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AUTO_INCREMENT
Column Works in InnoDB
After a database startup, when a user first does an insert to a
table T
where an auto-increment column has been defined, and the user does not provide
an explicit value for the column, then InnoDB executes SELECT
MAX(auto-inc-column) FROM T
, and assigns that value incremented
by one to the column and the auto-increment counter of the table.
We say that
the auto-increment counter for table T
has been initialized.
InnoDB follows the same procedure in initializing the auto-increment counter for a freshly created table.
Note that if the user specifies in an insert the value 0 to the auto-increment column, then InnoDB treats the row like the value would not have been specified.
After the auto-increment counter has been initialized, if a user inserts a row where he explicitly specifies the column value, and the value is bigger than the current counter value, then the counter is set to the specified column value. If the user does not explicitly specify a value, then InnoDB increments the counter by one and assigns its new value to the column.
The auto-increment mechanism, when assigning values from the counter, bypasses locking and transaction handling. Therefore you may also get gaps in the number sequence if you roll back transactions which have got numbers from the counter.
The behavior of auto-increment is not defined if a user gives 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.