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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: two database transactions each holding a lock the other one is waiting for

Databases Bug Report

MySQL deadlock on concurrent INSERT ... ON DUPLICATE KEY UPDATE batches

Daniel Tran · Nov 26, 2024 · 9 min read

Stock updates from supplier feeds started failing with Deadlock found when trying to get lock during peak hours. Concurrent upsert batches touched the same unique keys in different orders under InnoDB next-key locking. Sorting, smaller batches and a retry fixed it.