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Data & Analytics · Intermediate

Blocking

A situation in which one session waits for a lock held by another session; it resolves when the holder finishes.

Definition

Blocking is normal and short-lived in healthy systems but becomes a problem with long transactions, missing indexes (which cause large scans and many locks) or user interaction inside transactions. In SQL Server, use sys.dm_exec_requests and sys.dm_tran_locks to diagnose it.

Exam tip

Blocking clears when the blocker finishes; a deadlock never clears without a victim being killed.

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