Why Does MySQL\'s Autoincrement Increment Even on Failed Inserts?

Autoincrement Anomalies in MySQL with Failed Inserts

In MySQL, tables with an auto_increment field coupled with unique constraints exhibit a peculiar behavior when insert operations fail due to duplicate unique values. While the insert fails as intended, the auto_increment value gets incremented nonetheless, a phenomenon triggered by accessing the auto-increment counter.

At the core of this behavior lies InnoDB's transactional nature. To ensure concurrency for inserts, InnoDB utilizes a special table-level AUTO-INC lock that remains locked until the end of the current SQL statement rather than the end of the transaction.

This means that even if an insert fails due to a duplicate unique value, the auto-increment counter gets incremented because the LOCK is released upon statement completion. Subsequent inserts will pick up the incremented value, leading to gaps in the auto-increment sequence.

InnoDB's decision to allow gaps aims to enhance concurrency for inserts into tables with auto_increment columns. However, it does raise concerns about potential table corruption if the auto-increment id column wraps around its maximum value.

To mitigate this risk, consider using a BIGINT data type for the id column, which offers an 8-byte storage capacity and a significantly reduced likelihood of wrap-around issues.

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