Why Your MySQL Queries Are Slow Even After Adding Indexes (And How We Fixed most of Them in Production)
At a fintech startup I worked at, we spent 3 weeks chasing a “mystery latency spike” in our subscription renewal service—query latency jumped from 12ms to 420ms during peak hour, only on Tuesdays. The `EXPLAIN` plan showed “Using index”, `innodb_buffer_pool_hit_ratio` was 99.8%, and the DBA said “it’s fine”. Turns out it was a silent index corruption caused by `ALTER TABLE ... ALGORITHM=INPLACE` on a 42GB `subscriptions` table with `ROW_FORMAT=COMPACT` + `utf8mb4_0900_as_cs` collation—triggered ...