PostgreSQL raises error 40P01 when transactions create a circular lock wait, forcing the server to cancel one to break the deadlock.
PostgreSQL deadlock_detected (error 40P01) occurs when transactions hold locks in a circular chain. The server aborts one transaction to free locks. Remove the cycle by accessing tables and rows in a consistent order or locking them with SELECT FOR UPDATE before updates.
PostgreSQL Error 40P01
PostgreSQL throws error code 40P01 with message "deadlock detected" when two or more transactions hold locks the others need, forming a circular wait. To protect consistency, the server cancels one transaction so the others can proceed. The aborted session receives this error.
The problem appears during concurrent UPDATE, DELETE, INSERT, or SELECT ... FOR UPDATE statements that lock rows or tables in differing orders. Left unresolved, repeated deadlocks slow applications and lose work.
Quick diagnosis and schema-safe fixes restore reliable throughput.
Circular lock chains arise when Session A locks row X then requests row Y while Session B already holds row Y and now waits for row X. PostgreSQL’s deadlock detector wakes every deadlock_timeout (default 1 s) to search for such patterns and abort a victim.
Complex multi-table transactions, missing indexes that prolong lock holding, explicit advisory locks, or long-running idle-in-transaction sessions magnify risk.
Heavy contention during bulk jobs and OLTP traffic often triggers the fault.
Inspect the log entry shown with the error. PostgreSQL lists the blocking statement and the cancelled statement. Rewrite the affected code so all transactions acquire locks in the same order, or split large operations into smaller, deterministic steps.
When multiple sessions update the same rows, lock them first with SELECT ... FOR UPDATE in a predictable ordering clause.
Add application-side retry logic using savepoints or full transaction retries with exponential backoff.
Concurrent UPSERT patterns that first INSERT then UPDATE the same table can deadlock. Replace the two-step logic with INSERT ... ON CONFLICT UPDATE, which obtains only one row lock.
A nightly batch updating accounts and an API endpoint reading those rows with FOR UPDATE can collide.
Add covering indexes so the API locks only target rows, or run the batch at a lower isolation level.
Keep transactions short and touch as few rows as possible. Always access tables and rows in a consistent order across every code path. Create needed indexes to eliminate full scans that hold locks longer.
Monitor pg_stat_activity, pg_locks, and logs for long waiters.
In Galaxy, the live session panel surfaces blocking locks, letting engineers terminate or tune queries before a deadlock occurs.
Error 55P03 (lock_not_available) signals a lock wait timeout, not a true deadlock; resolve by raising lock_timeout or ordering locks consistently.
Error 40001 (serialization_failure) appears under SERIALIZABLE isolation when concurrent writes cannot be reconciled; add retry loops similar to deadlock handling.
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No. It only disables automatic detection, causing sessions to hang indefinitely. Always keep a small timeout.
It may reduce row lock scope but cannot stop deadlocks entirely. Consistent lock ordering is still required.
Galaxy’s session view visualizes blocking locks in real time and offers one-click cancellation, shortening recovery time.
Retries are safe when your transaction is idempotent or wrapped in checks that prevent duplicate side effects.
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