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Database & ORM · Developer Error Resolution Lab

Fix PostgreSQL Error 55P03: 'canceling statement due to lock timeout' During Zero-Downtime Migrations

Prevent `ALTER TABLE` and `CREATE INDEX` statements from blocking production traffic or failing with lock timeouts in PostgreSQL.

Triggered Error Message:

ERROR: 55P03: canceling statement due to lock timeout (or deadlock detected 40P01)

Customize Solution for Your Environment
-- 1. Find blocking PIDs in PostgreSQL:
SELECT pid, age(clock_timestamp(), query_start), usename, query
FROM pg_stat_activity
WHERE state != 'idle' AND query NOT ILIKE '%pg_stat_activity%'
ORDER BY query_start ASC;

-- 2. Create index safely without table-wide write locks:
CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_events_timestamp ON events (created_at);

Root Cause & Mechanism Analysis

Why does this error occur?

Standard `CREATE INDEX` or `ALTER TABLE ... ADD CONSTRAINT` acquires an `ACCESS EXCLUSIVE` or `SHARE` lock that waits behind long-running queries and blocks all incoming reads/writes until `lock_timeout` expires.

Frequently Asked Questions

Can `CREATE INDEX CONCURRENTLY` run inside a `BEGIN ... COMMIT` transaction block?
No. PostgreSQL requires `CREATE INDEX CONCURRENTLY` to run outside an explicit transaction block.
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