17.6.1.6 AUTO_INCREMENT Handling in InnoDB

InnoDB provides a configurable locking mechanism that can significantly improve scalability and performance of SQL statements that add rows to tables with AUTO_INCREMENT columns. To use the AUTO_INCREMENT mechanism with an InnoDB table, an AUTO_INCREMENT column must be defined as the first or only column of some 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. The index is not required to be a PRIMARY KEY or UNIQUE, but to avoid duplicate values in the AUTO_INCREMENT column, those index types are recommended.

This section describes the AUTO_INCREMENT lock modes, usage implications of different AUTO_INCREMENT lock mode settings, and how InnoDB initializes the AUTO_INCREMENT counter.

• InnoDB AUTO_INCREMENT Lock Modes

• InnoDB AUTO_INCREMENT Lock Mode Usage Implications

• InnoDB AUTO_INCREMENT Counter Initialization

• Notes

InnoDB AUTO_INCREMENT Lock Modes

This section describes the AUTO_INCREMENT lock modes used to generate auto-increment values, and how each lock mode affects replication. The auto-increment lock mode is configured at startup using the innodb_autoinc_lock_mode variable.

The following terms are used in describing innodb_autoinc_lock_mode settings:

• “INSERT-like” statements

  All statements that generate new rows in a table, including INSERT, INSERT ... SELECT, REPLACE, REPLACE ... SELECT, and LOAD DATA. Includes “simple-inserts”, “bulk-inserts”, and “mixed-mode” inserts.

• “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 assigns 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 variable. The settings are 0, 1, or 2, for “traditional”, “consecutive”, or “interleaved” lock mode, respectively. Interleaved lock mode (innodb_autoinc_lock_mode=2) is the default.

The default setting of interleaved lock mode in MySQL 9.1 reflects the change from statement-based replication to row based replication as the default replication type. Statement-based replication requires the consecutive auto-increment lock mode to ensure that auto-increment values are assigned in a predictable and repeatable order for a given sequence of SQL statements, whereas row-based replication is not sensitive to the execution order of SQL statements.

• innodb_autoinc_lock_mode = 0 (“traditional” lock mode)

  The traditional lock mode provides the same behavior that existed before the innodb_autoinc_lock_mode variable was introduced. The traditional lock mode option is provided for backward compatibility, performance testing, and working around issues with “mixed-mode inserts”, due to possible differences in semantics.

  In this lock mode, all “INSERT-like” statements obtain a special 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 values are assigned in a predictable and repeatable order for a given sequence of INSERT statements, and to ensure that auto-increment values assigned by any given statement are consecutive.

  In the case of statement-based replication, this means that when an SQL statement is replicated on a replica server, the same values are used for the auto-increment column as on the source server. The result of execution of multiple INSERT statements is deterministic, and the replica reproduces the same data as on the source. 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 replica 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 are 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 values generated by the Tx1 INSERT ... SELECT statement are consecutive, and the (single) auto-increment value used by the INSERT statement in Tx2 is either 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 are 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 table-level 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.

  Under the consecutive lock mode, InnoDB can avoid using table-level AUTO-INC locks for “simple insert” statements where the number of rows is known in advance, and still preserve deterministic execution and safety for statement-based replication.

  If you are not using the binary log to replay SQL statements as part of recovery or replication, the interleaved lock mode can be used to eliminate all use of table-level AUTO-INC locks 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.

• innodb_autoinc_lock_mode = 1 (“consecutive” 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. If the source table of the bulk insert operation is different from the target table, the AUTO-INC lock on the target table is taken after a shared lock is taken on the first row selected from the source table. If the source and target of the bulk insert operation are the same table, the AUTO-INC lock is taken after shared locks are taken on all selected rows.

