14.2.6.5 AUTO_INCREMENT Handling in InnoDB

14.2.6.5.1 Traditional InnoDB Auto-Increment Locking
14.2.6.5.2 Configurable InnoDB Auto-Increment Locking

InnoDB provides an optimization that significantly improves scalability and performance of SQL statements that insert rows into tables with AUTO_INCREMENT columns. To use the AUTO_INCREMENT mechanism with an InnoDB table, an AUTO_INCREMENT column ai_col must be defined as part of an index such that it is possible to perform the equivalent of an indexed SELECT MAX(ai_col) lookup on the table to obtain the maximum column value. Typically, this is achieved by making the column the first column of some table index.

This section provides background information on the original (traditional) implementation of auto-increment locking in InnoDB, explains the configurable locking mechanism, documents the parameter for configuring the mechanism, and describes its behavior and interaction with replication.

14.2.6.5.1 Traditional InnoDB Auto-Increment Locking

The original implementation of auto-increment handling in InnoDB uses the following strategy to prevent problems when using the binary log for statement-based replication or for certain recovery scenarios.

If you specify an AUTO_INCREMENT column for an InnoDB table, the table handle in the InnoDB data dictionary contains a special counter called the auto-increment counter that is used in assigning new values for the column. This counter is stored only in main memory, not on disk.

InnoDB uses the following algorithm to initialize the auto-increment counter for a table t that contains an AUTO_INCREMENT column named ai_col: After a server startup, for the first insert into a table t, InnoDB executes the equivalent of this statement:

SELECT MAX(ai_col) FROM t FOR UPDATE;

InnoDB increments the value retrieved by the statement and assigns it to the column and to the auto-increment counter for the table. By default, the value is incremented by one. This default can be overridden by the auto_increment_increment configuration setting.

If the table is empty, InnoDB uses the value 1. This default can be overridden by the auto_increment_offset configuration setting.

If a SHOW TABLE STATUS statement examines the table t before the auto-increment counter is initialized, InnoDB initializes but does not increment the value and stores it for use by later inserts. This initialization uses a normal exclusive-locking read on the table and the lock lasts to the end of the transaction.

InnoDB follows the same procedure for initializing the auto-increment counter for a freshly created table.

After the auto-increment counter has been initialized, if you do not explicitly specify a value for an AUTO_INCREMENT column, InnoDB increments the counter and assigns the new value to the column. If you insert a row that explicitly specifies the column value, and the value is bigger than the current counter value, the counter is set to the specified column value.

If a user specifies NULL or 0 for the AUTO_INCREMENT column in an INSERT, InnoDB treats the row as if the value was not specified and generates a new value for it.

The behavior of the auto-increment mechanism is not defined if you assign a negative value to the column, or if the value becomes bigger than the maximum integer that can be stored in the specified integer type.

When accessing the auto-increment counter, InnoDB uses a special table-level AUTO-INC lock that it keeps to the end of the current SQL statement, not to the end of the transaction. The special lock release strategy was introduced to improve concurrency for inserts into a table containing an AUTO_INCREMENT column. Nevertheless, two transactions cannot have the AUTO-INC lock on the same table simultaneously, which can have a performance impact if the AUTO-INC lock is held for a long time. That might be the case for a statement such as INSERT INTO t1 ... SELECT ... FROM t2 that inserts all rows from one table into another.

InnoDB uses the in-memory auto-increment counter as long as the server runs. When the server is stopped and restarted, InnoDB reinitializes the counter for each table for the first INSERT to the table, as described earlier.

A server restart also cancels the effect of the AUTO_INCREMENT = N table option in CREATE TABLE and ALTER TABLE statements, which you can use with InnoDB tables to set the initial counter value or alter the current counter value.

You may see gaps in the sequence of values assigned to the AUTO_INCREMENT column if you roll back transactions that have generated numbers using the counter.

14.2.6.5.2 Configurable InnoDB Auto-Increment Locking

As described in the previous section, InnoDB uses a special lock called the table-level AUTO-INC lock for inserts into tables with AUTO_INCREMENT columns. This lock is normally held to the end of the statement (not to the end of the transaction), to ensure that auto-increment numbers are assigned in a predictable and repeatable order for a given sequence of INSERT statements.

