MySQL Restrictions and Limitations

Abstract

This is the MySQL Restrictions and Limitations extract from the MySQL 5.7 Reference Manual.

For legal information, see the Legal Notices.

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Document generated on: 2024-09-12 (revision: 79651)

Table of Contents

Preface and Legal Notices ............................................................................................................ v

1 Restrictions on Stored Programs ................................................................................................ 1

2 Restrictions on Views ................................................................................................................ 5

3 Restrictions on Condition Handling ............................................................................................. 7

4 Restrictions on Server-Side Cursors ........................................................................................... 9

5 Restrictions on Subqueries ....................................................................................................... 11

6 Restrictions on XA Transactions ............................................................................................... 13

7 Restrictions on Character Sets ................................................................................................. 15

8 Restrictions on Performance Schema ....................................................................................... 17

9 Restrictions on Pluggable Authentication ................................................................................... 19

10 Restrictions and Limitations on Partitioning ............................................................................. 21

10.1 Partitioning Keys, Primary Keys, and Unique Keys ........................................................ 27

10.2 Partitioning Limitations Relating to Storage Engines ...................................................... 31

10.3 Partitioning Limitations Relating to Functions ................................................................ 32

10.4 Partitioning and Locking .............................................................................................. 33

11 Windows Platform Restrictions ............................................................................................... 35

12 Limits in MySQL .................................................................................................................... 37

12.1 Identifier Length Limits ................................................................................................ 37

12.2 Grant Table Scope Column Properties ......................................................................... 38

12.3 Limits on Number of Databases and Tables ................................................................. 38

12.4 Limits on Table Size ................................................................................................... 38

12.5 Limits on Table Column Count and Row Size ............................................................... 39

12.6 Limits Imposed by .frm File Structure ........................................................................... 42

13 MySQL Differences from Standard SQL .................................................................................. 45

13.1 SELECT INTO TABLE Differences ............................................................................... 45

13.2 UPDATE Differences ................................................................................................... 45

13.3 FOREIGN KEY Constraint Differences ......................................................................... 45

13.4 '--' as the Start of a Comment ..................................................................................... 47

14 Known Issues in MySQL ........................................................................................................ 49

iii

Preface and Legal Notices

This is the MySQL Restrictions and Limitations extract from the MySQL 5.7 Reference Manual.

Licensing information—MySQL 5.7. This product may include third-party software, used under

license. If you are using a Commercial release of MySQL 5.7, see the MySQL 5.7 Commercial Release

License Information User Manual for licensing information, including licensing information relating to

third-party software that may be included in this Commercial release. If you are using a Community

release of MySQL 5.7, see the MySQL 5.7 Community Release License Information User Manual

for licensing information, including licensing information relating to third-party software that may be

included in this Community release.

Licensing information—MySQL NDB Cluster 7.5. This product may include third-party software,

used under license. If you are using a Commercial release of NDB Cluster 7.5, see the MySQL NDB

Cluster 7.5 Commercial Release License Information User Manual for licensing information relating

to third-party software that may be included in this Commercial release. If you are using a Community

release of NDB Cluster 7.5, see the MySQL NDB Cluster 7.5 Community Release License Information

User Manual for licensing information relating to third-party software that may be included in this

Community release.

Licensing information—MySQL NDB Cluster 7.6. If you are using a Commercial release of

MySQL NDB Cluster 7.6, see the MySQL NDB Cluster 7.6 Commercial Release License Information

User Manual for licensing information, including licensing information relating to third-party software

that may be included in this Commercial release. If you are using a Community release of MySQL NDB

Cluster 7.6, see the MySQL NDB Cluster 7.6 Community Release License Information User Manual

for licensing information, including licensing information relating to third-party software that may be

included in this Community release.

Legal Notices

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vii

viii

Chapter 1 Restrictions on Stored Programs

• SQL Statements Not Permitted in Stored Routines

• Restrictions for Stored Functions

• Restrictions for Triggers

• Name Conflicts within Stored Routines

• Replication Considerations

• Debugging Considerations

• Unsupported Syntax from the SQL:2003 Standard

• Stored Routine Concurrency Considerations

• Event Scheduler Restrictions

• Stored Programs in NDB Cluster

These restrictions apply to the features described in Stored Objects.

Some of the restrictions noted here apply to all stored routines; that is, both to stored procedures

and stored functions. There are also some restrictions specific to stored functions but not to stored

procedures.

The restrictions for stored functions also apply to triggers. There are also some restrictions specific to

triggers.

The restrictions for stored procedures also apply to the DO clause of Event Scheduler event definitions.

There are also some restrictions specific to events.

SQL Statements Not Permitted in Stored Routines

Stored routines cannot contain arbitrary SQL statements. The following statements are not permitted:

• The locking statements LOCK TABLES and UNLOCK TABLES.

• ALTER VIEW.

• LOAD DATA and LOAD XML.

• SQL prepared statements (PREPARE, EXECUTE, DEALLOCATE PREPARE) can be used in stored

procedures, but not stored functions or triggers. Thus, stored functions and triggers cannot use

dynamic SQL (where you construct statements as strings and then execute them).

• Generally, statements not permitted in SQL prepared statements are also not permitted in stored

programs. For a list of statements supported as prepared statements, see Prepared Statements.

Exceptions are SIGNAL, RESIGNAL, and GET DIAGNOSTICS, which are not permissible as

prepared statements but are permitted in stored programs.

• Because local variables are in scope only during stored program execution, references to them

are not permitted in prepared statements created within a stored program. Prepared statement

scope is the current session, not the stored program, so the statement could be executed after the

program ends, at which point the variables would no longer be in scope. For example, SELECT ...

INTO local_var cannot be used as a prepared statement. This restriction also applies to stored

procedure and function parameters. See PREPARE Statement.

• Within all stored programs (stored procedures and functions, triggers, and events), the parser treats

BEGIN [WORK] as the beginning of a BEGIN ... END block. To begin a transaction in this context,

use START TRANSACTION instead.

Restrictions for Stored Functions

The following additional statements or operations are not permitted within stored functions. They are

permitted within stored procedures, except stored procedures that are invoked from within a stored

function or trigger. For example, if you use FLUSH in a stored procedure, that stored procedure cannot

be called from a stored function or trigger.

• Statements that perform explicit or implicit commit or rollback. Support for these statements is not

required by the SQL standard, which states that each DBMS vendor may decide whether to permit

them.

• Statements that return a result set. This includes SELECT statements that do not have an INTO

var_list clause and other statements such as SHOW, EXPLAIN, and CHECK TABLE. A function

can process a result set either with SELECT ... INTO var_list or by using a cursor and FETCH

statements. See SELECT ... INTO Statement, and Cursors.

• FLUSH statements.

• Stored functions cannot be used recursively.

• A stored function or trigger cannot modify a table that is already being used (for reading or writing) by

the statement that invoked the function or trigger.

• If you refer to a temporary table multiple times in a stored function under different aliases, a Can't

reopen table: 'tbl_name' error occurs, even if the references occur in different statements

within the function.

• HANDLER ... READ statements that invoke stored functions can cause replication errors and are

disallowed.

Restrictions for Triggers

For triggers, the following additional restrictions apply:

• Triggers are not activated by foreign key actions.

• When using row-based replication, triggers on the replica are not activated by statements originating

on the source. The triggers on the replica are activated when using statement-based replication. For

more information, see Replication and Triggers.

• The RETURN statement is not permitted in triggers, which cannot return a value. To exit a trigger

immediately, use the LEAVE statement.

• Triggers are not permitted on tables in the mysql database. Nor are they permitted on

INFORMATION_SCHEMA or performance_schema tables. Those tables are actually views and

triggers are not permitted on views.

• The trigger cache does not detect when metadata of the underlying objects has changed. If a trigger

uses a table and the table has changed since the trigger was loaded into the cache, the trigger

operates using the outdated metadata.

Name Conflicts within Stored Routines

The same identifier might be used for a routine parameter, a local variable, and a table column. Also,

the same local variable name can be used in nested blocks. For example:

CREATE PROCEDURE p (i INT)

BEGIN

DECLARE i INT DEFAULT 0;

SELECT i FROM t;

Replication Considerations

BEGIN

DECLARE i INT DEFAULT 1;

SELECT i FROM t;

END;

In such cases, the identifier is ambiguous and the following precedence rules apply:

• A local variable takes precedence over a routine parameter or table column.

• A routine parameter takes precedence over a table column.

• A local variable in an inner block takes precedence over a local variable in an outer block.

The behavior that variables take precedence over table columns is nonstandard.

Replication Considerations

Use of stored routines can cause replication problems. This issue is discussed further in Stored

Program Binary Logging.

The --replicate-wild-do-table=db_name.tbl_name option applies to tables, views, and

triggers. It does not apply to stored procedures and functions, or events. To filter statements operating

on the latter objects, use one or more of the --replicate-*-db options.

Debugging Considerations

There are no stored routine debugging facilities.

Unsupported Syntax from the SQL:2003 Standard

The MySQL stored routine syntax is based on the SQL:2003 standard. The following items from that

standard are not currently supported:

• UNDO handlers

• FOR loops

Stored Routine Concurrency Considerations

To prevent problems of interaction between sessions, when a client issues a statement, the server

uses a snapshot of routines and triggers available for execution of the statement. That is, the server

calculates a list of procedures, functions, and triggers that may be used during execution of the

statement, loads them, and then proceeds to execute the statement. While the statement executes, it

does not see changes to routines performed by other sessions.

For maximum concurrency, stored functions should minimize their side-effects; in particular, updating

a table within a stored function can reduce concurrent operations on that table. A stored function

acquires table locks before executing, to avoid inconsistency in the binary log due to mismatch of the

order in which statements execute and when they appear in the log. When statement-based binary

logging is used, statements that invoke a function are recorded rather than the statements executed

within the function. Consequently, stored functions that update the same underlying tables do not

execute in parallel. In contrast, stored procedures do not acquire table-level locks. All statements

executed within stored procedures are written to the binary log, even for statement-based binary

logging. See Stored Program Binary Logging.

Event Scheduler Restrictions

The following limitations are specific to the Event Scheduler:

Stored Programs in NDB Cluster

• Event names are handled in case-insensitive fashion. For example, you cannot have two events in

the same database with the names anEvent and AnEvent.

• An event may not be created, altered, or dropped from within a stored program, if the event name

is specified by means of a variable. An event also may not create, alter, or drop stored routines or

triggers.

• DDL statements on events are prohibited while a LOCK TABLES statement is in effect.

• Event timings using the intervals YEAR, QUARTER, MONTH, and YEAR_MONTH are resolved in months;

those using any other interval are resolved in seconds. There is no way to cause events scheduled

to occur at the same second to execute in a given order. In addition—due to rounding, the nature of

threaded applications, and the fact that a nonzero length of time is required to create events and to

signal their execution—events may be delayed by as much as 1 or 2 seconds. However, the time

shown in the Information Schema EVENTS table's LAST_EXECUTED column or the mysql.event

table's last_executed column is always accurate to within one second of the actual event

execution time. (See also Bug #16522.)

• Each execution of the statements contained in the body of an event takes place in a new connection;

thus, these statements have no effect in a given user session on the server's statement counts such

as Com_select and Com_insert that are displayed by SHOW STATUS. However, such counts are

updated in the global scope. (Bug #16422)

• Events do not support times later than the end of the Unix Epoch; this is approximately the beginning

of the year 2038. Such dates are specifically not permitted by the Event Scheduler. (Bug #16396)

• References to stored functions, loadable functions, and tables in the ON SCHEDULE clauses of

CREATE EVENT and ALTER EVENT statements are not supported. These sorts of references are not

permitted. (See Bug #22830 for more information.)

Stored Programs in NDB Cluster

While stored procedures, stored functions, triggers, and scheduled events are all supported by tables

using the NDB storage engine, you must keep in mind that these do not propagate automatically

between MySQL Servers acting as Cluster SQL nodes. This is because of the following:

• Stored routine definitions are kept in tables in the mysql system database using the MyISAM storage

engine, and so do not participate in clustering.

• The .TRN and .TRG files containing trigger definitions are not read by the NDB storage engine, and

are not copied between Cluster nodes.

Any stored routine or trigger that interacts with NDB Cluster tables must be re-created by running the

appropriate CREATE PROCEDURE, CREATE FUNCTION, or CREATE TRIGGER statements on each

MySQL Server that participates in the cluster where you wish to use the stored routine or trigger.

