Appendix A MySQL 8.0 Frequently Asked Questions

MySQL 8.0, 5.7, and MySQL 5.6 are supported for production use.

MySQL 8.0 achieved General Availability (GA) status with MySQL 8.0.11, which was released for production use on 19 April 2018.

MySQL 5.7 achieved General Availability (GA) status with MySQL 5.7.9, which was released for production use on 21 October 2015.

MySQL 5.6 achieved General Availability (GA) status with MySQL 5.6.10, which was released for production use on 5 February 2013.

MySQL 5.5 achieved General Availability (GA) status with MySQL 5.5.8, which was released for production use on 3 December 2010. The MySQL 5.5 series is no longer current, but still supported in production.

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

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

Due to the many new and important features we were introducing in this MySQL version, we decided to start a fresh new series. As the series numbers 6 and 7 had actually been used before by MySQL, we went to 8.0.

Yes. See Section 13.2.11, “Subquery Syntax”.

Yes. For the syntax required to perform multiple-table updates, see Section 13.2.12, “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 23.3, “Using Triggers”.

No. However, MySQL has an AUTO_INCREMENT system, which in MySQL 8.0 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.

Yes, see Section 11.3.6, “Fractional Seconds in Time Values”.

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

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.

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.

See Chapter 16, Alternative Storage Engines. That chapter contains information about all MySQL storage engines except for the InnoDB storage engine and the NDB storage engine (used for MySQL Cluster). InnoDB is covered in Chapter 15, The InnoDB Storage Engine. NDB is covered in MySQL NDB Cluster 7.5 and NDB Cluster 7.6.

No. InnoDB is the default storage engine for new tables. See Section 15.1, “Introduction to InnoDB” for details.

The PARTITION storage engine plugin which provided partitioning support is replaced by a native partitioning handler. As part of this change, the server can no longer be built using -DWITH_PARTITION_STORAGE_ENGINE. partition is also no longer displayed in the output of SHOW PLUGINS, or shown in the INFORMATION_SCHEMA.PLUGINS table.

In order to support partitioning of a given table, the storage engine used for the table must now provide its own (“native”) partitioning handler. InnoDB is the only storage engine supported in MySQL 8.0 that includes a native partitioning handler. An attempt to create partitioned tables in MySQL 8.0 using any other storage engine fails. (The NDB storage engine used by MySQL Cluster also provides its own partitioning handler, but is currently not supported by MySQL 8.0.)

Yes. The disabled_storage_engines configuration option defines which storage engines cannot be used to create tables or tablespaces. By default, disabled_storage_engines is empty (no engines disabled), but it can be set to a comma-separated list of one or more engines.

Yes. Using InnoDB tables exclusively can simplify backup and recovery operations. MySQL Enterprise Backup does a hot backup of all tables that use the InnoDB storage engine. For tables using MyISAM or other non-InnoDB storage engines, it does a “warm” backup, where the database continues to run, but those tables cannot be modified while being backed up. See Section 29.2, “MySQL Enterprise Backup Overview”.

The ARCHIVE storage engine stores large amounts of data without indexes; it has a small footprint, and performs selects using table scans. See Section 16.5, “The ARCHIVE Storage Engine”, for details.

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 5.1.10, “Server SQL Modes”.

Each mode can be independently switched on and off. See Section 5.1.10, “Server SQL Modes”, for a complete list of available modes.

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.

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'.

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 5.1.10, “Server SQL Modes”, for more information.

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.

The default SQL mode in MySQL 8.0 includes these modes: ONLY_FULL_GROUP_BY, STRICT_TRANS_TABLES, NO_ZERO_IN_DATE, NO_ZERO_DATE, ERROR_FOR_DIVISION_BY_ZERO, and NO_ENGINE_SUBSTITUTION.

For information about all available modes and default MySQL behavior, see Section 5.1.10, “Server SQL Modes”.

Yes. MySQL 8.0 supports two types of stored routines, stored procedures and stored functions.

See Section 23.2, “Using Stored Routines (Procedures and Functions)”.

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

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, that provide a comprehensive overview of the standard, including coverage of stored procedures.

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 24.20, “The INFORMATION_SCHEMA ROUTINES Table”).

