MySQL 5.6, cursors are available inside stored procedures only.

B.4.19: Can I return a cursor as an OUT parameter from a stored procedure?

In MySQL 5.6, cursors are available inside stored procedures only. However, if you do not open a cursor on a SELECT, the result will be sent directly to the client. You can also SELECT INTO variables. See Section 13.2.9, “SELECT Syntax”.

B.4.20: Can I print out a variable's value within a stored routine for debugging purposes?

Yes, you can do this in a stored procedure, but not in a stored function. If you perform an ordinary SELECT inside a stored procedure, the result set is returned directly to the client. You will need to use the MySQL 4.1 (or above) client/server protocol for this to work. This means that—for instance—in PHP, you need to use the mysqli extension rather than the old mysql extension.

B.4.21: Can I commit or roll back transactions inside a stored procedure?

Yes. However, you cannot perform transactional operations within a stored function.

B.4.22: Do MySQL 5.6 stored procedures and functions work with replication?

Yes, standard actions carried out in stored procedures and functions are replicated from a master MySQL server to a slave server. There are a few limitations that are described in detail in Section 18.7, “Binary Logging of Stored Programs”.

B.4.23: Are stored procedures and functions created on a master server replicated to a slave?

Yes, creation of stored procedures and functions carried out through normal DDL statements on a master server are replicated to a slave, so the objects will exist on both servers. ALTER and DROP statements for stored procedures and functions are also replicated.

B.4.24: How are actions that take place inside stored procedures and functions replicated?

MySQL records each DML event that occurs in a stored procedure and replicates those individual actions to a slave server. The actual calls made to execute stored procedures are not replicated.

Stored functions that change data are logged as function invocations, not as the DML events that occur inside each function.

B.4.25: Are there special security requirements for using stored procedures and functions together with replication?

Yes. Because a slave server has authority to execute any statement read from a master's binary log, special security constraints exist for using stored functions with replication. If replication or binary logging in general (for the purpose of point-in-time recovery) is active, then MySQL DBAs have two security options open to them:

B.4.26: What limitations exist for replicating stored procedure and function actions?

Nondeterministic (random) or time-based actions embedded in stored procedures may not replicate properly. By their very nature, randomly produced results are not predictable and cannot be exactly reproduced, and therefore, random actions replicated to a slave will not mirror those performed on a master. Note that declaring stored functions to be DETERMINISTIC or setting the log_bin_trust_function_creators system variable to 0 will not allow random-valued operations to be invoked.

In addition, time-based actions cannot be reproduced on a slave because the timing of such actions in a stored procedure is not reproducible through the binary log used for replication. It records only DML events and does not factor in timing constraints.

Finally, nontransactional tables for which errors occur during large DML actions (such as bulk inserts) may experience replication issues in that a master may be partially updated from DML activity, but no updates are done to the slave because of the errors that occurred. A workaround is for a function's DML actions to be carried out with the IGNORE keyword so that updates on the master that cause errors are ignored and updates that do not cause errors are replicated to the slave.

B.4.27: Do the preceding limitations affect MySQL's ability to do point-in-time recovery?

The same limitations that affect replication do affect point-in-time recovery.

B.4.28: What is being done to correct the aforementioned limitations?

You can choose either statement-based replication or row-based replication. The original replication implementation is based on statement-based binary logging. Row-based binary logging resolves the limitations mentioned earlier.

Mixed replication is also available (by starting the server with --binlog-format=mixed). This hybrid, “smart” form of replication “knows” whether statement-level replication can safely be used, or row-level replication is required.

For additional information, see Section 16.1.2, “Replication Formats”.

Questions

Questions and Answers

B.5.1: Where can I find the documentation for MySQL 5.6 triggers?

See Section 18.3, “Using Triggers”.

B.5.2: Is there a discussion forum for MySQL Triggers?

Yes. It is available at http://forums.mysql.com/list.php?99.

B.5.3: Does MySQL 5.6 have statement-level or row-level triggers?

In MySQL 5.6, all triggers are FOR EACH ROW—that is, the trigger is activated for each row that is inserted, updated, or deleted. MySQL 5.6 does not support triggers using FOR EACH STATEMENT.

B.5.4: Are there any default triggers?

Not explicitly. MySQL does have specific special behavior for some TIMESTAMP columns, as well as for columns which are defined using AUTO_INCREMENT.

B.5.5: How are triggers managed in MySQL?

In MySQL 5.6, triggers can be created using the CREATE TRIGGER statement, and dropped using DROP TRIGGER. See Section 13.1.15, “CREATE TRIGGER Syntax”, and Section 13.1.24, “DROP TRIGGER Syntax”, for more about these statements.

Information about triggers can be obtained by querying the INFORMATION_SCHEMA.TRIGGERS table. See Section 19.27, “The INFORMATION_SCHEMA TRIGGERS Table”.

B.5.6: Is there a way to view all triggers in a given database?

Yes. You can obtain a listing of all triggers defined on database dbname using a query on the INFORMATION_SCHEMA.TRIGGERS table such as the one shown here:

SELECT TRIGGER_NAME, EVENT_MANIPULATION, EVENT_OBJECT_TABLE, ACTION_STATEMENT
    FROM INFORMATION_SCHEMA.TRIGGERS
    WHERE TRIGGER_SCHEMA='dbname';

For more information about this table, see Section 19.27, “The INFORMATION_SCHEMA TRIGGERS Table”.

You can also use the SHOW TRIGGERS statement, which is specific to MySQL. See Section 13.7.5.39, “SHOW TRIGGERS Syntax”.