  “Simple inserts” (for which the number of rows to be inserted is known in advance) avoid table-level AUTO-INC locks by obtaining the required number of auto-increment values under the control of a mutex (a light-weight lock) that is only held for the duration of the allocation process, not until the statement completes. No table-level AUTO-INC lock is used unless an AUTO-INC lock is held by another transaction. If another transaction holds an AUTO-INC lock, a “simple insert” waits for the AUTO-INC lock, as if it 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, this lock mode significantly improves scalability while being safe for use with statement-based replication. Further, as with “traditional” lock mode, auto-increment numbers assigned by any given statement are consecutive. 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 allocates 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 are 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.

InnoDB AUTO_INCREMENT Lock Mode 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 source and its replicas. Auto-increment values are not ensured to be the same on the replicas as on the source if you use innodb_autoinc_lock_mode = 2 (“interleaved”) or configurations where the source and replicas 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.

• Specifying NULL or 0 for the AUTO_INCREMENT column

  In all lock modes (0, 1, and 2), 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.

• Assigning a negative value to the AUTO_INCREMENT column

  In all lock modes (0, 1, and 2), the behavior of the auto-increment mechanism is undefined if you assign a negative value to the AUTO_INCREMENT column.

• If the AUTO_INCREMENT value becomes larger than the maximum integer for the specified integer type

  In all lock modes (0, 1, and 2), the behavior of the auto-increment mechanism is undefined if the value becomes larger than the maximum integer that can be stored in the specified integer type.

• 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 are 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 behaves 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.

  mysql> CREATE TABLE t1 (
      -> c1 INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY, 
      -> c2 CHAR(1)
      -> ) ENGINE = INNODB;

  Now, consider the following “mixed-mode insert” statement:

  mysql> 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 are:

  mysql> SELECT c1, c2 FROM t1 ORDER BY c2;
  +-----+------+
  | c1  | c2   |
  +-----+------+
  |   1 | a    |
  | 101 | b    |
  |   5 | c    |
  | 102 | d    |
  +-----+------+

  The next available auto-increment value is 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 are also:

  mysql> SELECT c1, c2 FROM t1 ORDER BY c2;
  +-----+------+
  | c1  | c2   |
  +-----+------+
  |   1 | a    |
  | 101 | b    |
  |   5 | c    |
  | 102 | d    |
  +-----+------+

  However, in this case, the next available auto-increment value is 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 2 (“interleaved”), the four new rows are:

  mysql> SELECT c1, c2 FROM t1 ORDER BY c2;
  +-----+------+
  | c1  | c2   |
  +-----+------+
  |   1 | a    |
  |   x | b    |
  |   5 | c    |
  |   y | d    |
  +-----+------+

  The values of x and y are unique and larger than any previously generated rows. However, the specific values of x and y 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 is 100:

  mysql> INSERT INTO t1 (c1,c2) VALUES (1,'a'), (NULL,'b'), (101,'c'), (NULL,'d');

  With any innodb_autoinc_lock_mode setting, this statement generates a duplicate-key error 23000 (Can't write; duplicate key in table) because 101 is allocated for the row (NULL, 'b') and insertion of the row (101, 'c') fails.

• Modifying AUTO_INCREMENT column values in the middle of a sequence of INSERT statements

  If you modify an AUTO_INCREMENT column value to a value larger than the current maximum auto-increment value, the new value is persisted, and subsequent INSERT operations allocate auto-increment values starting from the new, larger value. This behavior is demonstrated in the following example:

  mysql> CREATE TABLE t1 (
      -> c1 INT NOT NULL AUTO_INCREMENT,
      -> PRIMARY KEY (c1)
      ->  ) ENGINE = InnoDB;
  
  mysql> INSERT INTO t1 VALUES(0), (0), (3);
  
  mysql> SELECT c1 FROM t1;
  +----+
  | c1 |
  +----+
  |  1 |
  |  2 |
  |  3 |
  +----+
  
  mysql> UPDATE t1 SET c1 = 4 WHERE c1 = 1;
  
  mysql> SELECT c1 FROM t1;
  +----+
  | c1 |
  +----+
  |  2 |
  |  3 |
  |  4 |
  +----+
  
  mysql> INSERT INTO t1 VALUES(0);
  
  mysql> SELECT c1 FROM t1;
  +----+
  | c1 |
  +----+
  |  2 |
  |  3 |
  |  4 |
  |  5 |
  +----+

InnoDB AUTO_INCREMENT Counter Initialization

This section describes how InnoDB initializes AUTO_INCREMENT counters.

