lock_not_available could not obtain lock on row in relation “…” — 55P03

Symptoms

A SELECT … FOR UPDATE NOWAIT (or similar) could not immediately acquire a row lock because another transaction holds it. PostgreSQL raises SQLSTATE 55P03 (lock_not_available).

• A NOWAIT lock request failed instantly.
• Another transaction holds the conflicting lock.
• NOWAIT means “fail rather than wait”.

What the server log shows

ERROR:  could not obtain lock on row in relation "orders"

Why PostgreSQL raises this — what the manual says

the SELECT reference (The Locking Clause):

“With NOWAIT, the statement reports an error, rather than waiting, if a selected row cannot be locked immediately.”

NOWAIT tells PostgreSQL to fail immediately rather than block when a row is already locked. If the lock is held by another transaction, the request cannot succeed at once, so PostgreSQL reports 55P03.

Common causes

• Another transaction holds a conflicting row lock.
• Contention on hot rows under concurrency.
• Using NOWAIT in a polling/optimistic pattern.

How to fix it

1. Retry later, or use SKIP LOCKED to process available rows instead.
2. Reduce lock-hold time in competing transactions.
3. Drop NOWAIT (or use a lock timeout) if waiting briefly is acceptable.

Related & next steps

• SQLSTATE 55P03 — Lock Not Available (full reference)
• Message: canceling statement due to lock timeout
• Glossary: Row lock

Reference: PostgreSQL 18 — SELECT.