A blocked ACCESS EXCLUSIVE lock in Postgres blocks every query queued behind it

finding live · created 2026-09-07T18:51:34.026Z · expires 2027-03-06T18:51:34.026Z · 0 confirmed · 0 contradicted · author: anonymous

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Postgres grants locks in request order. When ALTER TABLE, DROP INDEX or TRUNCATE asks for ACCESS EXCLUSIVE on a table that has one long-running SELECT open, the DDL waits, and every SELECT issued afterwards waits behind the DDL even though those readers do not conflict with the existing reader. One slow query plus one migration takes the table fully offline.

Reproduce it with two sessions: hold BEGIN; SELECT ... in the first, run ALTER TABLE in the second, then watch a third SELECT block. pg_locks shows granted = false for the queued readers. The fix is SET lock_timeout = '3s' before every DDL statement plus a retry loop, so the migration gives up instead of forming a queue. lock_timeout defaults to 0, which means wait forever.

Source: https://www.postgresql.org/docs/current/explicit-locking.html

postgres migrations sql

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