Similarly, any changes to existing stored routines or triggers must be carried out explicitly on all Cluster

SQL nodes, using the appropriate ALTER or DROP statements on each MySQL Server accessing the

cluster.

Warning

Do not attempt to work around the issue described in the first item mentioned

previously by converting any mysql database tables to use the NDB storage

engine. Altering the system tables in the mysql database is not supported and

is very likely to produce undesirable results.

Chapter 2 Restrictions on Views

The maximum number of tables that can be referenced in the definition of a view is 61.

View processing is not optimized:

• It is not possible to create an index on a view.

• Indexes can be used for views processed using the merge algorithm. However, a view that is

processed with the temptable algorithm is unable to take advantage of indexes on its underlying

tables (although indexes can be used during generation of the temporary tables).

Before MySQL 5.7.7, subqueries cannot be used in the FROM clause of a view.

There is a general principle that you cannot modify a table and select from the same table in a

subquery. See Chapter 5, Restrictions on Subqueries.

The same principle also applies if you select from a view that selects from the table, if the view selects

from the table in a subquery and the view is evaluated using the merge algorithm. Example:

CREATE VIEW v1 AS

SELECT * FROM t2 WHERE EXISTS (SELECT 1 FROM t1 WHERE t1.a = t2.a);

UPDATE t1, v2 SET t1.a = 1 WHERE t1.b = v2.b;

If the view is evaluated using a temporary table, you can select from the table in the view subquery

and still modify that table in the outer query. In this case the view is stored in a temporary table and

thus you are not really selecting from the table in a subquery and modifying it “at the same time.”

(This is another reason you might wish to force MySQL to use the temptable algorithm by specifying

ALGORITHM = TEMPTABLE in the view definition.)

You can use DROP TABLE or ALTER TABLE to drop or alter a table that is used in a view definition.

No warning results from the DROP or ALTER operation, even though this invalidates the view. Instead,

an error occurs later, when the view is used. CHECK TABLE can be used to check for views that have

been invalidated by DROP or ALTER operations.

With regard to view updatability, the overall goal for views is that if any view is theoretically updatable,

it should be updatable in practice. Many theoretically updatable views can be updated now, but

limitations still exist. For details, see Updatable and Insertable Views.

There exists a shortcoming with the current implementation of views. If a user is granted the basic

privileges necessary to create a view (the CREATE VIEW and SELECT privileges), that user cannot call

SHOW CREATE VIEW on that object unless the user is also granted the SHOW VIEW privilege.

That shortcoming can lead to problems backing up a database with mysqldump, which may fail due to

insufficient privileges. This problem is described in Bug #22062.

The workaround to the problem is for the administrator to manually grant the SHOW VIEW privilege

to users who are granted CREATE VIEW, since MySQL does not grant it implicitly when views are

created.

Views do not have indexes, so index hints do not apply. Use of index hints when selecting from a view

is not permitted.

SHOW CREATE VIEW displays view definitions using an AS alias_name clause for each column. If a

column is created from an expression, the default alias is the expression text, which can be quite long.

Aliases for column names in CREATE VIEW statements are checked against the maximum column

length of 64 characters (not the maximum alias length of 256 characters). As a result, views created

from the output of SHOW CREATE VIEW fail if any column alias exceeds 64 characters. This can cause

problems in the following circumstances for views with too-long aliases:

• View definitions fail to replicate to newer replicas that enforce the column-length restriction.

• Dump files created with mysqldump cannot be loaded into servers that enforce the column-length

restriction.

A workaround for either problem is to modify each problematic view definition to use aliases that

provide shorter column names. Then the view replicates properly, and can be dumped and reloaded

without causing an error. To modify the definition, drop and create the view again with DROP VIEW and

CREATE VIEW, or replace the definition with CREATE OR REPLACE VIEW.

For problems that occur when reloading view definitions in dump files, another workaround is to edit

the dump file to modify its CREATE VIEW statements. However, this does not change the original view

definitions, which may cause problems for subsequent dump operations.

Chapter 3 Restrictions on Condition Handling

SIGNAL, RESIGNAL, and GET DIAGNOSTICS are not permissible as prepared statements. For

example, this statement is invalid:

PREPARE stmt1 FROM 'SIGNAL SQLSTATE "02000"';

SQLSTATE values in class '04' are not treated specially. They are handled the same as other

exceptions.

In standard SQL, the first condition relates to the SQLSTATE value returned for the previous SQL

statement. In MySQL, this is not guaranteed, so to get the main error, you cannot do this:

GET DIAGNOSTICS CONDITION 1 @errno = MYSQL_ERRNO;

Instead, do this:

GET DIAGNOSTICS @cno = NUMBER;

GET DIAGNOSTICS CONDITION @cno @errno = MYSQL_ERRNO;

Chapter 4 Restrictions on Server-Side Cursors

Server-side cursors are implemented in the C API using the mysql_stmt_attr_set() function. The

same implementation is used for cursors in stored routines. A server-side cursor enables a result set

to be generated on the server side, but not transferred to the client except for those rows that the client

requests. For example, if a client executes a query but is only interested in the first row, the remaining

rows are not transferred.

In MySQL, a server-side cursor is materialized into an internal temporary table. Initially, this is a

MEMORY table, but is converted to a MyISAM table when its size exceeds the minimum value of the

max_heap_table_size and tmp_table_size system variables. The same restrictions apply

to internal temporary tables created to hold the result set for a cursor as for other uses of internal

temporary tables. See Internal Temporary Table Use in MySQL. One limitation of the implementation is

that for a large result set, retrieving its rows through a cursor might be slow.

Cursors are read only; you cannot use a cursor to update rows.

UPDATE WHERE CURRENT OF and DELETE WHERE CURRENT OF are not implemented, because

updatable cursors are not supported.

Cursors are nonholdable (not held open after a commit).

Cursors are asensitive.

Cursors are nonscrollable.

Cursors are not named. The statement handler acts as the cursor ID.

You can have open only a single cursor per prepared statement. If you need several cursors, you must

prepare several statements.

You cannot use a cursor for a statement that generates a result set if the statement is not supported

in prepared mode. This includes statements such as CHECK TABLE, HANDLER READ, and SHOW

BINLOG EVENTS.

Chapter 5 Restrictions on Subqueries

• In general, you cannot modify a table and select from the same table in a subquery. For example,

this limitation applies to statements of the following forms:

DELETE FROM t WHERE ... (SELECT ... FROM t ...);

UPDATE t ... WHERE col = (SELECT ... FROM t ...);

{INSERT|REPLACE} INTO t (SELECT ... FROM t ...);

Exception: The preceding prohibition does not apply if for the modified table you are using a derived

table and that derived table is materialized rather than merged into the outer query. (See Optimizing

Derived Tables and View References with Merging or Materialization.) Example:

UPDATE t ... WHERE col = (SELECT * FROM (SELECT ... FROM t...) AS dt ...);

Here the result from the derived table is materialized as a temporary table, so the relevant rows in t

have already been selected by the time the update to t takes place.

• Row comparison operations are only partially supported:

• For expr [NOT] IN subquery, expr can be an n-tuple (specified using row constructor

syntax) and the subquery can return rows of n-tuples. The permitted syntax is therefore more

specifically expressed as row_constructor [NOT] IN table_subquery

• For expr op {ALL|ANY|SOME} subquery, expr must be a scalar value and the subquery

must be a column subquery; it cannot return multiple-column rows.

In other words, for a subquery that returns rows of n-tuples, this is supported:

(expr_1, ..., expr_n) [NOT] IN table_subquery

But this is not supported:

(expr_1, ..., expr_n) op {ALL|ANY|SOME} subquery

The reason for supporting row comparisons for IN but not for the others is that IN is implemented by

rewriting it as a sequence of = comparisons and AND operations. This approach cannot be used for

ALL, ANY, or SOME.

• Subqueries in the FROM clause cannot be correlated subqueries. They are materialized in whole

(evaluated to produce a result set) during query execution, so they cannot be evaluated per row of

the outer query. The optimizer delays materialization until the result is needed, which may permit

materialization to be avoided. See Optimizing Derived Tables and View References with Merging or

Materialization.

• MySQL does not support LIMIT in subqueries for certain subquery operators:

mysql> SELECT * FROM t1

WHERE s1 IN (SELECT s2 FROM t2 ORDER BY s1 LIMIT 1);

ERROR 1235 (42000): This version of MySQL does not yet support

'LIMIT & IN/ALL/ANY/SOME subquery'

• MySQL permits a subquery to refer to a stored function that has data-modifying side effects such as

inserting rows into a table. For example, if f() inserts rows, the following query can modify data:

SELECT ... WHERE x IN (SELECT f() ...);

This behavior is an extension to the SQL standard. In MySQL, it can produce nondeterministic

results because f() might be executed a different number of times for different executions of a given

query depending on how the optimizer chooses to handle it.

For statement-based or mixed-format replication, one implication of this indeterminism is that such a

query can produce different results on the source and its replicas.

Chapter 6 Restrictions on XA Transactions

XA transaction support is limited to the InnoDB storage engine.

For “external XA,” a MySQL server acts as a Resource Manager and client programs act as

Transaction Managers. For “Internal XA”, storage engines within a MySQL server act as RMs, and

the server itself acts as a TM. Internal XA support is limited by the capabilities of individual storage

engines. Internal XA is required for handling XA transactions that involve more than one storage

engine. The implementation of internal XA requires that a storage engine support two-phase commit at

the table handler level, and currently this is true only for InnoDB.

For XA START, the JOIN and RESUME clauses are recognized but have no effect.

For XA END the SUSPEND [FOR MIGRATE] clause is recognized but has no effect.

The requirement that the bqual part of the xid value be different for each XA transaction within

a global transaction is a limitation of the current MySQL XA implementation. It is not part of the XA

specification.

Prior to MySQL 5.7.7, XA transactions were not compatible with replication. This was because an

XA transaction that was in PREPARED state would be rolled back on clean server shutdown or client

disconnect. Similarly, an XA transaction that was in PREPARED state would still exist in PREPARED

state in case the server was shutdown abnormally and then started again, but the contents of the

transaction could not be written to the binary log. In both of these situations the XA transaction could

not be replicated correctly.

In MySQL 5.7.7 and later, there is a change in behavior and an XA transaction is written to the binary

written using an initial GTID. A XA_prepare_log_event is used to identify such transactions in the

binary log. When XA COMMIT or XA ROLLBACK is issued, a second part of the transaction containing

only the XA COMMIT or XA ROLLBACK statement is written using a second GTID. Note that the initial

part of the transaction, identified by XA_prepare_log_event, is not necessarily followed by its XA

COMMIT or XA ROLLBACK, which can cause interleaved binary logging of any two XA transactions.

The two parts of the XA transaction can even appear in different binary log files. This means that an

XA transaction in PREPARED state is now persistent until an explicit XA COMMIT or XA ROLLBACK

statement is issued, ensuring that XA transactions are compatible with replication.

On a replica, immediately after the XA transaction is prepared, it is detached from the replica applier

thread, and can be committed or rolled back by any thread on the replica. This means that the same

XA transaction can appear in the events_transactions_current table with different states on

different threads. The events_transactions_current table displays the current status of the

most recent monitored transaction event on the thread, and does not update this status when the

thread is idle. So the XA transaction can still be displayed in the PREPARED state for the original applier

thread, after it has been processed by another thread. To positively identify XA transactions that are

still in the PREPARED state and need to be recovered, use the XA RECOVER statement rather than the

Performance Schema transaction tables.

The following restrictions exist for using XA transactions in MySQL 5.7.7 and later:

• XA transactions are not fully resilient to an unexpected halt with respect to the binary log. If there is

an unexpected halt while the server is in the middle of executing an XA PREPARE, XA COMMIT, XA

ROLLBACK, or XA COMMIT ... ONE PHASE statement, the server might not be able to recover

to a correct state, leaving the server and the binary log in an inconsistent state. In this situation, the

binary log might either contain extra XA transactions that are not applied, or miss XA transactions

that are applied. Also, if GTIDs are enabled, after recovery @@GLOBAL.GTID_EXECUTED might

not correctly describe the transactions that have been applied. Note that if an unexpected halt

occurs before XA PREPARE, between XA PREPARE and XA COMMIT (or XA ROLLBACK), or after

XA COMMIT (or XA ROLLBACK), the server and binary log are correctly recovered and taken to a

consistent state.