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 24.20, “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 13.7.6.9, “SHOW CREATE PROCEDURE Syntax”, for more information.

Stored procedures are stored in the mysql.routines and mysql.parameters tables, which are part of the data dictionary. You cannot access these tables directly. Instead, query the INFORMATION_SCHEMA ROUTINES and PARAMETERS tables. See Section 24.20, “The INFORMATION_SCHEMA ROUTINES Table”, and Section 24.13, “The INFORMATION_SCHEMA PARAMETERS Table”.

You can also use SHOW CREATE FUNCTION to obtain information about stored functions, and SHOW CREATE PROCEDURE to obtain information about stored procedures. See Section 13.7.6.9, “SHOW CREATE PROCEDURE Syntax”.

No. This is not supported in MySQL 8.0.

Yes.

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

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

Yes. MySQL 8.0 implements the SQL standard SIGNAL and RESIGNAL statements. See Section 13.6.7, “Condition Handling”.

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

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 higher) client/server protocol for this to work. This means that, for example, in PHP, you need to use the mysqli extension rather than the old mysql extension.

Not in MySQL 8.0.

There is no equivalent in MySQL 8.0.

Not in MySQL 8.0.

In MySQL 8.0, cursors are available inside stored procedures only.

In MySQL 8.0, 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 13.2.10, “SELECT Syntax”.

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 example, in PHP, you need to use the mysqli extension rather than the old mysql extension.

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

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 23.7, “Binary Logging of Stored Programs”.

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.

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.

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:

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. 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.

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

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 form of replication “knows” whether statement-level replication can safely be used, or row-level replication is required.

For additional information, see Section 17.2.1, “Replication Formats”.

See Section 23.3, “Using Triggers”.

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

In MySQL 8.0, all triggers are FOR EACH ROW; that is, the trigger is activated for each row that is inserted, updated, or deleted. MySQL 8.0 does not support triggers using FOR EACH STATEMENT.

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

In MySQL 8.0, triggers can be created using the CREATE TRIGGER statement, and dropped using DROP TRIGGER. See Section 13.1.20, “CREATE TRIGGER Syntax”, and Section 13.1.31, “DROP TRIGGER Syntax”, for more about these statements.

Information about triggers can be obtained by querying the INFORMATION_SCHEMA.TRIGGERS table. See Section 24.30, “The INFORMATION_SCHEMA TRIGGERS Table”.

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 24.30, “The INFORMATION_SCHEMA TRIGGERS Table”.

You can also use the SHOW TRIGGERS statement, which is specific to MySQL. See Section 13.7.6.38, “SHOW TRIGGERS Syntax”.

Triggers are stored in the mysql.triggers system table, which is part of the data dictionary.

Yes.

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.

In MySQL 8.0, it is possible to define multiple triggers for a given table that have the same trigger event and action time. For example, you can have two BEFORE UPDATE triggers for a table. By default, triggers that have the same trigger event and action time activate in the order they were created. To affect trigger order, specify a clause after FOR EACH ROW that indicates FOLLOWS or PRECEDES and the name of an existing trigger that also has the same trigger event and action time. With FOLLOWS, the new trigger activates after the existing trigger. With PRECEDES, the new trigger activates before the existing trigger.

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

Yes. A table on a remote server could be updated using the FEDERATED storage engine. (See Section 16.8, “The FEDERATED Storage Engine”).

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

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 17.4.1.36, “Replication and Triggers”.

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 17.4.1.36, “Replication and Triggers”.

See Section 23.5, “Using Views”.

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

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 13.7.3.2, “CHECK TABLE Syntax”.)

No.

No.

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.

See Chapter 24, INFORMATION_SCHEMA Tables

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

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, that provide a comprehensive overview of the standard, including INFORMATION_SCHEMA.

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

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.

For detailed upgrade information, see Section 2.10.1, “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.

Storage engine support has changed as follows:

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:

The default authentication plugin in MySQL 8.0 is caching_sha2_password. For information about this plugin, see Section 6.5.1.3, “Caching SHA-2 Pluggable Authentication”.

The caching_sha2_password plugin provides more secure password encryption than the mysql_native_password plugin (the default plugin in previous MySQL series). For information about the implications of this change of default plugin for server operation and compatibility of the server with clients and connectors, see caching_sha2_password as the Preferred Authentication Plugin.

