Auto Increment Primary Key Gaps: An Explanation

When using an auto increment primary key, it's common to encounter gaps in the sequence of assigned IDs. This can occur even when insertions are performed without any deletions. The reason for this is due to MySQL's transaction handling and the possibility of rollbacks.

To illustrate this issue, consider two overlapping transactions performing insertions:

1. Transaction 1: Inserts a record and obtains ID 42.
2. Transaction 2: Inserts a record and obtains ID 43.

If Transaction 1 then fails and rolls back, ID 42 becomes unused. However, Transaction 2 still completes, resulting in ID 43 being assigned. This scenario creates a gap in the sequence, leaving ID 42 unused.

MySQL's commitment to scalability is the underlying reason for this behavior. If consecutive values were guaranteed, every transaction would need to occur sequentially, which would impact performance when dealing with large volumes of data.

To mitigate the impact of gaps in ID sequences, consider using a surrogate key or a custom logic to ensure consecutive values. However, it's important to understand that gaps in auto increment primary keys are an inherent feature of MySQL's transaction handling and are not a cause for concern unless they impact the functionality of your application.