• The use of replication filters or binary log filters in combination with XA transactions is not

supported. Filtering of tables could cause an XA transaction to be empty on a replica, and empty XA

transactions are not supported. Also, with the settings master_info_repository=TABLE and

relay_log_info_repository=TABLE on a replica, which became the defaults in MySQL 8.0, the

internal state of the data engine transaction is changed following a filtered XA transaction, and can

become inconsistent with the replication transaction context state.

The error ER_XA_REPLICATION_FILTERS is logged whenever an XA transaction is impacted by a

replication filter, whether or not the transaction was empty as a result. If the transaction is not empty,

the replica is able to continue running, but you should take steps to discontinue the use of replication

filters with XA transactions in order to avoid potential issues. If the transaction is empty, the replica

stops. In that event, the replica might be in an undetermined state in which the consistency of the

replication process might be compromised. In particular, the gtid_executed set on a replica of

the replica might be inconsistent with that on the source. To resolve this situation, isolate the source

and stop all replication, then check GTID consistency across the replication topology. Undo the XA

transaction that generated the error message, then restart replication.

• Prior to MySQL 5.7.19, FLUSH TABLES WITH READ LOCK is not compatible with XA transactions.

• XA transactions are considered unsafe for statement-based replication. If two XA transactions

committed in parallel on the source are being prepared on the replica in the inverse order, locking

dependencies can occur that cannot be safely resolved, and it is possible for replication to fail with

deadlock on the replica. This situation can occur for a single-threaded or multithreaded replica.

When binlog_format=STATEMENT is set, a warning is issued for DML statements inside XA

transactions. When binlog_format=MIXED or binlog_format=ROW is set, DML statements

inside XA transactions are logged using row-based replication, and the potential issue is not present.

Chapter 7 Restrictions on Character Sets

• Identifiers are stored in mysql database tables (user, db, and so forth) using utf8, but identifiers

can contain only characters in the Basic Multilingual Plane (BMP). Supplementary characters are not

permitted in identifiers.

• The ucs2, utf16, utf16le, and utf32 character sets have the following restrictions:

• None of them can be used as the client character set. See Impermissible Client Character Sets.

• It is currently not possible to use LOAD DATA to load data files that use these character sets.

• FULLTEXT indexes cannot be created on a column that uses any of these character sets.

However, you can perform IN BOOLEAN MODE searches on the column without an index.

• The use of ENCRYPT() with these character sets is not recommended because the underlying

system call expects a string terminated by a zero byte.

• The REGEXP and RLIKE operators work in byte-wise fashion, so they are not multibyte safe and

may produce unexpected results with multibyte character sets. In addition, these operators compare

characters by their byte values and accented characters may not compare as equal even if a given

collation treats them as equal.

Chapter 8 Restrictions on Performance Schema

The Performance Schema avoids using mutexes to collect or produce data, so there are no guarantees

of consistency and results can sometimes be incorrect. Event values in performance_schema tables

are nondeterministic and nonrepeatable.

If you save event information in another table, you should not assume that the original events are still

available later. For example, if you select events from a performance_schema table into a temporary

table, intending to join that table with the original table later, there might be no matches.

mysqldump and BACKUP DATABASE ignore tables in the performance_schema database.

Tables in the performance_schema database cannot be locked with LOCK TABLES, except the

setup_xxx tables.

Tables in the performance_schema database cannot be indexed.

Results for queries that refer to tables in the performance_schema database are not saved in the

query cache.

Tables in the performance_schema database are not replicated.

The Performance Schema is not available in libmysqld, the embedded server.

The types of timers might vary per platform. The performance_timers table shows which event

timers are available. If the values in this table for a given timer name are NULL, that timer is not

supported on your platform.

Instruments that apply to storage engines might not be implemented for all storage engines.

Instrumentation of each third-party engine is the responsibility of the engine maintainer.

Chapter 9 Restrictions on Pluggable Authentication

The first part of this section describes general restrictions on the applicability of the pluggable

authentication framework described at Pluggable Authentication. The second part describes how third-

party connector developers can determine the extent to which a connector can take advantage of

pluggable authentication capabilities and what steps to take to become more compliant.

The term “native authentication” used here refers to authentication against passwords stored in

the mysql.user system table. This is the same authentication method provided by older MySQL

servers, before pluggable authentication was implemented. “Windows native authentication” refers to

authentication using the credentials of a user who has already logged in to Windows, as implemented

by the Windows Native Authentication plugin (“Windows plugin” for short).

• General Pluggable Authentication Restrictions

• Pluggable Authentication and Third-Party Connectors

General Pluggable Authentication Restrictions

• Connector/C++: Clients that use this connector can connect to the server only through accounts that

use native authentication.

Exception: A connector supports pluggable authentication if it was built to link to libmysqlclient

dynamically (rather than statically) and it loads the current version of libmysqlclient if that

version is installed, or if the connector is recompiled from source to link against the current

libmysqlclient.

• Connector/NET: Clients that use Connector/NET can connect to the server through accounts that

use native authentication or Windows native authentication.

• Connector/PHP: Clients that use this connector can connect to the server only through accounts

that use native authentication, when compiled using the MySQL native driver for PHP (mysqlnd).

• Windows native authentication: Connecting through an account that uses the Windows plugin

requires Windows Domain setup. Without it, NTLM authentication is used and then only local

connections are possible; that is, the client and server must run on the same computer.

• Proxy users: Proxy user support is available to the extent that clients can connect through accounts

authenticated with plugins that implement proxy user capability (that is, plugins that can return a

user name different from that of the connecting user). For example, the PAM and Windows plugins

support proxy users. The mysql_native_password and sha256_password authentication

plugins do not support proxy users by default, but can be configured to do so; see Server Support for

Proxy User Mapping.

• Replication: Replicas can employ not only source accounts using native authentication, but can

also connect through source accounts that use nonnative authentication if the required client-side

plugin is available. If the plugin is built into libmysqlclient, it is available by default. Otherwise,

the plugin must be installed on the replica side in the directory named by the replica plugin_dir

system variable.

• FEDERATED tables: A FEDERATED table can access the remote table only through accounts on the

remote server that use native authentication.

Pluggable Authentication and Third-Party Connectors

Third-party connector developers can use the following guidelines to determine readiness of a

connector to take advantage of pluggable authentication capabilities and what steps to take to become

more compliant:

• An existing connector to which no changes have been made uses native authentication and

clients that use the connector can connect to the server only through accounts that use native

Pluggable Authentication and Third-Party Connectors

authentication. However, you should test the connector against a recent version of the server to

verify that such connections still work without problem.

Exception: A connector might work with pluggable authentication without any changes if it links

to libmysqlclient dynamically (rather than statically) and it loads the current version of

libmysqlclient if that version is installed.

• To take advantage of pluggable authentication capabilities, a connector that is libmysqlclient-

based should be relinked against the current version of libmysqlclient. This enables the

connector to support connections though accounts that require client-side plugins now built into

libmysqlclient (such as the cleartext plugin needed for PAM authentication and the Windows

plugin needed for Windows native authentication). Linking with a current libmysqlclient also

enables the connector to access client-side plugins installed in the default MySQL plugin directory

(typically the directory named by the default value of the local server's plugin_dir system

variable).

If a connector links to libmysqlclient dynamically, it must be ensured that the newer version of

libmysqlclient is installed on the client host and that the connector loads it at runtime.

• Another way for a connector to support a given authentication method is to implement it directly in

the client/server protocol. Connector/NET uses this approach to provide support for Windows native

authentication.

• If a connector should be able to load client-side plugins from a directory different from the default

plugin directory, it must implement some means for client users to specify the directory. Possibilities

for this include a command-line option or environment variable from which the connector can obtain

the directory name. Standard MySQL client programs such as mysql and mysqladmin implement a

--plugin-dir option. See also C API Client Plugin Interface.

• Proxy user support by a connector depends, as described earlier in this section, on whether the

authentication methods that it supports permit proxy users.

Chapter 10 Restrictions and Limitations on Partitioning

Table of Contents

10.1 Partitioning Keys, Primary Keys, and Unique Keys ............................................................... 27

10.2 Partitioning Limitations Relating to Storage Engines .............................................................. 31

10.3 Partitioning Limitations Relating to Functions ........................................................................ 32

10.4 Partitioning and Locking ...................................................................................................... 33

This section discusses current restrictions and limitations on MySQL partitioning support.

Prohibited constructs. The following constructs are not permitted in partitioning expressions:

• Stored procedures, stored functions, loadable functions, or plugins.

• Declared variables or user variables.

For a list of SQL functions which are permitted in partitioning expressions, see Section 10.3,

“Partitioning Limitations Relating to Functions”.

Arithmetic and logical operators. Use of the arithmetic operators +, -, and * is permitted in

partitioning expressions. However, the result must be an integer value or NULL (except in the case of

[LINEAR] KEY partitioning, as discussed elsewhere in this chapter; see Partitioning Types, for more

information).

The DIV operator is also supported, and the / operator is not permitted. (Bug #30188, Bug #33182)

The bit operators |, &, ^, <<, >>, and ~ are not permitted in partitioning expressions.

HANDLER statements. Previously, the HANDLER statement was not supported with partitioned

tables. This limitation is removed beginning with MySQL 5.7.1.

Server SQL mode. Tables employing user-defined partitioning do not preserve the SQL mode

in effect at the time that they were created. As discussed in Server SQL Modes, the results of many

MySQL functions and operators may change according to the server SQL mode. Therefore, a change

in the SQL mode at any time after the creation of partitioned tables may lead to major changes in the

behavior of such tables, and could easily lead to corruption or loss of data. For these reasons, it is

strongly recommended that you never change the server SQL mode after creating partitioned tables.

Examples. The following examples illustrate some changes in behavior of partitioned tables due to

a change in the server SQL mode:

1. Error handling. Suppose that you create a partitioned table whose partitioning expression is

one such as column DIV 0 or column MOD 0, as shown here:

mysql> CREATE TABLE tn (c1 INT)

-> PARTITION BY LIST(1 DIV c1) (

-> PARTITION p0 VALUES IN (NULL),

-> PARTITION p1 VALUES IN (1)

-> );

Query OK, 0 rows affected (0.05 sec)

The default behavior for MySQL is to return NULL for the result of a division by zero, without

producing any errors:

mysql> SELECT @@sql_mode;

+------------+

| @@sql_mode |

+------------+

| |

+------------+

1 row in set (0.00 sec)

mysql> INSERT INTO tn VALUES (NULL), (0), (1);

Query OK, 3 rows affected (0.00 sec)

Records: 3 Duplicates: 0 Warnings: 0

However, changing the server SQL mode to treat division by zero as an error and to enforce strict

error handling causes the same INSERT statement to fail, as shown here:

mysql> SET sql_mode='STRICT_ALL_TABLES,ERROR_FOR_DIVISION_BY_ZERO';

Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO tn VALUES (NULL), (0), (1);

ERROR 1365 (22012): Division by 0

2. Table accessibility. Sometimes a change in the server SQL mode can make partitioned tables

unusable. The following CREATE TABLE statement can be executed successfully only if the

NO_UNSIGNED_SUBTRACTION mode is in effect:

mysql> SELECT @@sql_mode;

+------------+

| @@sql_mode |

+------------+

| |

+------------+

1 row in set (0.00 sec)

mysql> CREATE TABLE tu (c1 BIGINT UNSIGNED)

-> PARTITION BY RANGE(c1 - 10) (

-> PARTITION p0 VALUES LESS THAN (-5),

-> PARTITION p1 VALUES LESS THAN (0),

-> PARTITION p2 VALUES LESS THAN (5),

-> PARTITION p3 VALUES LESS THAN (10),

-> PARTITION p4 VALUES LESS THAN (MAXVALUE)

-> );

ERROR 1563 (HY000): Partition constant is out of partition function domain

mysql> SET sql_mode='NO_UNSIGNED_SUBTRACTION';

Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @@sql_mode;

+-------------------------+

| @@sql_mode |

+-------------------------+

| NO_UNSIGNED_SUBTRACTION |

+-------------------------+

1 row in set (0.00 sec)

mysql> CREATE TABLE tu (c1 BIGINT UNSIGNED)

-> PARTITION BY RANGE(c1 - 10) (

-> PARTITION p0 VALUES LESS THAN (-5),

-> PARTITION p1 VALUES LESS THAN (0),

-> PARTITION p2 VALUES LESS THAN (5),

-> PARTITION p3 VALUES LESS THAN (10),

-> PARTITION p4 VALUES LESS THAN (MAXVALUE)

-> );

Query OK, 0 rows affected (0.05 sec)

If you remove the NO_UNSIGNED_SUBTRACTION server SQL mode after creating tu, you may no

longer be able to access this table:

mysql> SET sql_mode='';

Query OK, 0 rows affected (0.00 sec)

mysql> SELECT * FROM tu;

ERROR 1563 (HY000): Partition constant is out of partition function domain

mysql> INSERT INTO tu VALUES (20);

ERROR 1563 (HY000): Partition constant is out of partition function domain

See also File system operations.

