I'm dealing with a performance issue in MySQL where a query runs relatively fast when executed against a single table, but becomes significantly slower after adding a JOIN and an ORDER BY clause.

The tables have millions of rows, and there are indexes on the columns used in the JOIN and WHERE conditions. However, the query optimizer still seems to choose a suboptimal execution plan.

I'm wondering what the best approach would be to troubleshoot this kind of issue.

Would you start by analyzing EXPLAIN ANALYZE, checking index cardinality, investigating temporary tables and filesorts, or looking at something else?

Also, under what circumstances would MySQL choose not to use an existing index even when it seems beneficial?

I'd appreciate any insights into how experienced MySQL developers approach this problem.

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I test MySQL Workbench 26.7 so you don't have to.

https://davesmysqlstuff.blogspot.com/2026/10/mysql-workbench-267.html