For general information about pluggable authentication and other available authentication plugins, see Section 6.3.10, “Pluggable Authentication”, and Section 6.5.1, “Authentication Plugins”.

Most 8.0 binaries have support for SSL connections between the client and server. See Section 6.4, “Using Encrypted 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 6.4.7, “Connecting to MySQL Remotely from Windows with SSH”.

Most 8.0 binaries have SSL enabled for client/server connections that are secured, authenticated, or both. See Section 6.4, “Using Encrypted Connections”.

The Enterprise edition includes a PAM Authentication Plugin that supports authentication against an LDAP directory.

Not at this time.

NDB Cluster is not supported in MySQL Server 8.0 releases; it is released as a separate product. You are strongly advised to use NDB Cluster 7.5 for any new deployments; if you are using an older version of NDB Cluster, we recommend that you upgrade to this version soon as possible. For an overview of improvements made in NDB Cluster 7.5, see What is New in NDB Cluster 7.5.

NDB Cluster 7.6, based on MySQL Server 5.7 and version 7.6 of the NDB storage engine, is also now available as a Developer Preview release for evaluation and testing purposes. For information about new features and other important changes in this series, see What is New in NDB Cluster 7.6.

For detailed information about deploying and using NDB Cluster, see MySQL NDB Cluster 7.5 and NDB Cluster 7.6.

“NDB” stands for “Network Database”. NDB and NDBCLUSTER are both names for the storage engine that enables clustering support with MySQL. NDB is preferred, but either name is correct.

A minimum of three computers is required to run a viable cluster. However, the minimum recommended number of computers in an NDB Cluster is four: one each to run the management and SQL nodes, and two computers to serve as data nodes. The purpose of the two data nodes is to provide redundancy; the management node must run on a separate machine to guarantee continued arbitration services in the event that one of the data nodes fails.

To provide increased throughput and high availability, you should use multiple SQL nodes (MySQL Servers connected to the cluster). It is also possible (although not strictly necessary) to run multiple management servers.

The list of CJK character sets may vary depending on your MySQL version. For example, the gb18030 character set is not supported prior to MySQL 5.7.4. 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 '%Chin%'
       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                   |
| gb18030            | China National Standard GB18030 |
| gb2312             | GB2312 Simplified Chinese       |
| gbk                | GBK Simplified Chinese          |
| sjis               | Shift-JIS Japanese              |
| ujis               | EUC-JP Japanese                 |
+--------------------+---------------------------------+

(For more information, see Section 24.1, “The INFORMATION_SCHEMA CHARACTER_SETS Table”.)

MySQL supports three 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, gbk, and (as of MySQL 5.7.4) gb18030.

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 does not work. (For an example, see Bug #16072).

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:

It is also possible to store CJK characters in Unicode character sets, although the available collations may not sort characters quite as you expect:

The collation used for a Unicode character set determines the ability to sort (that is, distinguish) characters in the set:

Moreover, distinguishing characters is not the same as ordering them per the conventions of a given CJK language. Currently, MySQL has only one CJK-specific UCA collation, gb18030_unicode_520_ci (which requires use of the non-Unicode gb18030 character set).

For information about Unicode collations and their differentiating properties, including collation properties for supplementary characters, see Section 10.10.1, “Unicode Character Sets”.

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

MySQL supports the Big5 character set which is common in Hong Kong and Taiwan (Republic of China). The MySQL big5 character set is in reality Microsoft code page 950, which is very similar to the original big5 character set.

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.

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.)

Our answer is: “?”. There are disadvantages to this, and many people would prefer a “loose” conversion, so that 81CA (NOT SIGN) in sjis becomes 81CA (FULLWIDTH NOT SIGN) in cp932.

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), whereas 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 (\).

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       |
+-------+----------+

To see the problem, 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);

In nonstrict SQL mode, try to place the rare character 汌 in both columns.

