Why Does MySQL's Auto-Increment Value Anomalously Increment with ON DUPLICATE KEY UPDATE?

ON DUPLICATE KEY UPDATE Conundrum: Auto Increment Increment Anomaly

When utilizing the ON DUPLICATE KEY UPDATE clause in an INSERT statement, you might encounter an unexpected behavior with auto increment values. Inserting a new row that triggers the ON DUPLICATE KEY clause can result in a peculiar increment in the auto increment value. This article delves into the cause and offers a solution.

Understanding the Mechanism

As documented by MySQL, when ON DUPLICATE KEY UPDATE is specified, a duplicate key value in a UNIQUE index or PRIMARY KEY triggers an UPDATE of the existing row. This operation is analogous to an UPDATE statement.

However, a crucial difference arises when using auto-increment columns in InnoDB tables. Unlike INSERT statements that automatically increment auto-increment columns, UPDATE statements do not affect their values.

Implications of ON DUPLICATE KEY

In our example, MySQL attempts an INSERT operation first, which increments the auto-increment value. Subsequently, the duplicate key is detected, and the UPDATE occurs. However, the auto-increment value set during the INSERT is not updated. This results in an inconsistent increment in the auto-increment column.

Reliance on Auto Increment

It's important to avoid relying on auto-increment values to maintain a sequence. MySQL auto-increment values can experience gaps due to the mechanism described above. For applications requiring sequential values, a more labor-intensive approach involving locks and renumbering using triggers is necessary.

Alternative Solution

A preferable solution is to calculate and output incremental values. This ensures sequential values without the complexities and performance overhead associated with locking and renumbering.

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