Why is My COUNT(*) Query with a WHERE Clause Slow in MySQL?

Slow COUNT(*) Query in MySQL with Where Clause

When dealing with sluggish MySQL queries involving COUNT(*), understanding the underlying factors is paramount. In this particular case, the long execution time is not immediately apparent from the query itself or the EXPLAIN statement.

Understanding the Issue

The table in question, change_event, has a clustered primary key on the change_event_id column. In InnoDB, clustered primary keys are stored alongside the data in the data pages, unlike non-clustered indexes that reside in separate index pages.

The Problem of Range Scans on Clustered Primary Keys

The presented query performs a range scan on the clustered primary key, where it needs to scan a significant portion of the table. This can be time-consuming since the data pages may not be physically stored in sorted order, resulting in extra I/O operations.

Potential Solutions

To optimize this query, consider implementing one or both of the following strategies:

1. Optimize Table: Running OPTIMIZE TABLE reconstructs the table data pages, ensuring that they are sorted by the primary key, which could potentially speed up the range scan.
2. Create Additional Index: Adding a non-primary index solely on the change_event_id column would allow MySQL to quickly scan the index pages instead of the data pages. This could provide a significant performance boost.

Additional Note:

Consider modifying the change_event_id column to be BIGINT UNSIGNED if it is indeed an auto-incrementing column starting from zero. This would prevent potential issues with signed integer overflow.

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