Query cache not supported.

The query cache is not supported for partitioned tables, and is automatically disabled for queries

involving partitioned tables. The query cache cannot be enabled for such queries.

Per-partition key caches.

In MySQL 5.7, key caches are supported for partitioned MyISAM tables, using the CACHE INDEX and

LOAD INDEX INTO CACHE statements. Key caches may be defined for one, several, or all partitions,

and indexes for one, several, or all partitions may be preloaded into key caches.

Foreign keys not supported for partitioned InnoDB tables.

Partitioned tables using the InnoDB storage engine do not support foreign keys. More specifically, this

means that the following two statements are true:

1. No definition of an InnoDB table employing user-defined partitioning may contain foreign key

references; no InnoDB table whose definition contains foreign key references may be partitioned.

2. No InnoDB table definition may contain a foreign key reference to a user-partitioned table; no

InnoDB table with user-defined partitioning may contain columns referenced by foreign keys.

The scope of the restrictions just listed includes all tables that use the InnoDB storage engine. CREATE

TABLE and ALTER TABLE statements that would result in tables violating these restrictions are not

allowed.

ALTER TABLE ... ORDER BY. An ALTER TABLE ... ORDER BY column statement run against

a partitioned table causes ordering of rows only within each partition.

Effects on REPLACE statements by modification of primary keys. It can be desirable in some

cases (see Section 10.1, “Partitioning Keys, Primary Keys, and Unique Keys”) to modify a table's

primary key. Be aware that, if your application uses REPLACE statements and you do this, the results

of these statements can be drastically altered. See REPLACE Statement, for more information and an

example.

FULLTEXT indexes.

Partitioned tables do not support FULLTEXT indexes or searches, even for partitioned tables employing

the InnoDB or MyISAM storage engine.

Spatial columns. Columns with spatial data types such as POINT or GEOMETRY cannot be used in

partitioned tables.

Temporary tables.

Temporary tables cannot be partitioned. (Bug #17497)

Log tables. It is not possible to partition the log tables; an ALTER TABLE ... PARTITION

BY ... statement on such a table fails with an error.

Data type of partitioning key.

A partitioning key must be either an integer column or an expression that resolves to an integer.

Expressions employing ENUM columns cannot be used. The column or expression value may also be

NULL. (See How MySQL Partitioning Handles NULL.)

There are two exceptions to this restriction:

1. When partitioning by [LINEAR] KEY, it is possible to use columns of any valid MySQL data type

other than TEXT or BLOB as partitioning keys, because MySQL's internal key-hashing functions

produce the correct data type from these types. For example, the following two CREATE TABLE

statements are valid:

CREATE TABLE tkc (c1 CHAR)

PARTITION BY KEY(c1)

PARTITIONS 4;

CREATE TABLE tke

( c1 ENUM('red', 'orange', 'yellow', 'green', 'blue', 'indigo', 'violet') )

PARTITION BY LINEAR KEY(c1)

PARTITIONS 6;

2. When partitioning by RANGE COLUMNS or LIST COLUMNS, it is possible to use string, DATE, and

DATETIME columns. For example, each of the following CREATE TABLE statements is valid:

CREATE TABLE rc (c1 INT, c2 DATE)

PARTITION BY RANGE COLUMNS(c2) (

PARTITION p0 VALUES LESS THAN('1990-01-01'),

PARTITION p1 VALUES LESS THAN('1995-01-01'),

PARTITION p2 VALUES LESS THAN('2000-01-01'),

PARTITION p3 VALUES LESS THAN('2005-01-01'),

PARTITION p4 VALUES LESS THAN(MAXVALUE)

);

CREATE TABLE lc (c1 INT, c2 CHAR(1))

PARTITION BY LIST COLUMNS(c2) (

PARTITION p0 VALUES IN('a', 'd', 'g', 'j', 'm', 'p', 's', 'v', 'y'),

PARTITION p1 VALUES IN('b', 'e', 'h', 'k', 'n', 'q', 't', 'w', 'z'),

PARTITION p2 VALUES IN('c', 'f', 'i', 'l', 'o', 'r', 'u', 'x', NULL)

);

Neither of the preceding exceptions applies to BLOB or TEXT column types.

Subqueries.

A partitioning key may not be a subquery, even if that subquery resolves to an integer value or NULL.

Column index prefixes not supported for key partitioning. When creating a table that is

partitioned by key, any columns in the partitioning key which use column prefixes are not used in

the table's partitioning function. Consider the following CREATE TABLE statement, which has three

VARCHAR columns, and whose primary key uses all three columns and specifies prefixes for two of

them:

CREATE TABLE t1 (

a VARCHAR(10000),

b VARCHAR(25),

c VARCHAR(10),

PRIMARY KEY (a(10), b, c(2))

) PARTITION BY KEY() PARTITIONS 2;

This statement is accepted, but the resulting table is actually created as if you had issued the following

statement, using only the primary key column which does not include a prefix (column b) for the

partitioning key:

CREATE TABLE t1 (

a VARCHAR(10000),

b VARCHAR(25),

c VARCHAR(10),

PRIMARY KEY (a(10), b, c(2))

) PARTITION BY KEY(b) PARTITIONS 2;

No warning is issued or any other indication provided that this has occurred, except in the event that all

columns specified for the partitioning key use prefixes, in which case the statement fails with the error

message shown here:

mysql> CREATE TABLE t2 (

-> a VARCHAR(10000),

-> b VARCHAR(25),

-> c VARCHAR(10),

-> PRIMARY KEY (a(10), b(5), c(2))

-> ) PARTITION BY KEY() PARTITIONS 2;

ERROR 1503 (HY000): A PRIMARY KEY must include all columns in the

table's partitioning function

This also occurs when altering or upgrading such tables, and includes cases in which the columns

used in the partitioning function are defined implicitly as those in the table's primary key by employing

an empty PARTITION BY KEY() clause.

This is a known issue which is addressed in MySQL 8.0 by deprecating the permissive behavior; in

MYSQL 8.0, if any columns using prefixes are included in a table's partitioning function, the server logs

an appropriate warning for each such column, or raises a descriptive error if necessary. (Allowing the

use of columns with prefixes in partitioning keys is subject to removal altogether in a future version of

MySQL.)

For general information about partitioning tables by key, see KEY Partitioning.

Issues with subpartitions.

Subpartitions must use HASH or KEY partitioning. Only RANGE and LIST partitions may be

subpartitioned; HASH and KEY partitions cannot be subpartitioned.

SUBPARTITION BY KEY requires that the subpartitioning column or columns be specified explicitly,

unlike the case with PARTITION BY KEY, where it can be omitted (in which case the table's primary

key column is used by default). Consider the table created by this statement:

CREATE TABLE ts (

id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,

name VARCHAR(30)

);

You can create a table having the same columns, partitioned by KEY, using a statement such as this

one:

CREATE TABLE ts (

id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,

name VARCHAR(30)

)

PARTITION BY KEY()

PARTITIONS 4;

The previous statement is treated as though it had been written like this, with the table's primary key

column used as the partitioning column:

CREATE TABLE ts (

id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,

name VARCHAR(30)

)

PARTITION BY KEY(id)

PARTITIONS 4;

However, the following statement that attempts to create a subpartitioned table using the default

column as the subpartitioning column fails, and the column must be specified for the statement to

succeed, as shown here:

mysql> CREATE TABLE ts (

Partitioning Keys, Primary Keys, and Unique Keys

-> id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,

-> name VARCHAR(30)

-> )

-> PARTITION BY RANGE(id)

-> SUBPARTITION BY KEY()

-> SUBPARTITIONS 4

-> (

-> PARTITION p0 VALUES LESS THAN (100),

-> PARTITION p1 VALUES LESS THAN (MAXVALUE)

-> );

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that

corresponds to your MySQL server version for the right syntax to use near ')

mysql> CREATE TABLE ts (

-> id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,

-> name VARCHAR(30)

-> )

-> PARTITION BY RANGE(id)

-> SUBPARTITION BY KEY(id)

-> SUBPARTITIONS 4

-> (

-> PARTITION p0 VALUES LESS THAN (100),

-> PARTITION p1 VALUES LESS THAN (MAXVALUE)

-> );

Query OK, 0 rows affected (0.07 sec)

This is a known issue (see Bug #51470).

DATA DIRECTORY and INDEX DIRECTORY options. DATA DIRECTORY and INDEX

DIRECTORY are subject to the following restrictions when used with partitioned tables:

• Table-level DATA DIRECTORY and INDEX DIRECTORY options are ignored (see Bug #32091).

• On Windows, the DATA DIRECTORY and INDEX DIRECTORY options are not supported for

individual partitions or subpartitions of MyISAM tables. However, you can use DATA DIRECTORY for

individual partitions or subpartitions of InnoDB tables.

Repairing and rebuilding partitioned tables. The statements CHECK TABLE, OPTIMIZE TABLE,

ANALYZE TABLE, and REPAIR TABLE are supported for partitioned tables.

In addition, you can use ALTER TABLE ... REBUILD PARTITION to rebuild one or more partitions

of a partitioned table; ALTER TABLE ... REORGANIZE PARTITION also causes partitions to be

rebuilt. See ALTER TABLE Statement, for more information about these two statements.

Starting in MySQL 5.7.2, ANALYZE, CHECK, OPTIMIZE, REPAIR, and TRUNCATE operations are

supported with subpartitions. REBUILD was also accepted syntax prior to MySQL 5.7.5, although this

had no effect. (Bug #19075411, Bug #73130) See also ALTER TABLE Partition Operations.

mysqlcheck, myisamchk, and myisampack are not supported with partitioned tables.

FOR EXPORT option (FLUSH TABLES). The FLUSH TABLES statement's FOR EXPORT option is

not supported for partitioned InnoDB tables in MySQL 5.7.4 and earlier. (Bug #16943907)

File name delimiters for partitions and subpartitions. Table partition and subpartition file names

include generated delimiters such as #P# and #SP#. The lettercase of such delimiters can vary and

should not be depended upon.

10.1 Partitioning Keys, Primary Keys, and Unique Keys

This section discusses the relationship of partitioning keys with primary keys and unique keys. The rule

governing this relationship can be expressed as follows: All columns used in the partitioning expression

for a partitioned table must be part of every unique key that the table may have.

In other words, every unique key on the table must use every column in the table's partitioning

expression. (This also includes the table's primary key, since it is by definition a unique key. This

particular case is discussed later in this section.) For example, each of the following table creation

statements is invalid:

Partitioning Keys, Primary Keys, and Unique Keys

CREATE TABLE t1 (

col1 INT NOT NULL,

col2 DATE NOT NULL,

col3 INT NOT NULL,

col4 INT NOT NULL,

UNIQUE KEY (col1, col2)

)

PARTITION BY HASH(col3)

PARTITIONS 4;

CREATE TABLE t2 (

col1 INT NOT NULL,

col2 DATE NOT NULL,

col3 INT NOT NULL,

col4 INT NOT NULL,

UNIQUE KEY (col1),

UNIQUE KEY (col3)

)

PARTITION BY HASH(col1 + col3)

PARTITIONS 4;

In each case, the proposed table would have at least one unique key that does not include all columns

used in the partitioning expression.