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

The INSERT produces 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

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      |
+-------+--------------+--------+-------------+

Several things need explanation here:

Obtain a direct connection to the server using the mysql client, and try the same query there. If mysql responds correctly, 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, you are most likely connecting with Connector/ODBC. In this case, you should check Configuring Connector/ODBC. If, for example, you use big5, you would enter SET NAMES 'big5'. (In this case, no ; character is required.) 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, you must specify the character set in the connection string. See Connecting to MySQL Using Connector/Net, for more information.

If you are using PHP, try this:

<?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'");
?>

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

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, include <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> in the <head> section of the HTML page.

If you are using Connector/J, see Using Character Sets and Unicode.

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 10.4, “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 Asian customers 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 resulting 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/ |
+--------------------------+----------------------------------------+

Now stop the client, and 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 resulting 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/ |
+--------------------------+----------------------------------------+

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 option is used.

For LIKE searches, there is a very simple problem with binary string column types such as BINARY and BLOB: we must know where characters end. With multibyte 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 |
+-------------------------+---------------------------+

If we do not know where the first character in a string ends, we do not know where the second character begins, in which case even very simple searches such as LIKE '_A%' fail. The solution is to use a nonbinary string column type defined to have the proper CJK character set. For example: mycol TEXT CHARACTER SET sjis. Alternatively, convert to a CJK character set before comparing.

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

For FULLTEXT searches, we must 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.

The majority of simplified Chinese and basic nonhalfwidth Japanese Kana characters appear in all CJK character sets. The following stored procedure accepts a UCS-2 Unicode character, converts it to 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 are conversion problems, UPDATE produces warnings. */

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//

DELIMITER ;

The input can be any single ucs2 character, or it can be the code 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 value is X'30DA'. If we use this value as the argument to p_convert(), the result is as shown here:

mysql> CALL p_convert(X'30DA');
+------+--------+------+-------+---------+-------+--------+------+------+------+
| ucs2 | utf8   | big5 | cp932 | eucjpms | euckr | gb2312 | gbk  | sjis | ujis |
+------+--------+------+-------+---------+-------+--------+------+------+------+
| 30DA | E3839A | C772 | 8379  | A5DA    | ABDA  | A5DA   | A5DA | 8379 | A5DA |
+------+--------+------+-------+---------+-------+--------+------+------+------+

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

CJK sorting problems that occurred in older MySQL versions can be solved as of MySQL 8.0 by using the utf8mb4 character set and the utf8mb4_ja_0900_as_cs collation.

CJK sorting problems that occurred in older MySQL versions can be solved as of MySQL 8.0 by using the utf8mb4 character set and the utf8mb4_ja_0900_as_cs collation.

Supplementary characters lie outside the Unicode Basic Multilingual Plane / Plane 0. BMP characters have code point values between U+0000 and U+FFFF. Supplementary characters have code point values between U+10000 and U+10FFFF.

To store supplementary characters, you must use a character set that permits them:

No. The term “CJKV” (Chinese Japanese Korean Vietnamese) refers to Vietnamese character sets which contain Han (originally Chinese) characters. MySQL supports the modern Vietnamese script with Western characters, but does not support the old Vietnamese script using Han characters.

As of MySQL 5.6, there are Vietnamese collations for Unicode character sets, as described in Section 10.10.1, “Unicode Character Sets”.

Yes.

The Japanese translation of the MySQL 5.6 manual can be downloaded from http://dev.mysql.com/doc/.

The following resources are available:

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.

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.

Check the Seconds_Behind_Master column in the output from SHOW SLAVE STATUS. See Section 17.1.7.1, “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 17.2.2, “Replication Implementation Details”.

Use the following procedure:

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.

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.

See the guide to using replication as a scale-out solution, Section 17.3.5, “Using Replication for Scale-Out”.

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:

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:

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 17.3.8, “Switching Masters During Failover”.

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, statement-based logging is used by default but replication switches automatically to row-based logging under certain conditions, such as unsafe statements. For information about when this may occur, see Section 5.4.4.3, “Mixed Binary Logging Format”.

Slaves automatically know which format to use.

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

Yes.

Yes.

The MySQL Thread Pool is a MySQL server plugin that extends the default connection-handling capabilities of the MySQL server to limit the number of concurrently executing statements/queries and transactions to ensure that each has sufficient CPU and memory resources to fulfill its task. Commercial distributions of MySQL 8.0 include the Thread Pool plugin.