B.5.7: Where are triggers stored?

Triggers for a table are currently stored in .TRG files, with one such file one per table.

B.5.8: Can a trigger call a stored procedure?

Yes.

B.5.9: Can triggers access tables?

A trigger can access both old and new data in its own table. A trigger can also affect other tables, but it is not permitted to modify a table that is already being used (for reading or writing) by the statement that invoked the function or trigger.

B.5.10: Can triggers call an external application through a UDF?

Yes. For example, a trigger could invoke the sys_exec() UDF.

B.5.11: Is it possible for a trigger to update tables on a remote server?

Yes. A table on a remote server could be updated using the FEDERATED storage engine. (See Section 14.9, “The FEDERATED Storage Engine”).

B.5.12: Do triggers work with replication?

Yes. However, the way in which they work depends whether you are using MySQL's “classic” statement-based replication available in all versions of MySQL, or the row-based replication format introduced in MySQL 5.1.

When using statement-based replication, triggers on the slave are executed by statements that are executed on the master (and replicated to the slave).

When using row-based replication, triggers are not executed on the slave due to statements that were run on the master and then replicated to the slave. Instead, when using row-based replication, the changes caused by executing the trigger on the master are applied on the slave.

For more information, see Section 16.4.1.31, “Replication and Triggers”.

B.5.13: How are actions carried out through triggers on a master replicated to a slave?

Again, this depends on whether you are using statement-based or row-based replication.

Statement-based replication.  First, the triggers that exist on a master must be re-created on the slave server. Once this is done, the replication flow works as any other standard DML statement that participates in replication. For example, consider a table EMP that has an AFTER insert trigger, which exists on a master MySQL server. The same EMP table and AFTER insert trigger exist on the slave server as well. The replication flow would be:

Row-based replication.  When you use row-based replication, the changes caused by executing the trigger on the master are applied on the slave. However, the triggers themselves are not actually executed on the slave under row-based replication. This is because, if both the master and the slave applied the changes from the master and—in addition—the trigger causing these changes were applied on the slave, the changes would in effect be applied twice on the slave, leading to different data on the master and the slave.

In most cases, the outcome is the same for both row-based and statement-based replication. However, if you use different triggers on the master and slave, you cannot use row-based replication. (This is because the row-based format replicates the changes made by triggers executing on the master to the slaves, rather than the statements that caused the triggers to execute, and the corresponding triggers on the slave are not executed.) Instead, any statements causing such triggers to be executed must be replicated using statement-based replication.

For more information, see Section 16.4.1.31, “Replication and Triggers”.

Questions

Questions and Answers

B.6.1: Where can I find documentation covering MySQL Views?

See Section 18.5, “Using Views”.

B.6.2: Is there a discussion forum for MySQL Views?

Yes. See http://forums.mysql.com/list.php?100

B.6.3: What happens to a view if an underlying table is dropped or renamed?

After a view has been created, it is possible to drop or alter a table or view to which the definition refers. To check a view definition for problems of this kind, use the CHECK TABLE statement. (See Section 13.7.2.2, “CHECK TABLE Syntax”.)

B.6.4: Does MySQL 5.6 have table snapshots?

No.

B.6.5: Does MySQL 5.6 have materialized views?

No.

B.6.6: Can you insert into views that are based on joins?

It is possible, provided that your INSERT statement has a column list that makes it clear there is only one table involved.

You cannot insert into multiple tables with a single insert on a view.

Questions

Questions and Answers

B.7.1: Where can I find documentation for the MySQL INFORMATION_SCHEMA database?

See Chapter 19, INFORMATION_SCHEMA Tables

B.7.2: Is there a discussion forum for INFORMATION_SCHEMA?

See http://forums.mysql.com/list.php?101.

B.7.3: Where can I find the ANSI SQL 2003 specification for INFORMATION_SCHEMA?

Unfortunately, the official specifications are not freely available. (ANSI makes them available for purchase.) However, there are books available—such as SQL-99 Complete, Really by Peter Gulutzan and Trudy Pelzer—which give a comprehensive overview of the standard, including INFORMATION_SCHEMA.

B.7.4: What is the difference between the Oracle Data Dictionary and MySQL's INFORMATION_SCHEMA?

Both Oracle and MySQL provide metadata in tables. However, Oracle and MySQL use different table names and column names. MySQL's implementation is more similar to those found in DB2 and SQL Server, which also support INFORMATION_SCHEMA as defined in the SQL standard.

B.7.5: Can I add to or otherwise modify the tables found in the INFORMATION_SCHEMA database?

No. Since applications may rely on a certain standard structure, this should not be modified. For this reason, we cannot support bugs or other issues which result from modifying INFORMATION_SCHEMA tables or data.

Questions

Questions and Answers

B.8.1: Where can I find information on how to migrate from MySQL 5.5 to MySQL 5.6?

For detailed upgrade information, see Section 2.11.1, “Upgrading MySQL”. Do not skip a major version when upgrading, but rather complete the process in steps, upgrading from one major version to the next in each step. This may seem more complicated, but it will you save time and trouble—if you encounter problems during the upgrade, their origin will be easier to identify, either by you or—if you have a MySQL Enterprise subscription—by MySQL support.

B.8.2: How has storage engine (table type) support changed in MySQL 5.6 from previous versions?

Storage engine support has changed as follows:

Questions

Questions and Answers

B.9.1: Where can I find documentation that addresses security issues for MySQL?

The best place to start is Chapter 6, Security.

