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May 22, 2026 · Databases

How a Single Slow Query Triggered a Database Avalanche – Full SQL Optimization Walkthrough

A real‑world MySQL incident where a batch UPDATE with an IN‑subquery caused a full‑table scan, connection pool exhaustion, and a system‑wide outage, and the step‑by‑step investigation, emergency mitigation, and comprehensive optimization that reduced query time from 45 seconds to 0.3 seconds.