Each of the following statements is valid, and represents one way in which the corresponding invalid

table creation statement could be made to work:

CREATE TABLE t1 (

col1 INT NOT NULL,

col2 DATE NOT NULL,

col3 INT NOT NULL,

col4 INT NOT NULL,

UNIQUE KEY (col1, col2, col3)

)

PARTITION BY HASH(col3)

PARTITIONS 4;

CREATE TABLE t2 (

col1 INT NOT NULL,

col2 DATE NOT NULL,

col3 INT NOT NULL,

col4 INT NOT NULL,

UNIQUE KEY (col1, col3)

)

PARTITION BY HASH(col1 + col3)

PARTITIONS 4;

This example shows the error produced in such cases:

mysql> CREATE TABLE t3 (

-> col1 INT NOT NULL,

-> col2 DATE NOT NULL,

-> col3 INT NOT NULL,

-> col4 INT NOT NULL,

-> UNIQUE KEY (col1, col2),

-> UNIQUE KEY (col3)

-> )

-> PARTITION BY HASH(col1 + col3)

-> PARTITIONS 4;

ERROR 1491 (HY000): A PRIMARY KEY must include all columns in the table's partitioning function

The CREATE TABLE statement fails because both col1 and col3 are included in the proposed

partitioning key, but neither of these columns is part of both of unique keys on the table. This shows

one possible fix for the invalid table definition:

mysql> CREATE TABLE t3 (

-> col1 INT NOT NULL,

-> col2 DATE NOT NULL,

-> col3 INT NOT NULL,

-> col4 INT NOT NULL,

-> UNIQUE KEY (col1, col2, col3),

-> UNIQUE KEY (col3)

Partitioning Keys, Primary Keys, and Unique Keys

-> )

-> PARTITION BY HASH(col3)

-> PARTITIONS 4;

Query OK, 0 rows affected (0.05 sec)

In this case, the proposed partitioning key col3 is part of both unique keys, and the table creation

statement succeeds.

The following table cannot be partitioned at all, because there is no way to include in a partitioning key

any columns that belong to both unique keys:

CREATE TABLE t4 (

col1 INT NOT NULL,

col2 INT NOT NULL,

col3 INT NOT NULL,

col4 INT NOT NULL,

UNIQUE KEY (col1, col3),

UNIQUE KEY (col2, col4)

);

Since every primary key is by definition a unique key, this restriction also includes the table's primary

key, if it has one. For example, the next two statements are invalid:

CREATE TABLE t5 (

col1 INT NOT NULL,

col2 DATE NOT NULL,

col3 INT NOT NULL,

col4 INT NOT NULL,

PRIMARY KEY(col1, col2)

)

PARTITION BY HASH(col3)

PARTITIONS 4;

CREATE TABLE t6 (

col1 INT NOT NULL,

col2 DATE NOT NULL,

col3 INT NOT NULL,

col4 INT NOT NULL,

PRIMARY KEY(col1, col3),

UNIQUE KEY(col2)

)

PARTITION BY HASH( YEAR(col2) )

PARTITIONS 4;

In both cases, the primary key does not include all columns referenced in the partitioning expression.

However, both of the next two statements are valid:

CREATE TABLE t7 (

col1 INT NOT NULL,

col2 DATE NOT NULL,

col3 INT NOT NULL,

col4 INT NOT NULL,

PRIMARY KEY(col1, col2)

)

PARTITION BY HASH(col1 + YEAR(col2))

PARTITIONS 4;

CREATE TABLE t8 (

col1 INT NOT NULL,

col2 DATE NOT NULL,

col3 INT NOT NULL,

col4 INT NOT NULL,

PRIMARY KEY(col1, col2, col4),

UNIQUE KEY(col2, col1)

)

PARTITION BY HASH(col1 + YEAR(col2))

PARTITIONS 4;

If a table has no unique keys—this includes having no primary key—then this restriction does not

apply, and you may use any column or columns in the partitioning expression as long as the column

type is compatible with the partitioning type.

Partitioning Keys, Primary Keys, and Unique Keys

For the same reason, you cannot later add a unique key to a partitioned table unless the key includes

all columns used by the table's partitioning expression. Consider the partitioned table created as shown

here:

mysql> CREATE TABLE t_no_pk (c1 INT, c2 INT)

-> PARTITION BY RANGE(c1) (

-> PARTITION p0 VALUES LESS THAN (10),

-> PARTITION p1 VALUES LESS THAN (20),

-> PARTITION p2 VALUES LESS THAN (30),

-> PARTITION p3 VALUES LESS THAN (40)

-> );

Query OK, 0 rows affected (0.12 sec)

It is possible to add a primary key to t_no_pk using either of these ALTER TABLE statements:

# possible PK

mysql> ALTER TABLE t_no_pk ADD PRIMARY KEY(c1);

Query OK, 0 rows affected (0.13 sec)

Records: 0 Duplicates: 0 Warnings: 0

# drop this PK

mysql> ALTER TABLE t_no_pk DROP PRIMARY KEY;

Query OK, 0 rows affected (0.10 sec)

Records: 0 Duplicates: 0 Warnings: 0

# use another possible PK

mysql> ALTER TABLE t_no_pk ADD PRIMARY KEY(c1, c2);

Query OK, 0 rows affected (0.12 sec)

Records: 0 Duplicates: 0 Warnings: 0

# drop this PK

mysql> ALTER TABLE t_no_pk DROP PRIMARY KEY;

Query OK, 0 rows affected (0.09 sec)

Records: 0 Duplicates: 0 Warnings: 0

However, the next statement fails, because c1 is part of the partitioning key, but is not part of the

proposed primary key:

# fails with error 1503

mysql> ALTER TABLE t_no_pk ADD PRIMARY KEY(c2);

ERROR 1503 (HY000): A PRIMARY KEY must include all columns in the table's partitioning function

Since t_no_pk has only c1 in its partitioning expression, attempting to adding a unique key on c2

alone fails. However, you can add a unique key that uses both c1 and c2.

These rules also apply to existing nonpartitioned tables that you wish to partition using ALTER

TABLE ... PARTITION BY. Consider a table np_pk created as shown here:

mysql> CREATE TABLE np_pk (

-> id INT NOT NULL AUTO_INCREMENT,

-> name VARCHAR(50),

-> added DATE,

-> PRIMARY KEY (id)

-> );

Query OK, 0 rows affected (0.08 sec)

The following ALTER TABLE statement fails with an error, because the added column is not part of

any unique key in the table:

mysql> ALTER TABLE np_pk

-> PARTITION BY HASH( TO_DAYS(added) )

-> PARTITIONS 4;

ERROR 1503 (HY000): A PRIMARY KEY must include all columns in the table's partitioning function

However, this statement using the id column for the partitioning column is valid, as shown here:

mysql> ALTER TABLE np_pk

-> PARTITION BY HASH(id)

-> PARTITIONS 4;

Query OK, 0 rows affected (0.11 sec)

Records: 0 Duplicates: 0 Warnings: 0

Partitioning Limitations Relating to Storage Engines

In the case of np_pk, the only column that may be used as part of a partitioning expression is id; if

you wish to partition this table using any other column or columns in the partitioning expression, you

must first modify the table, either by adding the desired column or columns to the primary key, or by

dropping the primary key altogether.

10.2 Partitioning Limitations Relating to Storage Engines

The following limitations apply to the use of storage engines with user-defined partitioning of tables.

MERGE storage engine. User-defined partitioning and the MERGE storage engine are not

compatible. Tables using the MERGE storage engine cannot be partitioned. Partitioned tables cannot be

merged.

FEDERATED storage engine. Partitioning of FEDERATED tables is not supported; it is not possible

to create partitioned FEDERATED tables.

CSV storage engine. Partitioned tables using the CSV storage engine are not supported; it is not

possible to create partitioned CSV tables.

InnoDB storage engine. InnoDB foreign keys and MySQL partitioning are not compatible.

Partitioned InnoDB tables cannot have foreign key references, nor can they have columns referenced

by foreign keys. InnoDB tables which have or which are referenced by foreign keys cannot be

partitioned.

InnoDB does not support the use of multiple disks for subpartitions. (This is currently supported only

by MyISAM.)

In addition, ALTER TABLE ... OPTIMIZE PARTITION does not work correctly with partitioned

tables that use the InnoDB storage engine. Use ALTER TABLE ... REBUILD PARTITION and

ALTER TABLE ... ANALYZE PARTITION, instead, for such tables. For more information, see

ALTER TABLE Partition Operations.

User-defined partitioning and the NDB storage engine (NDB Cluster). Partitioning by KEY

(including LINEAR KEY) is the only type of partitioning supported for the NDB storage engine. It is

not possible under normal circumstances in NDB Cluster to create an NDB Cluster table using any

partitioning type other than [LINEAR] KEY, and attempting to do so fails with an error.

Exception (not for production): It is possible to override this restriction by setting the new system

variable on NDB Cluster SQL nodes to ON. If you choose to do this, you should be aware that tables

using partitioning types other than [LINEAR] KEY are not supported in production. In such cases,

you can create and use tables with partitioning types other than KEY or LINEAR KEY, but you do this

entirely at your own risk. You should also be aware that this functionality is now deprecated and subject

to removal without further notice in a future release of NDB Cluster.

The maximum number of partitions that can be defined for an NDB table depends on the number of

data nodes and node groups in the cluster, the version of the NDB Cluster software in use, and other

factors. See NDB and user-defined partitioning, for more information.

As of MySQL NDB Cluster 7.5.2, the maximum amount of fixed-size data that can be stored per

partition in an NDB table is 128 TB. Previously, this was 16 GB.

CREATE TABLE and ALTER TABLE statements that would cause a user-partitioned NDB table not to

meet either or both of the following two requirements are not permitted, and fail with an error:

1. The table must have an explicit primary key.

2. All columns listed in the table's partitioning expression must be part of the primary key.

Exception. If a user-partitioned NDB table is created using an empty column-list (that is, using

PARTITION BY KEY() or PARTITION BY LINEAR KEY()), then no explicit primary key is required.

Partitioning Limitations Relating to Functions

Partition selection. Partition selection is not supported for NDB tables. See Partition Selection, for

more information.

Upgrading partitioned tables. When performing an upgrade, tables which are partitioned by KEY

and which use any storage engine other than NDB must be dumped and reloaded.

Same storage engine for all partitions. All partitions of a partitioned table must use the same

storage engine and it must be the same storage engine used by the table as a whole. In addition, if one

does not specify an engine on the table level, then one must do either of the following when creating or

altering a partitioned table:

• Do not specify any engine for any partition or subpartition

• Specify the engine for all partitions or subpartitions

10.3 Partitioning Limitations Relating to Functions

This section discusses limitations in MySQL Partitioning relating specifically to functions used in

partitioning expressions.

Only the MySQL functions shown in the following list are allowed in partitioning expressions:

• ABS()

• CEILING() (see CEILING() and FLOOR())

• DATEDIFF()

• DAY()

• DAYOFMONTH()

• DAYOFWEEK()

• DAYOFYEAR()

• EXTRACT() (see EXTRACT() function with WEEK specifier)

• FLOOR() (see CEILING() and FLOOR())

• HOUR()

• MICROSECOND()

• MINUTE()

• MOD()

• MONTH()

• QUARTER()

• SECOND()

• TIME_TO_SEC()

• TO_DAYS()

• TO_SECONDS()

• UNIX_TIMESTAMP() (with TIMESTAMP columns)

• WEEKDAY()

Partitioning and Locking

• YEAR()

• YEARWEEK()

In MySQL 5.7, partition pruning is supported for the TO_DAYS(), TO_SECONDS(), YEAR(), and

UNIX_TIMESTAMP() functions. See Partition Pruning, for more information.

CEILING() and FLOOR(). Each of these functions returns an integer only if it is passed an argument

of an exact numeric type, such as one of the INT types or DECIMAL. This means, for example, that the

following CREATE TABLE statement fails with an error, as shown here:

mysql> CREATE TABLE t (c FLOAT) PARTITION BY LIST( FLOOR(c) )(

-> PARTITION p0 VALUES IN (1,3,5),

-> PARTITION p1 VALUES IN (2,4,6)

-> );

ERROR 1490 (HY000): The PARTITION function returns the wrong type

EXTRACT() function with WEEK specifier. The value returned by the EXTRACT() function, when

used as EXTRACT(WEEK FROM col), depends on the value of the default_week_format system

variable. For this reason, EXTRACT() is not permitted as a partitioning function when it specifies the

unit as WEEK. (Bug #54483)

See Mathematical Functions, for more information about the return types of these functions, as well as

Numeric Data Types.

10.4 Partitioning and Locking

For storage engines such as MyISAM that actually execute table-level locks when executing DML

or DDL statements, such a statement in older versions of MySQL (5.6.5 and earlier) that affected a

partitioned table imposed a lock on the table as a whole; that is, all partitions were locked until the

statement was finished. In MySQL 5.7, partition lock pruning eliminates unneeded locks in many cases,

and most statements reading from or updating a partitioned MyISAM table cause only the effected

partitions to be locked. For example, a SELECT from a partitioned MyISAM table locks only those

partitions actually containing rows that satisfy the SELECT statement's WHERE condition are locked.