The default thread-handling model in MySQL Server executes statements using one thread per client connection. As more clients connect to the server and execute statements, overall performance degrades. The Thread Pool plugin provides an alternative thread-handling model designed to reduce overhead and improve performance. The Thread Pool plugin increases server performance by efficiently managing statement execution threads for large numbers of client connections, especially on modern multi-CPU/Core systems.

For more information, see Section 5.6.3, “MySQL Enterprise Thread Pool”.

The Thread Pool uses a “divide and conquer” approach to limiting and balancing concurrency. Unlike the default connection handling of the MySQL Server, the Thread Pool separates connections and threads, so there is no fixed relationship between connections and the threads that execute statements received from those connections. The Thread Pool then manages client connections within configurable thread groups, where they are prioritized and queued based on the nature of the work they were submitted to accomplish.

For more information, see Section 5.6.3.3, “Thread Pool Operation”.

The MySQL Connection Pool operates on the client side to ensure that a MySQL client does not constantly connect to and disconnect from the MySQL server. It is designed to cache idle connections in the MySQL client for use by other users as they are needed. This minimizes the overhead and expense of establishing and tearing down connections as queries are submitted to the MySQL server. The MySQL Connection Pool has no visibility as to the query handling capabilities or load of the back-end MySQL server. By contrast, the Thread Pool operates on the MySQL server side and is designed to manage the execution of inbound concurrent connections and queries as they are received from the client connections accessing the back-end MySQL database. Because of the separation of duties, the MySQL Connection Pool and Thread Pool are orthogonal and can be used independent of each other.

MySQL Connection Pooling via the MySQL Connectors is covered in Chapter 27, Connectors and APIs.

There are a few rules of thumb to consider for optimal Thread Pool use cases:

The MySQL Threads_running variable keeps track of the number of concurrent statements currently executing in the MySQL Server. If this variable consistently exceeds a region where the server won't operate optimally (usually going beyond 40 for InnoDB workloads), the Thread Pool will be beneficial, especially in extreme parallel overload situations.

If you are using the innodb_thread_concurrency to limit the number of concurrently executing statements, you will find the Thread Pool solves the same problem, only better, by assigning connections to thread groups, then queuing executions based on transactional content, user defined designations, and so forth.

Lastly, if your workload comprises mainly short queries, the Thread Pool will be beneficial.

To learn more, see Section 5.6.3.4, “Thread Pool Tuning”.

The Thread Pool has a number of user case driven configuration parameters that affect its performance. To learn about these and tips on tuning, see Section 5.6.3.4, “Thread Pool Tuning”.

INSERT, UPDATE, and DELETE operations can modify secondary indexes. If an affected index page is not in the buffer pool, the changes can be buffered in the change buffer.

Buffering secondary index changes when secondary index pages are not in the buffer pool avoids expensive random access I/O operations that would be required to immediately read in affected index pages from disk. Buffered changes can be applied later, in batches, as pages are read into the buffer pool by other read operations.

No. The change buffer only supports secondary indexes. Clustered indexes, full-text indexes, and spatial indexes are not supported. Full-text indexes have their own caching mechanism.

Prior to the introduction of the innodb_change_buffer_max_size configuration option in MySQL 5.6, the maximum size of the on-disk change buffer in the system tablespace was 1/3 of the InnoDB buffer pool size.

In MySQL 5.6 and later, the innodb_change_buffer_max_size configuration option defines the maximum size of the change buffer as a percentage of the total buffer pool size. By default, innodb_change_buffer_max_size is set to 25. The maximum setting is 50.

InnoDB does not buffer an operation if it would cause the on-disk change buffer to exceed the defined limit.

Change buffer pages are not required to persist in the buffer pool and may be evicted by LRU operations.

The current size of the change buffer is reported by SHOW ENGINE INNODB STATUS \G, under the INSERT BUFFER AND ADAPTIVE HASH INDEX heading. For example:

-------------------------------------
INSERT BUFFER AND ADAPTIVE HASH INDEX
-------------------------------------
Ibuf: size 1, free list len 0, seg size 2, 0 merges

Relevant data points include:

For information about monitoring change buffer status, see Section 15.4.2, “Change Buffer”.