Other portions of the MySQL Documentation which you may find useful with regard to specific security concerns include the following:

B.9.2: Does MySQL 5.6 have native support for SSL?

Most 5.6 binaries have support for SSL connections between the client and server. See Section 6.3.8, “Using SSL for Secure Connections”.

You can also tunnel a connection using SSH, if (for example) the client application does not support SSL connections. For an example, see Section 6.3.9, “Connecting to MySQL Remotely from Windows with SSH”.

B.9.3: Is SSL support be built into MySQL binaries, or must I recompile the binary myself to enable it?

Most 5.6 binaries have SSL enabled for client-server connections that are secured, authenticated, or both. See Section 6.3.8, “Using SSL for Secure Connections”.

B.9.4: Does MySQL 5.6 have built-in authentication against LDAP directories?

Not at this time.

B.9.5: Does MySQL 5.6 include support for Roles Based Access Control (RBAC)?

Not at this time.

In the following section, we answer questions that are frequently asked about MySQL Cluster and the NDBCLUSTER storage engine.

Questions

Questions and Answers

B.10.1: Which versions of the MySQL software support Cluster? Do I have to compile from source?

MySQL Cluster is not supported in MySQL Server 5.6 releases. Instead, MySQL Cluster is released as a separate product, available as MySQL Cluster NDB 7.1 and MySQL Cluster NDB 7.2. You should use MySQL Cluster NDB 7.2 for new deployments, and plan to upgrade if you are using a previous version of MySQL Cluster. For an overview of improvements made in MySQL Cluster NDB 7.1, see MySQL Cluster Development in MySQL Cluster NDB 7.1; for information about improvements made in MySQL Cluster NDB 7.2, see MySQL Cluster Development in MySQL Cluster NDB 7.2.

For detailed information about deploying and using MySQL Cluster, see MySQL Cluster NDB 6.1 - 7.1 and MySQL Cluster NDB 7.2.

This set of Frequently Asked Questions derives from the experience of MySQL's Support and Development groups in handling many inquiries about CJK (Chinese-Japanese-Korean) issues.

Questions

Questions and Answers

B.11.1: What CJK character sets are available in MySQL?

The list of CJK character sets may vary depending on your MySQL version. For example, the eucjpms character set was not supported prior to MySQL 5.0.3. However, since the name of the applicable language appears in the DESCRIPTION column for every entry in the INFORMATION_SCHEMA.CHARACTER_SETS table, you can obtain a current list of all the non-Unicode CJK character sets using this query:

mysql> SELECT CHARACTER_SET_NAME, DESCRIPTION
    -> FROM INFORMATION_SCHEMA.CHARACTER_SETS
    -> WHERE DESCRIPTION LIKE '%Chinese%'
    -> OR DESCRIPTION LIKE '%Japanese%'
    -> OR DESCRIPTION LIKE '%Korean%'
    -> ORDER BY CHARACTER_SET_NAME;
+--------------------+---------------------------+
| CHARACTER_SET_NAME | DESCRIPTION               |
+--------------------+---------------------------+
| big5               | Big5 Traditional Chinese  |
| cp932              | SJIS for Windows Japanese |
| eucjpms            | UJIS for Windows Japanese |
| euckr              | EUC-KR Korean             |
| gb2312             | GB2312 Simplified Chinese |
| gbk                | GBK Simplified Chinese    |
| sjis               | Shift-JIS Japanese        |
| ujis               | EUC-JP Japanese           |
+--------------------+---------------------------+
8 rows in set (0.01 sec)

(See Section 19.1, “The INFORMATION_SCHEMA CHARACTER_SETS Table”, for more information.)

MySQL supports the two common variants of the GB (Guojia Biaozhun, or National Standard, or Simplified Chinese) character sets which are official in the People's Republic of China: gb2312 and gbk. Sometimes people try to insert gbk characters into gb2312, and it works most of the time because gbk is a superset of gb2312—but eventually they try to insert a rarer Chinese character and it doesn't work. (See Bug #16072 for an example).

Here, we try to clarify exactly what characters are legitimate in gb2312 or gbk, with reference to the official documents. Please check these references before reporting gb2312 or gbk bugs.

B.11.2: I have inserted CJK characters into my table. Why does SELECT display them as “?” characters?

This problem is usually due to a setting in MySQL that doesn't match the settings for the application program or the operating system. Here are some common steps for correcting these types of issues:

B.11.3: What problems should I be aware of when working with the Big5 Chinese character set?

MySQL supports the Big5 character set which is common in Hong Kong and Taiwan (Republic of China). MySQL's big5 is in reality Microsoft code page 950, which is very similar to the original big5 character set. We changed to this character set starting with MySQL version 4.1.16 / 5.0.16 (as a result of Bug #12476). For example, the following statements work in current versions of MySQL, but not in old versions:

mysql> CREATE TABLE big5 (BIG5 CHAR(1) CHARACTER SET BIG5);
Query OK, 0 rows affected (0.13 sec)

mysql> INSERT INTO big5 VALUES (0xf9dc);
Query OK, 1 row affected (0.00 sec)

mysql> SELECT * FROM big5;
+------+
| big5 |
+------+
| 嫺  |
+------+
1 row in set (0.02 sec)

A feature request for adding HKSCS extensions has been filed. People who need this extension may find the suggested patch for Bug #13577 to be of interest.

B.11.4: Why do Japanese character set conversions fail?

MySQL supports the sjis, ujis, cp932, and eucjpms character sets, as well as Unicode. A common need is to convert between character sets. For example, there might be a Unix server (typically with sjis or ujis) and a Windows client (typically with cp932).