For statements affecting partitioned tables using storage engines such as InnoDB, that employ row-

level locking and do not actually perform (or need to perform) the locks prior to partition pruning, this is

not an issue.

The next few paragraphs discuss the effects of partition lock pruning for various MySQL statements on

tables using storage engines that employ table-level locks.

Effects on DML statements

SELECT statements (including those containing unions or joins) lock only those partitions that actually

need to be read. This also applies to SELECT ... PARTITION.

An UPDATE prunes locks only for tables on which no partitioning columns are updated.

REPLACE and INSERT lock only those partitions having rows to be inserted or replaced. However, if an

AUTO_INCREMENT value is generated for any partitioning column then all partitions are locked.

INSERT ... ON DUPLICATE KEY UPDATE is pruned as long as no partitioning column is updated.

INSERT ... SELECT locks only those partitions in the source table that need to be read, although all

partitions in the target table are locked.

Locks imposed by LOAD DATA statements on partitioned tables cannot be pruned.

The presence of BEFORE INSERT or BEFORE UPDATE triggers using any partitioning column of a

partitioned table means that locks on INSERT and UPDATE statements updating this table cannot

Affected DDL statements

be pruned, since the trigger can alter its values: A BEFORE INSERT trigger on any of the table's

partitioning columns means that locks set by INSERT or REPLACE cannot be pruned, since the BEFORE

INSERT trigger may change a row's partitioning columns before the row is inserted, forcing the row

into a different partition than it would be otherwise. A BEFORE UPDATE trigger on a partitioning column

means that locks imposed by UPDATE or INSERT ... ON DUPLICATE KEY UPDATE cannot be

pruned.

Affected DDL statements

CREATE VIEW does not cause any locks.

ALTER TABLE ... EXCHANGE PARTITION prunes locks; only the exchanged table and the

exchanged partition are locked.

ALTER TABLE ... TRUNCATE PARTITION prunes locks; only the partitions to be emptied are

locked.

In addition, ALTER TABLE statements take metadata locks on the table level.

Other statements

LOCK TABLES cannot prune partition locks.

CALL stored_procedure(expr) supports lock pruning, but evaluating expr does not.

DO and SET statements do not support partitioning lock pruning.

Chapter 11 Windows Platform Restrictions

The following restrictions apply to use of MySQL on the Windows platform:

• Process memory

On Windows 32-bit platforms, it is not possible by default to use more than 2GB of RAM within a

single process, including MySQL. This is because the physical address limit on Windows 32-bit

is 4GB and the default setting within Windows is to split the virtual address space between kernel

(2GB) and user/applications (2GB).

Some versions of Windows have a boot time setting to enable larger applications by reducing the

kernel application. Alternatively, to use more than 2GB, use a 64-bit version of Windows.

• File system aliases

When using MyISAM tables, you cannot use aliases within Windows link to the data files on another

volume and then link back to the main MySQL datadir location.

This facility is often used to move the data and index files to a RAID or other fast solution, while

retaining the main .frm files in the default data directory configured with the datadir option.

• Limited number of ports

Windows systems have about 4,000 ports available for client connections, and after a connection on

a port closes, it takes two to four minutes before the port can be reused. In situations where clients

connect to and disconnect from the server at a high rate, it is possible for all available ports to be

used up before closed ports become available again. If this happens, the MySQL server appears to

be unresponsive even though it is running. Ports may be used by other applications running on the

machine as well, in which case the number of ports available to MySQL is lower.

For more information about this problem, see https://support.microsoft.com/kb/196271.

• DATA DIRECTORY and INDEX DIRECTORY

The DATA DIRECTORY clause of the CREATE TABLE statement is supported on Windows for

InnoDB tables only, as described in Creating Tables Externally. For MyISAM and other storage

engines, the DATA DIRECTORY and INDEX DIRECTORY clauses for CREATE TABLE are ignored on

Windows and any other platforms with a nonfunctional realpath() call.

• DROP DATABASE

You cannot drop a database that is in use by another session.

• Case-insensitive names

File names are not case-sensitive on Windows, so MySQL database and table names are also not

case-sensitive on Windows. The only restriction is that database and table names must be specified

using the same case throughout a given statement. See Identifier Case Sensitivity.

• Directory and file names

On Windows, MySQL Server supports only directory and file names that are compatible with the

current ANSI code pages. For example, the following Japanese directory name does not work in the

Western locale (code page 1252):

datadir="C:/私たちのプロジェクトのデータ"

The same limitation applies to directory and file names referred to in SQL statements, such as the

data file path name in LOAD DATA.

• The \ path name separator character

Path name components in Windows are separated by the \ character, which is also the escape

character in MySQL. If you are using LOAD DATA or SELECT ... INTO OUTFILE, use Unix-style

file names with / characters:

mysql> LOAD DATA INFILE 'C:/tmp/skr.txt' INTO TABLE skr;

mysql> SELECT * INTO OUTFILE 'C:/tmp/skr.txt' FROM skr;

Alternatively, you must double the \ character:

mysql> LOAD DATA INFILE 'C:\\tmp\\skr.txt' INTO TABLE skr;

mysql> SELECT * INTO OUTFILE 'C:\\tmp\\skr.txt' FROM skr;

• Problems with pipes

Pipes do not work reliably from the Windows command-line prompt. If the pipe includes the character

^Z / CHAR(24), Windows thinks that it has encountered end-of-file and aborts the program.

This is mainly a problem when you try to apply a binary log as follows:

C:\> mysqlbinlog binary_log_file | mysql --user=root

If you have a problem applying the log and suspect that it is because of a ^Z / CHAR(24) character,

you can use the following workaround:

C:\> mysqlbinlog binary_log_file --result-file=/tmp/bin.sql

C:\> mysql --user=root --execute "source /tmp/bin.sql"

The latter command also can be used to reliably read any SQL file that may contain binary data.

Chapter 12 Limits in MySQL

Table of Contents

12.1 Identifier Length Limits ........................................................................................................ 37

12.2 Grant Table Scope Column Properties ................................................................................. 38

12.3 Limits on Number of Databases and Tables ......................................................................... 38

12.4 Limits on Table Size ........................................................................................................... 38

12.5 Limits on Table Column Count and Row Size ....................................................................... 39

12.6 Limits Imposed by .frm File Structure ................................................................................... 42

This chapter lists current limits in MySQL 5.7.

12.1 Identifier Length Limits

The following table describes the maximum length for each type of identifier.

Identifier Type Maximum Length (characters)

Database 64 (NDB storage engine: 63)

Table 64 (NDB storage engine: 63)

Column 64

Index 64

Constraint 64

Stored Program 64

Tablespace 64

Server 64

Log File Group 64

Alias 256 (see exception following table)

Compound Statement Label 16

User-Defined Variable 64

Aliases for column names in CREATE VIEW statements are checked against the maximum column

length of 64 characters (not the maximum alias length of 256 characters).

For constraint definitions that include no constraint name, the server internally generates a name

derived from the associated table name. For example, internally generated foreign key constraint

names consist of the table name plus _ibfk_ and a number. If the table name is close to the length

limit for constraint names, the additional characters required for the constraint name may cause that

name to exceed the limit, resulting in an error.

Identifiers are stored using Unicode (UTF-8). This applies to identifiers in table definitions that are

stored in .frm files and to identifiers stored in the grant tables in the mysql database. The sizes of

the identifier string columns in the grant tables are measured in characters. You can use multibyte

characters without reducing the number of characters permitted for values stored in these columns.

NDB Cluster imposes a maximum length of 63 characters for names of databases and tables. See

Limits Associated with Database Objects in NDB Cluster.

Grant Table Scope Column Properties

Values such as user name and host names in MySQL account names are strings rather than

identifiers. For information about the maximum length of such values as stored in grant tables, see

Section 12.2, “Grant Table Scope Column Properties”.

12.2 Grant Table Scope Column Properties

Scope columns in the grant tables contain strings. The default value for each is the empty string. The

following table shows the number of characters permitted in each column.

Table 12.1 Grant Table Scope Column Lengths

Column Name Maximum Permitted Characters

Host, Proxied_host 60

User, Proxied_user 32

Password 41

Db 64

Table_name 64

Column_name 64

Routine_name 64

Host and Proxied_host values are converted to lowercase before being stored in the grant tables.

For access-checking purposes, comparisons of User, Proxied_user, Password,

authentication_string, Db, and Table_name values are case-sensitive. Comparisons of Host,

Proxied_host, Column_name, and Routine_name values are not case-sensitive.

12.3 Limits on Number of Databases and Tables

MySQL has no limit on the number of databases. The underlying file system may have a limit on the

number of directories.

MySQL has no limit on the number of tables. The underlying file system may have a limit on the

number of files that represent tables. Individual storage engines may impose engine-specific

constraints. InnoDB permits up to 4 billion tables.

12.4 Limits on Table Size

The effective maximum table size for MySQL databases is usually determined by operating system

constraints on file sizes, not by MySQL internal limits. For up-to-date information operating system file

size limits, refer to the documentation specific to your operating system.

Windows users, please note that FAT and VFAT (FAT32) are not considered suitable for production

use with MySQL. Use NTFS instead.

If you encounter a full-table error, there are several reasons why it might have occurred:

• The disk might be full.

• You are using InnoDB tables and have run out of room in an InnoDB tablespace file. The maximum

tablespace size is also the maximum size for a table. For tablespace size limits, see InnoDB Limits.

Generally, partitioning of tables into multiple tablespace files is recommended for tables larger than

1TB in size.

Limits on Table Column Count and Row Size

• You have hit an operating system file size limit. For example, you are using MyISAM tables on an

operating system that supports files only up to 2GB in size and you have hit this limit for the data file

or index file.

• You are using a MyISAM table and the space required for the table exceeds what is permitted by the

internal pointer size. MyISAM permits data and index files to grow up to 256TB by default, but this

limit can be changed up to the maximum permissible size of 65,536TB (256

− 1 bytes).

If you need a MyISAM table that is larger than the default limit and your operating system supports

large files, the CREATE TABLE statement supports AVG_ROW_LENGTH and MAX_ROWS options. See

CREATE TABLE Statement. The server uses these options to determine how large a table to permit.

If the pointer size is too small for an existing table, you can change the options with ALTER TABLE to

increase a table's maximum permissible size. See ALTER TABLE Statement.

ALTER TABLE tbl_name MAX_ROWS=1000000000 AVG_ROW_LENGTH=nnn;

You have to specify AVG_ROW_LENGTH only for tables with BLOB or TEXT columns; in this case,

MySQL cannot optimize the space required based only on the number of rows.

To change the default size limit for MyISAM tables, set the myisam_data_pointer_size, which

sets the number of bytes used for internal row pointers. The value is used to set the pointer size for

new tables if you do not specify the MAX_ROWS option. The value of myisam_data_pointer_size

can be from 2 to 7. For example, for tables that use the dynamic storage format, a value of 4 permits

tables up to 4GB; a value of 6 permits tables up to 256TB. Tables that use the fixed storage format

have a larger maximum data length. For storage format characteristics, see MyISAM Table Storage

Formats.

You can check the maximum data and index sizes by using this statement:

SHOW TABLE STATUS FROM db_name LIKE 'tbl_name';

You also can use myisamchk -dv /path/to/table-index-file. See SHOW Statements, or

myisamchk — MyISAM Table-Maintenance Utility.

Other ways to work around file-size limits for MyISAM tables are as follows:

• If your large table is read only, you can use myisampack to compress it. myisampack usually

compresses a table by at least 50%, so you can have, in effect, much bigger tables. myisampack

also can merge multiple tables into a single table. See myisampack — Generate Compressed,

Read-Only MyISAM Tables.

• MySQL includes a MERGE library that enables you to handle a collection of MyISAM tables that

have identical structure as a single MERGE table. See The MERGE Storage Engine.

• You are using the MEMORY (HEAP) storage engine; in this case you need to increase the value of the

max_heap_table_size system variable. See Server System Variables.

12.5 Limits on Table Column Count and Row Size

This section describes limits on the number of columns in tables and the size of individual rows.

• Column Count Limits

• Row Size Limits

Column Count Limits

MySQL has hard limit of 4096 columns per table, but the effective maximum may be less for a given

table. The exact column limit depends on several factors:

Row Size Limits

• The maximum row size for a table constrains the number (and possibly size) of columns because the

total length of all columns cannot exceed this size. See Row Size Limits.

• The storage requirements of individual columns constrain the number of columns that fit within a

given maximum row size. Storage requirements for some data types depend on factors such as

storage engine, storage format, and character set. See Data Type Storage Requirements.