In the case of statement-based replication, this means that when an SQL statement is replicated on a slave server, the same values are used for the auto-increment column as on the master server. The result of execution of multiple INSERT statements is deterministic, and the slave reproduces the same data as on the master. If auto-increment values generated by multiple INSERT statements were interleaved, the result of two concurrent INSERT statements would be nondeterministic, and could not reliably be propagated to a slave server using statement-based replication.

To make this clear, consider an example that uses this table:

CREATE TABLE t1 (
  c1 INT(11) NOT NULL AUTO_INCREMENT,
  c2 VARCHAR(10) DEFAULT NULL,
  PRIMARY KEY (c1)
) ENGINE=InnoDB;

Suppose that there are two transactions running, each inserting rows into a table with an AUTO_INCREMENT column. One transaction is using an INSERT ... SELECT statement that inserts 1000 rows, and another is using a simple INSERT statement that inserts one row:

Tx1: INSERT INTO t1 (c2) SELECT 1000 rows from another table ...
Tx2: INSERT INTO t1 (c2) VALUES ('xxx');

InnoDB cannot tell in advance how many rows will be retrieved from the SELECT in the INSERT statement in Tx1, and it assigns the auto-increment values one at a time as the statement proceeds. With a table-level lock, held to the end of the statement, only one INSERT statement referring to table t1 can execute at a time, and the generation of auto-increment numbers by different statements is not interleaved. The auto-increment value generated by the Tx1 INSERT ... SELECT statement will be consecutive, and the (single) auto-increment value used by the INSERT statement in Tx2 will either be smaller or larger than all those used for Tx1, depending on which statement executes first.

As long as the SQL statements execute in the same order when replayed from the binary log (when using statement-based replication, or in recovery scenarios), the results will be the same as they were when Tx1 and Tx2 first ran. Thus, table-level locks held until the end of a statement make INSERT statements using auto-increment safe for use with statement-based replication. However, those locks limit concurrency and scalability when multiple transactions are executing insert statements at the same time.

In the preceding example, if there were no table-level lock, the value of the auto-increment column used for the INSERT in Tx2 depends on precisely when the statement executes. If the INSERT of Tx2 executes while the INSERT of Tx1 is running (rather than before it starts or after it completes), the specific auto-increment values assigned by the two INSERT statements are nondeterministic, and may vary from run to run.

InnoDB can avoid using the table-level AUTO-INC lock for a class of INSERT statements where the number of rows is known in advance, and still preserve deterministic execution and safety for statement-based replication. Further, if you are not using the binary log to replay SQL statements as part of recovery or replication, you can entirely eliminate use of the table-level AUTO-INC lock for even greater concurrency and performance, at the cost of permitting gaps in auto-increment numbers assigned by a statement and potentially having the numbers assigned by concurrently executing statements interleaved.

For INSERT statements where the number of rows to be inserted is known at the beginning of processing the statement, InnoDB quickly allocates the required number of auto-increment values without taking any lock, but only if there is no concurrent session already holding the table-level AUTO-INC lock (because that other statement will be allocating auto-increment values one-by-one as it proceeds). More precisely, such an INSERT statement obtains auto-increment values under the control of a mutex (a light-weight lock) that is not held until the statement completes, but only for the duration of the allocation process.

This new locking scheme enables much greater scalability, but it does introduce some subtle differences in how auto-increment values are assigned compared to the original mechanism. To describe the way auto-increment works in InnoDB, the following discussion defines some terms, and explains how InnoDB behaves using different settings of the innodb_autoinc_lock_mode configuration parameter, which you can set at server startup. Additional considerations are described following the explanation of auto-increment locking behavior.

First, some definitions:

  • INSERT-like statements

    All statements that generate new rows in a table, including INSERT, INSERT ... SELECT, REPLACE, REPLACE ... SELECT, and LOAD DATA.

