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#performance-schema

2 stories

Banner: a MySQL ALTER TABLE blocked by a long transaction with a queue of queries waiting behind it

Databases Bug Report

MySQL ALTER TABLE stuck on "Waiting for table metadata lock" took the site down

Daniel Tran · Aug 19, 2025 · 10 min read

A one-second ALTER TABLE ... ALGORITHM=INSTANT hung for 11 minutes and every query on the table queued behind it. The blocker was a reporting job holding an open transaction. How we found it with performance_schema and made migrations fail fast.

Banner: finding the query that is overloading MySQL with processlist, performance_schema and pt-query-digest

Databases

Finding the query that is overloading MySQL: processlist, performance_schema and pt-query-digest

Le Minh Khoa · Mar 16, 2025 · 7 min read

When MySQL sits at 100% CPU, the guilty query is rarely the slowest one; it is usually a fast query run millions of times. A practical sequence for finding it: the processlist for right now, performance_schema statement digests for the last hours, the sys schema for shortcuts, and pt-query-digest for slow log history.