In the following conversion table, the ucs2 column represents the source, and the sjis, cp932, ujis, and eucjpms columns represent the destinations—that is, the last 4 columns provide the hexadecimal result when we use CONVERT(ucs2) or we assign a ucs2 column containing the value to an sjis, cp932, ujis, or eucjpms column.

Now consider the following portion of the table.

This means that MySQL converts the NOT SIGN (Unicode U+00AC) to sjis code point 0x81CA and to cp932 code point 3F. (3F is the question mark (“?”)—this is what is always used when the conversion cannot be performed.

B.11.5: What should I do if I want to convert SJIS 81CA to cp932?

Our answer is: “?”. There are serious complaints about this: many people would prefer a “loose” conversion, so that 81CA (NOT SIGN) in sjis becomes 81CA (FULLWIDTH NOT SIGN) in cp932. We are considering a change to this behavior.

B.11.6: How does MySQL represent the Yen (¥) sign?

A problem arises because some versions of Japanese character sets (both sjis and euc) treat 5C as a reverse solidus (\—also known as a backslash), and others treat it as a yen sign (¥).

MySQL follows only one version of the JIS (Japanese Industrial Standards) standard description. In MySQL, 5C is always the reverse solidus (\).

B.11.7: Does MySQL plan to make a separate character set where 5C is the Yen sign, as at least one other major DBMS does?

This is one possible solution to the Yen sign issue; however, this will not happen in MySQL 5.1 or 6.0.

B.11.8: Of what issues should I be aware when working with Korean character sets in MySQL?

In theory, while there have been several versions of the euckr (Extended Unix Code Korea) character set, only one problem has been noted.

We use the “ASCII” variant of EUC-KR, in which the code point 0x5c is REVERSE SOLIDUS, that is \, instead of the “KS-Roman” variant of EUC-KR, in which the code point 0x5c is WON SIGN(₩). This means that you cannot convert Unicode U+20A9 to euckr:

mysql> SELECT
    ->     CONVERT('₩' USING euckr) AS euckr,
    ->     HEX(CONVERT('₩' USING euckr)) AS hexeuckr;
+-------+----------+
| euckr | hexeuckr |
+-------+----------+
| ?     | 3F       |
+-------+----------+
1 row in set (0.00 sec)

MySQL's graphic Korean chart is here: euckr.

B.11.9: Why do I get Incorrect string value error messages?

For illustration, we'll create a table with one Unicode (ucs2) column and one Chinese (gb2312) column.

mysql> CREATE TABLE ch
    -> (ucs2 CHAR(3) CHARACTER SET ucs2,
    -> gb2312 CHAR(3) CHARACTER SET gb2312);
Query OK, 0 rows affected (0.05 sec)

We'll try to place the rare character 汌 in both columns.

mysql> INSERT INTO ch VALUES ('A汌B','A汌B');
Query OK, 1 row affected, 1 warning (0.00 sec)

Ah, there is a warning. Use the following statement to see what it is:

mysql> SHOW WARNINGS\G
*************************** 1. row ***************************
  Level: Warning
   Code: 1366
Message: Incorrect string value: '\xE6\xB1\x8CB' for column 'gb2312' at row 1
1 row in set (0.00 sec)

So it is a warning about the gb2312 column only.

mysql> SELECT ucs2,HEX(ucs2),gb2312,HEX(gb2312) FROM ch;
+-------+--------------+--------+-------------+
| ucs2  | HEX(ucs2)    | gb2312 | HEX(gb2312) |
+-------+--------------+--------+-------------+
| A汌B | 00416C4C0042 | A?B    | 413F42      |
+-------+--------------+--------+-------------+
1 row in set (0.00 sec)

Several things need explanation here:

B.11.10: Why does my GUI front end or browser not display CJK characters correctly in my application using Access, PHP, or another API?

Obtain a direct connection to the server using the mysql client (Windows: mysql.exe), and try the same query there. If mysql responds correctly, then the trouble may be that your application interface requires initialization. Use mysql to tell you what character set or sets it uses with the statement SHOW VARIABLES LIKE 'char%';. If you are using Access, then you are most likely connecting with Connector/ODBC. In this case, you should check Section 21.1.4, “Configuring Connector/ODBC”. If, for instance, you use big5, you would enter SET NAMES 'big5'. (Note that no ; is required in this case). If you are using ASP, you might need to add SET NAMES in the code. Here is an example that has worked in the past:

<%
Session.CodePage=0
Dim strConnection
Dim Conn
strConnection="driver={MySQL ODBC 3.51 Driver};server=server;uid=username;" \
               & "pwd=password;database=database;stmt=SET NAMES 'big5';"
Set Conn = Server.CreateObject("ADODB.Connection")
Conn.Open strConnection
%>

In much the same way, if you are using any character set other than latin1 with Connector/Net, then you must specify the character set in the connection string. See Section 21.2.5.1, “Connecting to MySQL Using Connector/Net”, for more information.

If you are using PHP, try this:

<?php
  $link = mysql_connect($host, $usr, $pwd);

  mysql_select_db($db);

  if( mysql_error() ) { print "Database ERROR: " . mysql_error(); }
  mysql_query("SET NAMES 'utf8'", $link);
?>

In this case, we used SET NAMES to change character_set_client and character_set_connection and character_set_results.