  • Simple inserts

    Statements for which the number of rows to be inserted can be determined in advance (when the statement is initially processed). This includes single-row and multiple-row INSERT and REPLACE statements that do not have a nested subquery, but not INSERT ... ON DUPLICATE KEY UPDATE.

  • Bulk inserts

    Statements for which the number of rows to be inserted (and the number of required auto-increment values) is not known in advance. This includes INSERT ... SELECT, REPLACE ... SELECT, and LOAD DATA statements, but not plain INSERT. InnoDB will assign new values for the AUTO_INCREMENT column one at a time as each row is processed.

  • Mixed-mode inserts

    These are simple insert statements that specify the auto-increment value for some (but not all) of the new rows. An example follows, where c1 is an AUTO_INCREMENT column of table t1:

    INSERT INTO t1 (c1,c2) VALUES (1,'a'), (NULL,'b'), (5,'c'), (NULL,'d');
    

    Another type of mixed-mode insert is INSERT ... ON DUPLICATE KEY UPDATE, which in the worst case is in effect an INSERT followed by a UPDATE, where the allocated value for the AUTO_INCREMENT column may or may not be used during the update phase.

There are three possible settings for the innodb_autoinc_lock_mode parameter:

  • innodb_autoinc_lock_mode = 0 (traditional lock mode)

    This lock mode provides the same behavior as before innodb_autoinc_lock_mode existed. For all INSERT-like statements, a special table-level AUTO-INC lock is obtained and held to the end of the statement. This assures that the auto-increment values assigned by any given statement are consecutive.

    This lock mode is provided for:

    • Backward compatibility.

    • Performance testing.

    • Working around issues with mixed-mode inserts, due to the possible differences in semantics described later.

  • innodb_autoinc_lock_mode = 1 (consecutive lock mode)

    This is the default lock mode. In this mode, bulk inserts use the special AUTO-INC table-level lock and hold it until the end of the statement. This applies to all INSERT ... SELECT, REPLACE ... SELECT, and LOAD DATA statements. Only one statement holding the AUTO-INC lock can execute at a time.

    With this lock mode, simple inserts (only) use a new locking model where a light-weight mutex is used during the allocation of auto-increment values, and no table-level AUTO-INC lock is used, unless an AUTO-INC lock is held by another transaction. If another transaction does hold an AUTO-INC lock, a simple insert waits for the AUTO-INC lock, as if it too were a bulk insert.

    This lock mode ensures that, in the presence of INSERT statements where the number of rows is not known in advance (and where auto-increment numbers are assigned as the statement progresses), all auto-increment values assigned by any INSERT-like statement are consecutive, and operations are safe for statement-based replication.

    Simply put, the important impact of this lock mode is significantly better scalability. This mode is safe for use with statement-based replication. Further, as with traditional lock mode, auto-increment numbers assigned by any given statement are consecutive. In this mode, there is no change in semantics compared to traditional mode for any statement that uses auto-increment, with one important exception.

    The exception is for mixed-mode inserts, where the user provides explicit values for an AUTO_INCREMENT column for some, but not all, rows in a multiple-row simple insert. For such inserts, InnoDB will allocate more auto-increment values than the number of rows to be inserted. However, all values automatically assigned are consecutively generated (and thus higher than) the auto-increment value generated by the most recently executed previous statement. Excess numbers are lost.

  • innodb_autoinc_lock_mode = 2 (interleaved lock mode)

    In this lock mode, no INSERT-like statements use the table-level AUTO-INC lock, and multiple statements can execute at the same time. This is the fastest and most scalable lock mode, but it is not safe when using statement-based replication or recovery scenarios when SQL statements are replayed from the binary log.

    In this lock mode, auto-increment values are guaranteed to be unique and monotonically increasing across all concurrently executing INSERT-like statements. However, because multiple statements can be generating numbers at the same time (that is, allocation of numbers is interleaved across statements), the values generated for the rows inserted by any given statement may not be consecutive.

    If the only statements executing are simple inserts where the number of rows to be inserted is known ahead of time, there will be no gaps in the numbers generated for a single statement, except for mixed-mode inserts. However, when bulk inserts are executed, there may be gaps in the auto-increment values assigned by any given statement.

