Why Do Auto-Increment Primary Keys Have Gaps?

Auto-Increment Gaps in Primary Key

When utilizing an auto-increment primary key on a table designed for substantial data storage, it's crucial to be aware that gaps in the numerical sequence may occur. This is not an error but rather a deliberate design choice due to the following reasons:

Overlapping transactions may result in some insertions being assigned IDs while others fail and roll back, leaving unused gaps in the sequence.

For example, if two concurrent transactions attempt to insert records and acquire consecutive auto-increment IDs (e.g., 42 and 43), and one of the transactions subsequently fails, the unused ID (42) will create a gap in the sequence.

Ensuring consecutive values for all insertions would require enforcing sequential transaction execution, which is not practical for scalable systems.

To avoid inserting duplicate entries in a unique column, you're using INSERT IGNORE. However, this practice does not affect the behavior of the auto-increment primary key.

Therefore, it's essential to understand that gaps in auto-increment IDs are unavoidable and do not indicate any errors or data corruption.

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