We encourage the use of the newer mysqli extension, rather than mysql. Using mysqli, the previous example could be rewritten as shown here:

<?php
  $link = new mysqli($host, $usr, $pwd, $db);

  if( mysqli_connect_errno() )
  {
    printf("Connect failed: %s\n", mysqli_connect_error());
    exit();
  }

  $link->query("SET NAMES 'utf8'");
?>

Another issue often encountered in PHP applications has to do with assumptions made by the browser. Sometimes adding or changing a <meta> tag suffices to correct the problem: for example, to insure that the user agent interprets page content as UTF-8, you should include <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> in the <head> of the HTML page.

If you are using Connector/J, see Section 21.3.5.4, “Using Character Sets and Unicode”.

B.11.11: I've upgraded to MySQL 5.6. How can I revert to behavior like that in MySQL 4.0 with regard to character sets?

In MySQL Version 4.0, there was a single “global” character set for both server and client, and the decision as to which character to use was made by the server administrator. This changed starting with MySQL Version 4.1. What happens now is a “handshake”, as described in Section 10.1.4, “Connection Character Sets and Collations”:

The effect of this is that you cannot control the client character set by starting mysqld with --character-set-server=utf8. However, some of our Asian customers have said that they prefer the MySQL 4.0 behavior. To make it possible to retain this behavior, we added a mysqld switch, --character-set-client-handshake, which can be turned off with --skip-character-set-client-handshake. If you start mysqld with --skip-character-set-client-handshake, then, when a client connects, it sends to the server the name of the character set that it wants to use—however, the server ignores this request from the client.

By way of example, suppose that your favorite server character set is latin1 (unlikely in a CJK area, but this is the default value). Suppose further that the client uses utf8 because this is what the client's operating system supports. Now, start the server with latin1 as its default character set:

mysqld --character-set-server=latin1

And then start the client with the default character set utf8:

mysql --default-character-set=utf8

The current settings can be seen by viewing the output of SHOW VARIABLES:

mysql> SHOW VARIABLES LIKE 'char%';
+--------------------------+----------------------------------------+
| Variable_name            | Value                                  |
+--------------------------+----------------------------------------+
| character_set_client     | utf8                                   |
| character_set_connection | utf8                                   |
| character_set_database   | latin1                                 |
| character_set_filesystem | binary                                 |
| character_set_results    | utf8                                   |
| character_set_server     | latin1                                 |
| character_set_system     | utf8                                   |
| character_sets_dir       | /usr/local/mysql/share/mysql/charsets/ |
+--------------------------+----------------------------------------+
8 rows in set (0.01 sec)

Now stop the client, and then stop the server using mysqladmin. Then start the server again, but this time tell it to skip the handshake like so:

mysqld --character-set-server=utf8 --skip-character-set-client-handshake

Start the client with utf8 once again as the default character set, then display the current settings:

mysql> SHOW VARIABLES LIKE 'char%';
+--------------------------+----------------------------------------+
| Variable_name            | Value                                  |
+--------------------------+----------------------------------------+
| character_set_client     | latin1                                 |
| character_set_connection | latin1                                 |
| character_set_database   | latin1                                 |
| character_set_filesystem | binary                                 |
| character_set_results    | latin1                                 |
| character_set_server     | latin1                                 |
| character_set_system     | utf8                                   |
| character_sets_dir       | /usr/local/mysql/share/mysql/charsets/ |
+--------------------------+----------------------------------------+
8 rows in set (0.01 sec)

As you can see by comparing the differing results from SHOW VARIABLES, the server ignores the client's initial settings if the --skip-character-set-client-handshake is used.

B.11.12: Why do some LIKE and FULLTEXT searches with CJK characters fail?

There is a very simple problem with LIKE searches on BINARY and BLOB columns: we need to know the end of a character. With multi-byte character sets, different characters might have different octet lengths. For example, in utf8, A requires one byte but ペ requires three bytes, as shown here:

+-------------------------+---------------------------+
| OCTET_LENGTH(_utf8 'A') | OCTET_LENGTH(_utf8 'ペ') |
+-------------------------+---------------------------+
|                       1 |                         3 |
+-------------------------+---------------------------+
1 row in set (0.00 sec)

If we don't know where the first character ends, then we don't know where the second character begins, in which case even very simple searches such as LIKE '_A%' fail. The solution is to use a regular CJK character set in the first place, or to convert to a CJK character set before comparing.

This is one reason why MySQL cannot allow encodings of nonexistent characters. If it is not strict about rejecting bad input, then it has no way of knowing where characters end.

For FULLTEXT searches, we need to know where words begin and end. With Western languages, this is rarely a problem because most (if not all) of these use an easy-to-identify word boundary—the space character. However, this is not usually the case with Asian writing. We could use arbitrary halfway measures, like assuming that all Han characters represent words, or (for Japanese) depending on changes from Katakana to Hiragana due to grammatical endings. However, the only sure solution requires a comprehensive word list, which means that we would have to include a dictionary in the server for each Asian language supported. This is simply not feasible.

B.11.13: How do I know whether character X is available in all character sets?

The majority of simplified Chinese and basic nonhalfwidth Japanese Kana characters appear in all CJK character sets. This stored procedure accepts a UCS-2 Unicode character, converts it to all other character sets, and displays the results in hexadecimal.

