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

MySQL Autoincrement Behavior on Failed Inserts

Question:

Why does the autoincrement value in MySQL increase despite failed inserts when a unique field exists?

Explanation:

InnoDB, the default engine in MySQL, operates transactionally. Hence, when multiple sessions attempt simultaneous insertions into a table with an autoincrement column, certain scenarios may arise:

• If Session A inserts a record and subsequently rolls back, a gap may occur or Session B, which attempted to insert a record concurrently, may be locked until Session A's transaction concludes.
• To enhance concurrency for insertions into such tables, InnoDB utilizes a special table-level AUTO-INC lock. This lock is released at the end of the SQL statement, not the transaction, allowing other sessions to access the autoincrement counter more efficiently.

Consequence:

When an insert fails due to a unique field constraint violation, the autoincrement value is still incremented because InnoDB maintains the in-memory autoincrement counter throughout the server's operation. When the server restarts, InnoDB resets the counter for each table during the first insert into that table.

Mitigation:

If the potential for ID column wrapping is a concern, consider using a BIGINT data type (8-byte long) to provide a larger range of values.

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