The auto-increment locking modes provided by innodb_autoinc_lock_mode have several usage implications:

  • Using auto-increment with replication

    If you are using statement-based replication, set innodb_autoinc_lock_mode to 0 or 1 and use the same value on the master and its slaves. Auto-increment values are not ensured to be the same on the slaves as on the master if you use innodb_autoinc_lock_mode = 2 (interleaved) or configurations where the master and slaves do not use the same lock mode.

    If you are using row-based or mixed-format replication, all of the auto-increment lock modes are safe, since row-based replication is not sensitive to the order of execution of the SQL statements (and the mixed format uses row-based replication for any statements that are unsafe for statement-based replication).

  • Lost auto-increment values and sequence gaps

    In all lock modes (0, 1, and 2), if a transaction that generated auto-increment values rolls back, those auto-increment values are lost. Once a value is generated for an auto-increment column, it cannot be rolled back, whether or not the INSERT-like statement is completed, and whether or not the containing transaction is rolled back. Such lost values are not reused. Thus, there may be gaps in the values stored in an AUTO_INCREMENT column of a table.

  • Gaps in auto-increment values for bulk inserts

    With innodb_autoinc_lock_mode set to 0 (traditional) or 1 (consecutive), the auto-increment values generated by any given statement will be consecutive, without gaps, because the table-level AUTO-INC lock is held until the end of the statement, and only one such statement can execute at a time.

    With innodb_autoinc_lock_mode set to 2 (interleaved), there may be gaps in the auto-increment values generated by bulk inserts, but only if there are concurrently executing INSERT-like statements.

    For lock modes 1 or 2, gaps may occur between successive statements because for bulk inserts the exact number of auto-increment values required by each statement may not be known and overestimation is possible.

  • Auto-increment values assigned by mixed-mode inserts

    Consider a mixed-mode insert, where a simple insert specifies the auto-increment value for some (but not all) resulting rows. Such a statement will behave differently in lock modes 0, 1, and 2. For example, assume c1 is an AUTO_INCREMENT column of table t1, and that the most recent automatically generated sequence number is 100. Consider the following mixed-mode insert statement:

    INSERT INTO t1 (c1,c2) VALUES (1,'a'), (NULL,'b'), (5,'c'), (NULL,'d');
    

    With innodb_autoinc_lock_mode set to 0 (traditional), the four new rows will be:

    +-----+------+
    | c1  | c2   |
    +-----+------+
    |   1 | a    |
    | 101 | b    |
    |   5 | c    |
    | 102 | d    |
    +-----+------+
    

    The next available auto-increment value will be 103 because the auto-increment values are allocated one at a time, not all at once at the beginning of statement execution. This result is true whether or not there are concurrently executing INSERT-like statements (of any type).

    With innodb_autoinc_lock_mode set to 1 (consecutive), the four new rows will also be:

    +-----+------+
    | c1  | c2   |
    +-----+------+
    |   1 | a    |
    | 101 | b    |
    |   5 | c    |
    | 102 | d    |
    +-----+------+
    

    However, in this case, the next available auto-increment value will be 105, not 103 because four auto-increment values are allocated at the time the statement is processed, but only two are used. This result is true whether or not there are concurrently executing INSERT-like statements (of any type).

    With innodb_autoinc_lock_mode set to mode 2 (interleaved), the four new rows will be:

    +-----+------+
    | c1  | c2   |
    +-----+------+
    |   1 | a    |
    |   x | b    |
    |   5 | c    |
    |   y | d    |
    +-----+------+
    

    The values of x and y will be unique and larger than any previously generated rows. However, the specific values of x and y will depend on the number of auto-increment values generated by concurrently executing statements.

    Finally, consider the following statement, issued when the most-recently generated sequence number was the value 4:

    INSERT INTO t1 (c1,c2) VALUES (1,'a'), (NULL,'b'), (5,'c'), (NULL,'d');
    

    With any innodb_autoinc_lock_mode setting, this statement will generate a duplicate-key error 23000 (Can't write; duplicate key in table) because 5 will be allocated for the row (NULL, 'b') and insertion of the row (5, 'c') will fail.