DELIMITER //

CREATE PROCEDURE p_convert(ucs2_char CHAR(1) CHARACTER SET ucs2)
BEGIN

CREATE TABLE tj
             (ucs2 CHAR(1) character set ucs2,
              utf8 CHAR(1) character set utf8,
              big5 CHAR(1) character set big5,
              cp932 CHAR(1) character set cp932,
              eucjpms CHAR(1) character set eucjpms,
              euckr CHAR(1) character set euckr,
              gb2312 CHAR(1) character set gb2312,
              gbk CHAR(1) character set gbk,
              sjis CHAR(1) character set sjis,
              ujis CHAR(1) character set ujis);

INSERT INTO tj (ucs2) VALUES (ucs2_char);

UPDATE tj SET utf8=ucs2,
              big5=ucs2,
              cp932=ucs2,
              eucjpms=ucs2,
              euckr=ucs2,
              gb2312=ucs2,
              gbk=ucs2,
              sjis=ucs2,
              ujis=ucs2;

/* If there is a conversion problem, UPDATE will produce a warning. */

SELECT hex(ucs2) AS ucs2,
       hex(utf8) AS utf8,
       hex(big5) AS big5,
       hex(cp932) AS cp932,
       hex(eucjpms) AS eucjpms,
       hex(euckr) AS euckr,
       hex(gb2312) AS gb2312,
       hex(gbk) AS gbk,
       hex(sjis) AS sjis,
       hex(ujis) AS ujis
FROM tj;

DROP TABLE tj;

END//

The input can be any single ucs2 character, or it can be the code point value (hexadecimal representation) of that character. For example, from Unicode's list of ucs2 encodings and names (http://www.unicode.org/Public/UNIDATA/UnicodeData.txt), we know that the Katakana character Pe appears in all CJK character sets, and that its code point value is 0x30da. If we use this value as the argument to p_convert(), the result is as shown here:

mysql> CALL p_convert(0x30da)//
+------+--------+------+-------+---------+-------+--------+------+------+------+
| ucs2 | utf8   | big5 | cp932 | eucjpms | euckr | gb2312 | gbk  | sjis | ujis |
+------+--------+------+-------+---------+-------+--------+------+------+------+
| 30DA | E3839A | C772 | 8379  | A5DA    | ABDA  | A5DA   | A5DA | 8379 | A5DA |
+------+--------+------+-------+---------+-------+--------+------+------+------+
1 row in set (0.04 sec)

Since none of the column values is 3F—that is, the question mark character (?)—we know that every conversion worked.

B.11.14: Why do CJK strings sort incorrectly in Unicode? (I)

Sometimes people observe that the result of a utf8_unicode_ci or ucs2_unicode_ci search, or of an ORDER BY sort is not what they think a native would expect. Although we never rule out the possibility that there is a bug, we have found in the past that many people do not read correctly the standard table of weights for the Unicode Collation Algorithm. MySQL uses the table found at http://www.unicode.org/Public/UCA/4.0.0/allkeys-4.0.0.txt. This is not the first table you will find by navigating from the unicode.org home page, because MySQL uses the older 4.0.0 “allkeys” table, rather than the more recent 4.1.0 table. (The newer '520' collations in MySQL 5.6 use the 5.2 “allkeys” table.) This is because we are very wary about changing ordering which affects indexes, lest we bring about situations such as that reported in Bug #16526, illustrated as follows:

mysql< CREATE TABLE tj (s1 CHAR(1) CHARACTER SET utf8 COLLATE utf8_unicode_ci);
Query OK, 0 rows affected (0.05 sec)

mysql> INSERT INTO tj VALUES ('が'),('か');
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM tj WHERE s1 = 'か';
+------+
| s1   |
+------+
| が  |
| か  |
+------+
2 rows in set (0.00 sec)

The character in the first result row is not the one that we searched for. Why did MySQL retrieve it? First we look for the Unicode code point value, which is possible by reading the hexadecimal number for the ucs2 version of the characters:

mysql> SELECT s1, HEX(CONVERT(s1 USING ucs2)) FROM tj;
+------+-----------------------------+
| s1   | HEX(CONVERT(s1 USING ucs2)) |
+------+-----------------------------+
| が  | 304C                        |
| か  | 304B                        |
+------+-----------------------------+
2 rows in set (0.03 sec)

Now we search for 304B and 304C in the 4.0.0 allkeys table, and find these lines:

304B  ; [.1E57.0020.000E.304B] # HIRAGANA LETTER KA
304C  ; [.1E57.0020.000E.304B][.0000.0140.0002.3099] # HIRAGANA LETTER GA; QQCM

The official Unicode names (following the “#” mark) tell us the Japanese syllabary (Hiragana), the informal classification (letter, digit, or punctuation mark), and the Western identifier (KA or GA, which happen to be voiced and unvoiced components of the same letter pair). More importantly, the primary weight (the first hexadecimal number inside the square brackets) is 1E57 on both lines. For comparisons in both searching and sorting, MySQL pays attention to the primary weight only, ignoring all the other numbers. This means that we are sorting が and か correctly according to the Unicode specification. If we wanted to distinguish them, we'd have to use a non-UCA (Unicode Collation Algorithm) collation (utf8_bin or utf8_general_ci), or to compare the HEX() values, or use ORDER BY CONVERT(s1 USING sjis). Being correct “according to Unicode” isn't enough, of course: the person who submitted the bug was equally correct. We plan to add another collation for Japanese according to the JIS X 4061 standard, in which voiced/unvoiced letter pairs like KA/GA are distinguishable for ordering purposes.

