Cookbook recipe

Online schema changes without long locks

Applies to PostgreSQL 13–17 Last reviewed May 2026 Grounded in source

What This Is

A production experience capsule based on real incident patterns.

Why It Matters

Builds production judgment under pressure.

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Estimated investigation4 min

Scenario

An ALTER TABLE blocks all reads/writes because it waits for (and then holds) an ACCESS EXCLUSIVE lock behind a long query. Do it safely. Diagnose it See what the DDL would wait behind: SELECT pid, now()-query_start…

Investigation Path

An ALTER TABLE blocks all reads/writes because it waits for (and then holds) an ACCESS EXCLUSIVE lock behind a long query. Do it safely.

Diagnose it

See what the DDL would wait behind:

SELECT pid, now()-query_start AS runtime, left(query,60)
FROM pg_stat_activity WHERE state='active' ORDER BY runtime DESC;

Why it happens

Most ALTERs need a brief ACCESS EXCLUSIVE lock, but they must first wait for conflicting transactions — and while waiting they queue everyone behind them. A long-running query can turn a fast ALTER into a system-wide stall.

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Concepts on this page

Schema Lock
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