Updated pages are flushed by the same flushing mechanism that flushes the other pages that occupy the buffer pool.

The change buffer is a feature designed to reduce random I/O to secondary indexes as indexes grow larger and no longer fit in the InnoDB buffer pool. Generally, the change buffer should be used when the entire data set does not fit into the buffer pool, when there is substantial DML activity that modifies secondary index pages, or when there are lots of secondary indexes that are regularly changed by DML activity.

You might consider disabling the change buffer if the entire data set fits within the InnoDB buffer pool, if you have relatively few secondary indexes, or if you are using solid-state storage, where random reads are about as fast as sequential reads. Before making configuration changes, it is recommended that you run tests using a representative workload to determine if disabling the change buffer provides any benefit.

The following sections and blog post provide additional information about the InnoDB change buffer:

Yes. InnoDB tablespace encryption is designed to provide customers with the ability to transparently apply encryption within the database without impacting existing applications. Returning data in encrypted format would break most existing applications. InnoDB tablespace encryption provides the benefit of encryption without the overhead associated with traditional database encryption solutions, which would typically require expensive and substantial changes to applications, database triggers, and views.

There is no additional storage overhead. According to internal benchmarks, performance overhead amounts to a single digit percentage difference.

InnoDB tablespace encryption supports the Advanced Encryption Standard (AES256) block-based encryption algorithm. It uses Electronic Codebook (ECB) block encryption mode for tablespace key encryption and Cipher Block Chaining (CBC) block encryption mode for data encryption.

No, it is not possible to use other encryption algorithms. The provided encryption algorithm is broadly accepted.

InnoDB tablespace encryption supports all indexes transparently.

InnoDB tablespace encryption supports all supported data types. There is no data length limitation.

Data encrypted by the InnoDB tablespace encryption feature is decrypted when it is read from the tablespace file. Thus, if the data is on the network, it is in clear-text form. However, data on the network can be encrypted using MySQL network encryption, which encrypts data traveling to and from a database using SSL/TLS.

With InnoDB tablespace encryption, in-memory data is decrypted, which provides 100% transparency.

Compliance with the PCI-DSS standard requires that credit card numbers (Primary Account Number, or 'PAN') be stored in encrypted form. Breach Notification Laws (for example, CA SB 1386, CA AB 1950, and similar laws in 43+ more US states) require encryption of first name, last name, driver license number, and other PII data. In early 2008, CA AB 1298 added medical and health insurance information to PII data. Additionally, industry specific privacy and security standards may require encryption of certain assets. For example, assets such as pharmaceutical research results, oil field exploration results, financial contracts, or personal data of law enforcement informants may require encryption. In the health care industry, the privacy of patient data, health records and X-ray images is of the highest importance.

There are symmetric and asymmetric encryption APIs in MySQL that can be used to manually encrypt data within the database. However, the application must manage encryption keys and perform required encryption and decryption operations by calling API functions. InnoDB tablespace encryption requires no application changes, is transparent to end users, and provides automated, built-in key management.

Yes. For more information, see Exporting Encrypted Tables.

Customers using InnoDB tablespace encryption receive the full benefit of compression because compression is applied before data blocks are encrypted.

Yes. Because these utilities create logical backups, the data dumped from encrypted tables is not encrypted.

InnoDB tablespace encryption uses a two tier key mechanism. When tablespace encryption is used, individual tablespace keys are stored in the header of the underlying tablespace data file. Tablespace keys are encrypted using the master encryption key. The master encryption key is generated when tablespace encryption is enabled, and is stored outside the database. The master encryption key is rotated using the ALTER INSTANCE ROTATE INNODB MASTER KEY statement, which generates a new master encryption key, stores the key, and rotates the key into use.

Transferring data from one tablespace to another is not required. To encrypt data in an InnoDB tablespace, run ALTER TABLE tbl_name ENCRYPTION='Y'. InnoDB tablespace encryption is only supported with file-per-table tablespaces.

MySQL is supported on virtualized environments, but is certified only for Oracle VM. Contact Oracle Support for more information.

Be aware of potential problems when using virtualization software. The usual ones are related to performance, performance degradations, slowness, or unpredictability of disk, I/O, network, and memory.