B.11.15: Why do CJK strings sort incorrectly in Unicode? (II)

If you are using Unicode (ucs2 or utf8), and you know what the Unicode sort order is (see Section B.11, “MySQL 5.6 FAQ: MySQL Chinese, Japanese, and Korean Character Sets”), but MySQL still seems to sort your table incorrectly, then you should first verify the table character set:

mysql> SHOW CREATE TABLE t\G
******************** 1. row ******************
Table: t
Create Table: CREATE TABLE `t` (
`s1` char(1) CHARACTER SET ucs2 DEFAULT NULL
) ENGINE=MyISAM DEFAULT CHARSET=latin1
1 row in set (0.00 sec)

Since the character set appears to be correct, let's see what information the INFORMATION_SCHEMA.COLUMNS table can provide about this column:

mysql> SELECT COLUMN_NAME, CHARACTER_SET_NAME, COLLATION_NAME
    -> FROM INFORMATION_SCHEMA.COLUMNS
    -> WHERE COLUMN_NAME = 's1'
    -> AND TABLE_NAME = 't';
+-------------+--------------------+-----------------+
| COLUMN_NAME | CHARACTER_SET_NAME | COLLATION_NAME  |
+-------------+--------------------+-----------------+
| s1          | ucs2               | ucs2_general_ci |
+-------------+--------------------+-----------------+
1 row in set (0.01 sec)

(See Section 19.4, “The INFORMATION_SCHEMA COLUMNS Table”, for more information.)

You can see that the collation is ucs2_general_ci instead of ucs2_unicode_ci. The reason why this is so can be found using SHOW CHARSET, as shown here:

mysql> SHOW CHARSET LIKE 'ucs2%';
+---------+---------------+-------------------+--------+
| Charset | Description   | Default collation | Maxlen |
+---------+---------------+-------------------+--------+
| ucs2    | UCS-2 Unicode | ucs2_general_ci   |      2 |
+---------+---------------+-------------------+--------+
1 row in set (0.00 sec)

For ucs2 and utf8, the default collation is “general”. To specify a Unicode collation, use COLLATE ucs2_unicode_ci.

B.11.16: Why are my supplementary characters rejected by MySQL?

Before MySQL 5.5.3, MySQL does not support supplementary characters—that is, characters which need more than 3 bytes—for UTF-8. We support only what Unicode calls the Basic Multilingual Plane / Plane 0. Only a few very rare Han characters are supplementary; support for them is uncommon. This has led to reports such as that found in Bug #12600, which we rejected as “not a bug”. With utf8, we must truncate an input string when we encounter bytes that we don't understand. Otherwise, we wouldn't know how long the bad multi-byte character is.

One possible workaround is to use ucs2 instead of utf8, in which case the “bad” characters are changed to question marks; however, no truncation takes place. You can also change the data type to BLOB or BINARY, which perform no validity checking.

As of MySQL 5.5.3, Unicode support is extended to include supplementary characters by means of additional Unicode character sets: utf16, utf32, and 4-byte utf8mb4. These character sets support supplementary Unicode characters outside the Basic Multilingual Plane (BMP).

B.11.17: Shouldn't it be “CJKV”?

No. The term “CJKV” (Chinese Japanese Korean Vietnamese) refers to Vietnamese character sets which contain Han (originally Chinese) characters. MySQL has no plan to support the old Vietnamese script using Han characters. MySQL does of course support the modern Vietnamese script with Western characters.

As of MySQL 5.6, there are Vietnamese collations for Unicode character sets, as described in Section 10.1.14.1, “Unicode Character Sets”.

B.11.18: Does MySQL allow CJK characters to be used in database and table names?

This issue is fixed in MySQL 5.1, by automatically rewriting the names of the corresponding directories and files.

For example, if you create a database named 楮 on a server whose operating system does not support CJK in directory names, MySQL creates a directory named @0w@00a5@00ae. which is just a fancy way of encoding E6A5AE—that is, the Unicode hexadecimal representation for the 楮 character. However, if you run a SHOW DATABASES statement, you can see that the database is listed as 楮.

B.11.19: Where can I find translations of the MySQL Manual into Chinese, Japanese, and Korean?

A Simplified Chinese version of the Manual, current for MySQL 5.1.12, can be found at http://dev.mysql.com/doc/. The Japanese translation of the MySQL 4.1 manual can be downloaded from http://dev.mysql.com/doc/.

B.11.20: Where can I get help with CJK and related issues in MySQL?

The following resources are available:

For common questions, issues, and answers relating to the MySQL Connectors and other APIs, see the following areas of the Manual:

In the following section, we provide answers to questions that are most frequently asked about MySQL Replication.

Questions

Questions and Answers

B.13.1: Must the slave be connected to the master all the time?

No, it does not. The slave can go down or stay disconnected for hours or even days, and then reconnect and catch up on updates. For example, you can set up a master/slave relationship over a dial-up link where the link is up only sporadically and for short periods of time. The implication of this is that, at any given time, the slave is not guaranteed to be in synchrony with the master unless you take some special measures.

To ensure that catchup can occur for a slave that has been disconnected, you must not remove binary log files from the master that contain information that has not yet been replicated to the slaves. Asynchronous replication can work only if the slave is able to continue reading the binary log from the point where it last read events.

B.13.2: Must I enable networking on my master and slave to enable replication?

Yes, networking must be enabled on the master and slave. If networking is not enabled, the slave cannot connect to the master and transfer the binary log. Check that the skip-networking option has not been enabled in the configuration file for either server.

B.13.3: How do I know how late a slave is compared to the master? In other words, how do I know the date of the last statement replicated by the slave?

Check the Seconds_Behind_Master column in the output from SHOW SLAVE STATUS. See Section 16.1.5.1, “Checking Replication Status”.