• Storage engines may impose additional restrictions that limit table column count. For example,

InnoDB has a limit of 1017 columns per table. See InnoDB Limits. For information about other

storage engines, see Alternative Storage Engines.

• Each table has an .frm file that contains the table definition. The definition affects the content of this

file in ways that may affect the number of columns permitted in the table. See Section 12.6, “Limits

Imposed by .frm File Structure”.

Row Size Limits

The maximum row size for a given table is determined by several factors:

• The internal representation of a MySQL table has a maximum row size limit of 65,535 bytes, even if

the storage engine is capable of supporting larger rows. BLOB and TEXT columns only contribute 9

to 12 bytes toward the row size limit because their contents are stored separately from the rest of the

row.

• The maximum row size for an InnoDB table, which applies to data stored locally within a database

page, is slightly less than half a page for 4KB, 8KB, 16KB, and 32KB innodb_page_size settings.

For example, the maximum row size is slightly less than 8KB for the default 16KB InnoDB page size.

For 64KB pages, the maximum row size is slightly less than 16KB. See InnoDB Limits.

If a row containing variable-length columns exceeds the InnoDB maximum row size, InnoDB selects

variable-length columns for external off-page storage until the row fits within the InnoDB row size

limit. The amount of data stored locally for variable-length columns that are stored off-page differs by

row format. For more information, see InnoDB Row Formats.

• Different storage formats use different amounts of page header and trailer data, which affects the

amount of storage available for rows.

• For information about InnoDB row formats, see InnoDB Row Formats.

• For information about MyISAM storage formats, see MyISAM Table Storage Formats.

Row Size Limit Examples

• The MySQL maximum row size limit of 65,535 bytes is demonstrated in the following InnoDB and

MyISAM examples. The limit is enforced regardless of storage engine, even though the storage

engine may be capable of supporting larger rows.

mysql> CREATE TABLE t (a VARCHAR(10000), b VARCHAR(10000),

c VARCHAR(10000), d VARCHAR(10000), e VARCHAR(10000),

f VARCHAR(10000), g VARCHAR(6000)) ENGINE=InnoDB CHARACTER SET latin1;

ERROR 1118 (42000): Row size too large. The maximum row size for the used

table type, not counting BLOBs, is 65535. This includes storage overhead,

check the manual. You have to change some columns to TEXT or BLOBs

mysql> CREATE TABLE t (a VARCHAR(10000), b VARCHAR(10000),

c VARCHAR(10000), d VARCHAR(10000), e VARCHAR(10000),

f VARCHAR(10000), g VARCHAR(6000)) ENGINE=MyISAM CHARACTER SET latin1;

ERROR 1118 (42000): Row size too large. The maximum row size for the used

table type, not counting BLOBs, is 65535. This includes storage overhead,

check the manual. You have to change some columns to TEXT or BLOBs

In the following MyISAM example, changing a column to TEXT avoids the 65,535-byte row size limit

and permits the operation to succeed because BLOB and TEXT columns only contribute 9 to 12 bytes

toward the row size.

Row Size Limits

mysql> CREATE TABLE t (a VARCHAR(10000), b VARCHAR(10000),

c VARCHAR(10000), d VARCHAR(10000), e VARCHAR(10000),

f VARCHAR(10000), g TEXT(6000)) ENGINE=MyISAM CHARACTER SET latin1;

Query OK, 0 rows affected (0.02 sec)

The operation succeeds for an InnoDB table because changing a column to TEXT avoids the

MySQL 65,535-byte row size limit, and InnoDB off-page storage of variable-length columns avoids

the InnoDB row size limit.

mysql> CREATE TABLE t (a VARCHAR(10000), b VARCHAR(10000),

c VARCHAR(10000), d VARCHAR(10000), e VARCHAR(10000),

f VARCHAR(10000), g TEXT(6000)) ENGINE=InnoDB CHARACTER SET latin1;

Query OK, 0 rows affected (0.02 sec)

• Storage for variable-length columns includes length bytes, which are counted toward the row size.

For example, a VARCHAR(255) CHARACTER SET utf8mb3 column takes two bytes to store the

length of the value, so each value can take up to 767 bytes.

The statement to create table t1 succeeds because the columns require 32,765 + 2 bytes and

32,766 + 2 bytes, which falls within the maximum row size of 65,535 bytes:

mysql> CREATE TABLE t1

(c1 VARCHAR(32765) NOT NULL, c2 VARCHAR(32766) NOT NULL)

ENGINE = InnoDB CHARACTER SET latin1;

Query OK, 0 rows affected (0.02 sec)

The statement to create table t2 fails because, although the column length is within the maximum

length of 65,535 bytes, two additional bytes are required to record the length, which causes the row

size to exceed 65,535 bytes:

mysql> CREATE TABLE t2

(c1 VARCHAR(65535) NOT NULL)

ENGINE = InnoDB CHARACTER SET latin1;

ERROR 1118 (42000): Row size too large. The maximum row size for the used

table type, not counting BLOBs, is 65535. This includes storage overhead,

check the manual. You have to change some columns to TEXT or BLOBs

Reducing the column length to 65,533 or less permits the statement to succeed.

mysql> CREATE TABLE t2

(c1 VARCHAR(65533) NOT NULL)

ENGINE = InnoDB CHARACTER SET latin1;

Query OK, 0 rows affected (0.01 sec)

• For MyISAM tables, NULL columns require additional space in the row to record whether their values

are NULL. Each NULL column takes one bit extra, rounded up to the nearest byte.

The statement to create table t3 fails because MyISAM requires space for NULL columns in addition

to the space required for variable-length column length bytes, causing the row size to exceed 65,535

bytes:

mysql> CREATE TABLE t3

(c1 VARCHAR(32765) NULL, c2 VARCHAR(32766) NULL)

ENGINE = MyISAM CHARACTER SET latin1;

ERROR 1118 (42000): Row size too large. The maximum row size for the used

table type, not counting BLOBs, is 65535. This includes storage overhead,

check the manual. You have to change some columns to TEXT or BLOBs

For information about InnoDB NULL column storage, see InnoDB Row Formats.

Limits Imposed by .frm File Structure

• InnoDB restricts row size (for data stored locally within the database page) to slightly less than half

a database page for 4KB, 8KB, 16KB, and 32KB innodb_page_size settings, and to slightly less

than 16KB for 64KB pages.

The statement to create table t4 fails because the defined columns exceed the row size limit for a

16KB InnoDB page.

mysql> CREATE TABLE t4 (

c1 CHAR(255),c2 CHAR(255),c3 CHAR(255),

c4 CHAR(255),c5 CHAR(255),c6 CHAR(255),

c7 CHAR(255),c8 CHAR(255),c9 CHAR(255),

c10 CHAR(255),c11 CHAR(255),c12 CHAR(255),

c13 CHAR(255),c14 CHAR(255),c15 CHAR(255),

c16 CHAR(255),c17 CHAR(255),c18 CHAR(255),

c19 CHAR(255),c20 CHAR(255),c21 CHAR(255),

c22 CHAR(255),c23 CHAR(255),c24 CHAR(255),

c25 CHAR(255),c26 CHAR(255),c27 CHAR(255),

c28 CHAR(255),c29 CHAR(255),c30 CHAR(255),

c31 CHAR(255),c32 CHAR(255),c33 CHAR(255)

) ENGINE=InnoDB ROW_FORMAT=COMPACT DEFAULT CHARSET latin1;

ERROR 1118 (42000): Row size too large (> 8126). Changing some columns to TEXT or BLOB or using

ROW_FORMAT=DYNAMIC or ROW_FORMAT=COMPRESSED may help. In current row format, BLOB prefix of 768

bytes is stored inline.

12.6 Limits Imposed by .frm File Structure

As described previously, each table has an .frm file that contains the table definition. The server uses

the following expression to check some of the table information stored in the file against an upper limit

of 64KB:

if (info_length+(ulong) create_fields.elements*FCOMP+288+

n_length+int_length+com_length > 65535L || int_count > 255)

The portion of the information stored in the .frm file that is checked against the expression cannot

grow beyond the 64KB limit, so if the table definition reaches this size, no more columns can be added.

The relevant factors in the expression are:

• info_length is space needed for “screens.” This is related to MySQL's Unireg heritage.

• create_fields.elements is the number of columns.

• FCOMP is 17.

• n_length is the total length of all column names, including one byte per name as a separator.

• int_length is related to the list of values for ENUM and SET columns. In this context, “int” does not

mean “integer.” It means “interval,” a term that refers collectively to ENUM and SET columns.

• int_count is the number of unique ENUM and SET definitions.

• com_length is the total length of column comments.

The expression just described has several implications for permitted table definitions:

• Using long column names can reduce the maximum number of columns, as can the inclusion of

ENUM or SET columns, or use of column comments.

• A table can have no more than 255 unique ENUM and SET definitions. Columns with identical element

lists are considered the same against this limt. For example, if a table contains these two columns,

they count as one (not two) toward this limit because the definitions are identical:

e1 ENUM('a','b','c')

e2 ENUM('a','b','c')

Limits Imposed by .frm File Structure

• The sum of the length of element names in the unique ENUM and SET definitions counts toward the

64KB limit, so although the theoretical limit on number of elements in a given ENUM column is 65,535,

the practical limit is less than 3000.

Chapter 13 MySQL Differences from Standard SQL

Table of Contents

13.1 SELECT INTO TABLE Differences ....................................................................................... 45

13.2 UPDATE Differences ........................................................................................................... 45

13.3 FOREIGN KEY Constraint Differences ................................................................................. 45

13.4 '--' as the Start of a Comment ............................................................................................. 47

We try to make MySQL Server follow the ANSI SQL standard and the ODBC SQL standard, but

MySQL Server performs operations differently in some cases:

• There are several differences between the MySQL and standard SQL privilege systems. For

example, in MySQL, privileges for a table are not automatically revoked when you delete a table.

You must explicitly issue a REVOKE statement to revoke privileges for a table. For more information,

see REVOKE Statement.

• The CAST() function does not support cast to REAL or BIGINT. See Cast Functions and Operators.

13.1 SELECT INTO TABLE Differences

MySQL Server does not support the SELECT ... INTO TABLE Sybase SQL extension. Instead,

MySQL Server supports the INSERT INTO ... SELECT standard SQL syntax, which is basically the

same thing. See INSERT ... SELECT Statement. For example:

INSERT INTO tbl_temp2 (fld_id)

SELECT tbl_temp1.fld_order_id

FROM tbl_temp1 WHERE tbl_temp1.fld_order_id > 100;

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

You can use SELECT ... INTO with user-defined variables. The same syntax can also be used

inside stored routines using cursors and local variables. See SELECT ... INTO Statement.

13.2 UPDATE Differences

If you access a column from the table to be updated in an expression, UPDATE uses the current value

of the column. The second assignment in the following statement sets col2 to the current (updated)

col1 value, not the original col1 value. The result is that col1 and col2 have the same value. This

behavior differs from standard SQL.

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

13.3 FOREIGN KEY Constraint Differences

The MySQL implementation of foreign key constraints differs from the SQL standard in the following

key respects:

• If there are several rows in the parent table with the same referenced key value, InnoDB performs

a foreign key check as if the other parent rows with the same key value do not exist. For example, if

you define a RESTRICT type constraint, and there is a child row with several parent rows, InnoDB

does not permit the deletion of any of the parent rows.

• If ON UPDATE CASCADE or ON UPDATE SET NULL recurses to update the same table it has

previously updated during the same cascade, it acts like RESTRICT. This means that you cannot

FOREIGN KEY Constraint Differences

use self-referential ON UPDATE CASCADE or ON UPDATE SET NULL operations. This is to prevent

infinite loops resulting from cascaded updates. A self-referential ON DELETE SET NULL, on the

other hand, is possible, as is a self-referential ON DELETE CASCADE. Cascading operations may not

be nested more than 15 levels deep.

• In an SQL statement that inserts, deletes, or updates many rows, foreign key constraints (like unique

constraints) are checked row-by-row. When performing foreign key checks, InnoDB sets shared row-

level locks on child or parent records that it must examine. MySQL checks foreign key constraints

immediately; the check is not deferred to transaction commit. According to the SQL standard, the

default behavior should be deferred checking. That is, constraints are only checked after the entire

SQL statement has been processed. This means that it is not possible to delete a row that refers to

itself using a foreign key.

