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This article examines how MySQL's FLUSH TABLE WITH READ LOCK command can cause blocking, detailing two cases where global read locks or table cache flushes stall other operations, explaining the underlying MDL lock mechanisms, table cache handling, and providing code examples and debugging insights.
The article explains how MySQL metadata locks, especially SHARED_UPGRADABLE and SHARED_READ_ONLY locks acquired during ALTER TABLE on tables with foreign keys, can cause long wait times under concurrent write transactions, and shows how adjusting the max_write_lock_count variable resolves the issue.
This article presents a comprehensive walkthrough of MySQL's two‑layer architecture, metadata (MDL) locks, InnoDB lock types—including gap and record locks—and practical debugging techniques, helping readers understand lock acquisition, release points, and source‑code entry points for effective database operation and development.