When the slave SQL thread executes an event read from the master, it modifies its own time to the event timestamp. (This is why TIMESTAMP is well replicated.) In the Time column in the output of SHOW PROCESSLIST, the number of seconds displayed for the slave SQL thread is the number of seconds between the timestamp of the last replicated event and the real time of the slave machine. You can use this to determine the date of the last replicated event. Note that if your slave has been disconnected from the master for one hour, and then reconnects, you may immediately see large Time values such as 3600 for the slave SQL thread in SHOW PROCESSLIST. This is because the slave is executing statements that are one hour old. See Section 16.2.1, “Replication Implementation Details”.

B.13.4: How do I force the master to block updates until the slave catches up?

Use the following procedure:

B.13.5: What issues should I be aware of when setting up two-way replication?

MySQL replication currently does not support any locking protocol between master and slave to guarantee the atomicity of a distributed (cross-server) update. In other words, it is possible for client A to make an update to co-master 1, and in the meantime, before it propagates to co-master 2, client B could make an update to co-master 2 that makes the update of client A work differently than it did on co-master 1. Thus, when the update of client A makes it to co-master 2, it produces tables that are different from what you have on co-master 1, even after all the updates from co-master 2 have also propagated. This means that you should not chain two servers together in a two-way replication relationship unless you are sure that your updates can safely happen in any order, or unless you take care of mis-ordered updates somehow in the client code.

You should also realize that two-way replication actually does not improve performance very much (if at all) as far as updates are concerned. Each server must do the same number of updates, just as you would have a single server do. The only difference is that there is a little less lock contention because the updates originating on another server are serialized in one slave thread. Even this benefit might be offset by network delays.

B.13.6: How can I use replication to improve performance of my system?

Set up one server as the master and direct all writes to it. Then configure as many slaves as you have the budget and rackspace for, and distribute the reads among the master and the slaves. You can also start the slaves with the --skip-innodb, --low-priority-updates, and --delay-key-write=ALL options to get speed improvements on the slave end. In this case, the slave uses nontransactional MyISAM tables instead of InnoDB tables to get more speed by eliminating transactional overhead.

B.13.7: What should I do to prepare client code in my own applications to use performance-enhancing replication?

See the guide to using replication as a scale-out solution, Section 16.3.3, “Using Replication for Scale-Out”.

B.13.8: When and how much can MySQL replication improve the performance of my system?

MySQL replication is most beneficial for a system that processes frequent reads and infrequent writes. In theory, by using a single-master/multiple-slave setup, you can scale the system by adding more slaves until you either run out of network bandwidth, or your update load grows to the point that the master cannot handle it.

To determine how many slaves you can use before the added benefits begin to level out, and how much you can improve performance of your site, you must know your query patterns, and determine empirically by benchmarking the relationship between the throughput for reads and writes on a typical master and a typical slave. The example here shows a rather simplified calculation of what you can get with replication for a hypothetical system. Let reads and writes denote the number of reads and writes per second, respectively.

Let's say that system load consists of 10% writes and 90% reads, and we have determined by benchmarking that reads is 1200 - 2 * writes. In other words, the system can do 1,200 reads per second with no writes, the average write is twice as slow as the average read, and the relationship is linear. Suppose that the master and each slave have the same capacity, and that we have one master and N slaves. Then we have for each server (master or slave):

reads = 1200 - 2 * writes

reads = 9 * writes / (N + 1) (reads are split, but writes replicated to all slaves)

9 * writes / (N + 1) + 2 * writes = 1200

writes = 1200 / (2 + 9/(N + 1))

The last equation indicates the maximum number of writes for N slaves, given a maximum possible read rate of 1,200 per second and a ratio of nine reads per write.

This analysis yields the following conclusions:

Note that these computations assume infinite network bandwidth and neglect several other factors that could be significant on your system. In many cases, you may not be able to perform a computation similar to the one just shown that accurately predicts what will happen on your system if you add N replication slaves. However, answering the following questions should help you decide whether and by how much replication will improve the performance of your system:

B.13.9: How can I use replication to provide redundancy or high availability?

How you implement redundancy is entirely dependent on your application and circumstances. High-availability solutions (with automatic failover) require active monitoring and either custom scripts or third party tools to provide the failover support from the original MySQL server to the slave.

To handle the process manually, you should be able to switch from a failed master to a pre-configured slave by altering your application to talk to the new server or by adjusting the DNS for the MySQL server from the failed server to the new server.

For more information and some example solutions, see Section 16.3.6, “Switching Masters During Failover”.

B.13.10: How do I tell whether a master server is using statement-based or row-based binary logging format?

Check the value of the binlog_format system variable:

mysql> SHOW VARIABLES LIKE 'binlog_format';

The value shown will be one of STATEMENT, ROW, or MIXED. For MIXED mode, row-based logging is preferred but replication switches automatically to statement-based logging under certain conditions; for information about when this may occur, see Section 5.2.4.3, “Mixed Binary Logging Format”.

B.13.11: How do I tell a slave to use row-based replication?

Slaves automatically know which format to use.

B.13.12: How do I prevent GRANT and REVOKE statements from replicating to slave machines?

Start the server with the --replicate-wild-ignore-table=mysql.% option to ignore replication for tables in the mysql database.

B.13.13: Does replication work on mixed operating systems (for example, the master runs on Linux while slaves run on Mac OS X and Windows)?

Yes.

B.13.14: Does replication work on mixed hardware architectures (for example, the master runs on a 64-bit machine while slaves run on 32-bit machines)?

Yes.