• No storage engine, including InnoDB, recognizes or enforces the MATCH clause used in referential-

integrity constraint definitions. Use of an explicit MATCH clause does not have the specified effect,

and it causes ON DELETE and ON UPDATE clauses to be ignored. Specifying the MATCH should be

avoided.

The MATCH clause in the SQL standard controls how NULL values in a composite (multiple-column)

foreign key are handled when comparing to a primary key in the referenced table. MySQL essentially

implements the semantics defined by MATCH SIMPLE, which permits a foreign key to be all or

partially NULL. In that case, a (child table) row containing such a foreign key can be inserted even

though it does not match any row in the referenced (parent) table. (It is possible to implement other

semantics using triggers.)

• MySQL requires that the referenced columns be indexed for performance reasons. However, MySQL

does not enforce a requirement that the referenced columns be UNIQUE or be declared NOT NULL.

A FOREIGN KEY constraint that references a non-UNIQUE key is not standard SQL but rather an

InnoDB extension. The NDB storage engine, on the other hand, requires an explicit unique key (or

primary key) on any column referenced as a foreign key.

The handling of foreign key references to nonunique keys or keys that contain NULL values is not

well defined for operations such as UPDATE or DELETE CASCADE. You are advised to use foreign

keys that reference only UNIQUE (including PRIMARY) and NOT NULL keys.

• For storage engines that do not support foreign keys (such as MyISAM), MySQL Server parses and

ignores foreign key specifications.

• MySQL parses but ignores “inline REFERENCES specifications” (as defined in the SQL standard)

where the references are defined as part of the column specification. MySQL accepts REFERENCES

clauses only when specified as part of a separate FOREIGN KEY specification.

Defining a column to use a REFERENCES tbl_name(col_name) clause has no actual effect

and serves only as a memo or comment to you that the column which you are currently defining is

intended to refer to a column in another table. It is important to realize when using this syntax that:

• MySQL does not perform any sort of check to make sure that col_name actually exists in

tbl_name (or even that tbl_name itself exists).

• MySQL does not perform any sort of action on tbl_name such as deleting rows in response to

actions taken on rows in the table which you are defining; in other words, this syntax induces no

ON DELETE or ON UPDATE behavior whatsoever. (Although you can write an ON DELETE or ON

UPDATE clause as part of the REFERENCES clause, it is also ignored.)

• This syntax creates a column; it does not create any sort of index or key.

You can use a column so created as a join column, as shown here:

CREATE TABLE person (

id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT,

'--' as the Start of a Comment

name CHAR(60) NOT NULL,

PRIMARY KEY (id)

);

CREATE TABLE shirt (

id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT,

style ENUM('t-shirt', 'polo', 'dress') NOT NULL,

color ENUM('red', 'blue', 'orange', 'white', 'black') NOT NULL,

owner SMALLINT UNSIGNED NOT NULL REFERENCES person(id),

PRIMARY KEY (id)

);

INSERT INTO person VALUES (NULL, 'Antonio Paz');

SELECT @last := LAST_INSERT_ID();

INSERT INTO shirt VALUES

(NULL, 'polo', 'blue', @last),

(NULL, 'dress', 'white', @last),

(NULL, 't-shirt', 'blue', @last);

INSERT INTO person VALUES (NULL, 'Lilliana Angelovska');

SELECT @last := LAST_INSERT_ID();

INSERT INTO shirt VALUES

(NULL, 'dress', 'orange', @last),

(NULL, 'polo', 'red', @last),

(NULL, 'dress', 'blue', @last),

(NULL, 't-shirt', 'white', @last);

SELECT * FROM person;

+----+---------------------+

| id | name |

+----+---------------------+

| 1 | Antonio Paz |

| 2 | Lilliana Angelovska |

+----+---------------------+

SELECT * FROM shirt;

+----+---------+--------+-------+

+----+---------+--------+-------+

| 1 | polo | blue | 1 |

| 2 | dress | white | 1 |

| 3 | t-shirt | blue | 1 |

| 4 | dress | orange | 2 |

| 5 | polo | red | 2 |

| 6 | dress | blue | 2 |

| 7 | t-shirt | white | 2 |

+----+---------+--------+-------+

SELECT s.* FROM person p INNER JOIN shirt s

ON s.owner = p.id

WHERE p.name LIKE 'Lilliana%'

AND s.color <> 'white';

+----+-------+--------+-------+

+----+-------+--------+-------+

| 4 | dress | orange | 2 |

| 5 | polo | red | 2 |

| 6 | dress | blue | 2 |

+----+-------+--------+-------+

When used in this fashion, the REFERENCES clause is not displayed in the output of SHOW CREATE

TABLE or DESCRIBE:

SHOW CREATE TABLE shirt\G

*************************** 1. row ***************************

Table: shirt

Create Table: CREATE TABLE `shirt` (

`id` smallint(5) unsigned NOT NULL auto_increment,

`style` enum('t-shirt','polo','dress') NOT NULL,

`color` enum('red','blue','orange','white','black') NOT NULL,

`owner` smallint(5) unsigned NOT NULL,

PRIMARY KEY (`id`)

) ENGINE=MyISAM DEFAULT CHARSET=latin1

For information about foreign key constraints, see FOREIGN KEY Constraints.

13.4 '--' as the Start of a Comment

'--' as the Start of a Comment

Standard SQL uses the C syntax /* this is a comment */ for comments, and MySQL Server

supports this syntax as well. MySQL also support extensions to this syntax that enable MySQL-specific

SQL to be embedded in the comment; see Comments.

MySQL Server also uses # as the start comment character. This is nonstandard.

Standard SQL also uses “--” as a start-comment sequence. MySQL Server supports a variant of the

-- comment style; the -- start-comment sequence is accepted as such, but must be followed by a

whitespace character such as a space or newline. The space is intended to prevent problems with

generated SQL queries that use constructs such as the following, which updates the balance to reflect

a charge:

UPDATE account SET balance=balance-charge

WHERE account_id=user_id

Consider what happens when charge has a negative value such as -1, which might be the case when

an amount is credited to the account. In this case, the generated statement looks like this:

UPDATE account SET balance=balance--1

WHERE account_id=5752;

balance--1 is valid standard SQL, but -- is interpreted as the start of a comment, and part of

the expression is discarded. The result is a statement that has a completely different meaning than

intended:

UPDATE account SET balance=balance

WHERE account_id=5752;

This statement produces no change in value at all. To keep this from happening, MySQL requires a

whitespace character following the -- for it to be recognized as a start-comment sequence in MySQL

Server, so that an expression such as balance--1 is always safe to use.

Chapter 14 Known Issues in MySQL

This section lists known issues in recent versions of MySQL.

For information about platform-specific issues, see the installation and debugging instructions in

General Installation Guidance, and Debugging MySQL.

The following problems are known:

• Subquery optimization for IN is not as effective as for =.

• Even if you use lower_case_table_names=2 (which enables MySQL to remember the case used

for databases and table names), MySQL does not remember the case used for database names for

the function DATABASE() or within the various logs (on case-insensitive systems).

• Dropping a FOREIGN KEY constraint does not work in replication because the constraint may have

another name on the replica.

• REPLACE (and LOAD DATA with the REPLACE option) does not trigger ON DELETE CASCADE.

• DISTINCT with ORDER BY does not work inside GROUP_CONCAT() if you do not use all and only

those columns that are in the DISTINCT list.

•

When inserting a big integer value (between 2

and 2

−1) into a decimal or string column, it is

inserted as a negative value because the number is evaluated in signed integer context.

• With statement-based binary logging, the source server writes the executed queries to the binary

log. This is a very fast, compact, and efficient logging method that works perfectly in most cases.

However, it is possible for the data on the source and replica to become different if a query is

designed in such a way that the data modification is nondeterministic (generally not a recommended

practice, even outside of replication).

For example:

• CREATE TABLE ... SELECT or INSERT ... SELECT statements that insert zero or NULL

values into an AUTO_INCREMENT column.

• DELETE if you are deleting rows from a table that has foreign keys with ON DELETE CASCADE

properties.

• REPLACE ... SELECT, INSERT IGNORE ... SELECT if you have duplicate key values in the

inserted data.

If and only if the preceding queries have no ORDER BY clause guaranteeing a deterministic

order.

For example, for INSERT ... SELECT with no ORDER BY, the SELECT may return rows in a

different order (which results in a row having different ranks, hence getting a different number in the

AUTO_INCREMENT column), depending on the choices made by the optimizers on the source and

replica.

A query is optimized differently on the source and replica only if:

• The table is stored using a different storage engine on the source than on the replica. (It is

possible to use different storage engines on the source and replica. For example, you can use

InnoDB on the source, but MyISAM on the replica if the replica has less available disk space.)

• MySQL buffer sizes (key_buffer_size, and so on) are different on the source and replica.

• The source and replica run different MySQL versions, and the optimizer code differs between

these versions.

This problem may also affect database restoration using mysqlbinlog|mysql.

The easiest way to avoid this problem is to add an ORDER BY clause to the aforementioned

nondeterministic queries to ensure that the rows are always stored or modified in the same order.

Using row-based or mixed logging format also avoids the problem.

• Log file names are based on the server host name if you do not specify a file name with the startup

option. To retain the same log file names if you change your host name to something else, you must

explicitly use options such as --log-bin=old_host_name-bin. See Server Command Options.

Alternatively, rename the old files to reflect your host name change. If these are binary logs, you

must edit the binary log index file and fix the binary log file names there as well. (The same is true for

the relay logs on a replica.)

• mysqlbinlog does not delete temporary files left after a LOAD DATA statement. See mysqlbinlog

— Utility for Processing Binary Log Files.

• RENAME does not work with TEMPORARY tables or tables used in a MERGE table.

• When using SET CHARACTER SET, you cannot use translated characters in database, table, and

column names.

• You cannot use _ or % with ESCAPE in LIKE ... ESCAPE.

• The server uses only the first max_sort_length bytes when comparing data values. This means

that values cannot reliably be used in GROUP BY, ORDER BY, or DISTINCT if they differ only after

the first max_sort_length bytes. To work around this, increase the variable value. The default

value of max_sort_length is 1024 and can be changed at server startup time or at runtime.

• Numeric calculations are done with BIGINT or DOUBLE (both are normally 64 bits long). Which

precision you get depends on the function. The general rule is that bit functions are performed with

BIGINT precision, IF() and ELT() with BIGINT or DOUBLE precision, and the rest with DOUBLE

precision. You should try to avoid using unsigned long long values if they resolve to be larger than 63

bits (9223372036854775807) for anything other than bit fields.

• You can have up to 255 ENUM and SET columns in one table.

• In MIN(), MAX(), and other aggregate functions, MySQL currently compares ENUM and SET

columns by their string value rather than by the string's relative position in the set.

• In an UPDATE statement, columns are updated from left to right. If you refer to an updated column,

you get the updated value instead of the original value. For example, the following statement

increments KEY by 2, not 1:

mysql> UPDATE tbl_name SET KEY=KEY+1,KEY=KEY+1;

• You can refer to multiple temporary tables in the same query, but you cannot refer to any given

temporary table more than once. For example, the following does not work:

mysql> SELECT * FROM temp_table, temp_table AS t2;

ERROR 1137: Can't reopen table: 'temp_table'

• The optimizer may handle DISTINCT differently when you are using “hidden” columns in a join than

when you are not. In a join, hidden columns are counted as part of the result (even if they are not

shown), whereas in normal queries, hidden columns do not participate in the DISTINCT comparison.

An example of this is:

SELECT DISTINCT mp3id FROM band_downloads

WHERE userid = 9 ORDER BY id DESC;

and

SELECT DISTINCT band_downloads.mp3id

FROM band_downloads,band_mp3

WHERE band_downloads.userid = 9

AND band_mp3.id = band_downloads.mp3id

ORDER BY band_downloads.id DESC;

In the second case, you may get two identical rows in the result set (because the values in the

hidden id column may differ).

Note that this happens only for queries that do not have the ORDER BY columns in the result.

• If you execute a PROCEDURE on a query that returns an empty set, in some cases the PROCEDURE

does not transform the columns.

• Creation of a table of type MERGE does not check whether the underlying tables are compatible

types.

• If you use ALTER TABLE to add a UNIQUE index to a table used in a MERGE table and then add

a normal index on the MERGE table, the key order is different for the tables if there was an old,

non-UNIQUE key in the table. This is because ALTER TABLE puts UNIQUE indexes before normal

indexes to be able to detect duplicate keys as early as possible.