SQL Server uses locks to protect data while concurrent sessions read and modify it. Locks preserve transaction correctness, but they also create waiting, blocking, and deadlocks. When evaluating a slow query, ask: "Is it waiting for another transaction, and what resource does it need?"

This cheat sheet explains the lock types you are most likely to encounter, how they affect query execution, and how to diagnose locking problems quickly.

Locks are not latches. Locks protect logical data and transaction consistency. Latches are lightweight internal synchronization mechanisms that protect in-memory structures such as data pages. A lock problem commonly appears as an LCK_M_* wait; a latch problem uses waits such as PAGELATCH_* .

The basic locking flow

A lock request is granted when compatible with locks already held on the resource. Otherwise, the session waits. Lock duration depends on its mode, isolation level, statement, and transaction scope.…