If you specify an AUTO_INCREMENT column for an InnoDB table, the in-memory table object contains a special counter called the auto-increment counter that is used when assigning new values for the column.

The current maximum auto-increment counter value is written to the redo log each time it changes and saved to the data dictionary on each checkpoint; this makes the current maximum auto-increment counter value persistent across server restarts.

On a server restart following a normal shutdown, InnoDB initializes the in-memory auto-increment counter using the current maximum auto-increment value stored in the data dictionary.

On a server restart during crash recovery, InnoDB initializes the in-memory auto-increment counter using the current maximum auto-increment value stored in the data dictionary and scans the redo log for auto-increment counter values written since the last checkpoint. If a redo-logged value is greater than the in-memory counter value, the redo-logged value is applied. However, in the case of an unexpected server exit, reuse of a previously allocated auto-increment value cannot be guaranteed. Each time the current maximum auto-increment value is changed due to an INSERT or UPDATE operation, the new value is written to the redo log, but if the unexpected exit occurs before the redo log is flushed to disk, the previously allocated value could be reused when the auto-increment counter is initialized after the server is restarted.

The only circumstance in which InnoDB uses the equivalent of a SELECT MAX(ai_col) FROM table_name FOR UPDATE statement to initialize an auto-increment counter is when importing a table without a .cfg metadata file. Otherwise, the current maximum auto-increment counter value is read from the .cfg metadata file if present. Aside from counter value initialization, the equivalent of a SELECT MAX(ai_col) FROM table_name statement is used to determine the current maximum auto-increment counter value of the table when attempting to set the counter value to one that is smaller than or equal to the persisted counter value using an ALTER TABLE ... AUTO_INCREMENT = N statement. For example, you might try to set the counter value to a lesser value after deleting some records. In this case, the table must be searched to ensure that the new counter value is not less than or equal to the actual current maximum counter value.

A server restart does not cancel the effect of the AUTO_INCREMENT = N table option. If you initialize the auto-increment counter to a specific value, or if you alter the auto-increment counter value to a larger value, the new value is persisted across server restarts.

Note

ALTER TABLE ... AUTO_INCREMENT = N can only change the auto-increment counter value to a value larger than the current maximum.

The current maximum auto-increment value is persisted, preventing the reuse of previously allocated values.

If a SHOW TABLE STATUS statement examines a table before the auto-increment counter is initialized, InnoDB opens the table and initializes the counter value using the current maximum auto-increment value that is stored in the data dictionary. The value is then stored in memory for use by later inserts or updates. Initialization of the counter value uses a normal exclusive-locking read on the table which lasts to the end of the transaction. InnoDB follows the same procedure when initializing the auto-increment counter for a newly created table that has a user-specified auto-increment value greater than 0.

After the auto-increment counter is initialized, if you do not explicitly specify an auto-increment value when inserting a row, InnoDB implicitly increments the counter and assigns the new value to the column. If you insert a row that explicitly specifies an auto-increment column value, and the value is greater than the current maximum counter value, the counter is set to the specified value.

InnoDB uses the in-memory auto-increment counter as long as the server runs. When the server is stopped and restarted, InnoDB reinitializes the auto-increment counter, as described earlier.

The auto_increment_offset variable determines the starting point for the AUTO_INCREMENT column value. The default setting is 1.

The auto_increment_increment variable controls the interval between successive column values. The default setting is 1.

Notes

When an AUTO_INCREMENT integer column runs out of values, a subsequent INSERT operation returns a duplicate-key error. This is general MySQL behavior.