Why Does Auto Increment Skip Numbers in MySQL?

Auto Increment Increments Skipping Numbers?

When using auto-increment columns in MySQL, it is expected that the column values will increment by 1. However, some users may encounter instances where the increments skip numbers, resulting in a higher value than expected.

原因

The default auto-increment behavior in MySQL versions 5.1 and later is to lose auto-increment values when an INSERT fails. This means that the auto-increment value is incremented even if the INSERT was unsuccessful, causing skipped numbers.

解决方案

There are a few possible solutions to address this issue:

1. Disable InnoDB Auto-Increment Locking:
   By setting innodb_autoinc_lock_mode to 0, MySQL will revert to the 5.0 behavior, which avoids losing values when inserts fail.
2. Check Auto-Increment Increment Value:
   The auto_increment_increment configuration variable controls how much the auto-increment value is incremented by each time. Ensure that this variable is set to 1 to increment by one.

Avoiding Consecutive Numbers

It is important to note that auto-increment columns are designed to ensure uniqueness, not necessarily consecutive ordering. Therefore, it may not always be necessary to address skipped numbers.

Other Considerations

If the auto-increment value is skipping significant numbers (e.g., 750 values), it is recommended to investigate the underlying cause of the insert failures. This could indicate incomplete data or unnecessary transaction handling.

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