Every MySQL schema that’s been around for a few years collects indexes nobody remembers adding. Some were for a report that was retired. Some came from an ORM migration. A few were added “just in case” and never used. They all cost something: extra writes on every INSERT and UPDATE, increased buffer pool pressure, larger backups, and slower schema changes.
MySQL makes it look easy to find them. Since 5.7, the sys schema ships a view called sys.schema_unused_indexes, and one query gives you a list of indexes with zero reads since the server started. It’s tempting to turn that list straight into DROP INDEX statements.
In an earlier post, How to Find Duplicate, Unused, and Invisible Indexes in MySQL, I showed how to use that view on a single server. This post covers what that approach misses once replicas are involved.
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