enable_nestloop

Definition PG 18 manual

Enables or disables the query planner's use of nested-loop join plans. It is impossible to suppress nested-loop joins entirely, but turning this variable off discourages the planner from using one if there are other methods available. The default is on.

How it works

enable_nestloop controls whether the planner may choose nested-loop joins, including parameterized inner scans that may be best for small outer relations.

It is consulted while a plan is built. A session-level change is useful for comparing EXPLAIN alternatives, but an already cached plan is not retroactively rewritten; invalidation or replanning is required to observe a different choice.

Turning it off only discourages nested loops because some joins have no other correct implementation. Its user context permits session- or transaction-local changes; newly performed or newly planned work sees the value.

Operational considerations

Treating enable_nestloop as a query hint even though it affects every plan built in its scope.

Testing an already cached prepared plan and concluding that the setting has no effect.

Masking stale statistics or cardinality errors by disabling a plan method globally.

Turning it off only discourages nested loops because some joins have no other correct implementation.

Workload guidance

OLAP: Benchmark representative analytical plans with and without enable_nestloop. Judge planning time, memory, spill, and execution time together; a win for one report is not evidence for a global setting.

OLTP: Keep the upstream default for routine OLTP. Use a transaction- or session-local change to diagnose whether choosing nested-loop joins, including parameterized inner scans that may be best for small outer relations explains a regression, then repair statistics, indexes, estimates, or query shape instead of leaving a cluster-wide method ban.

SMALL: Do not use enable_nestloop as a permanent hint on a small host. Resource pressure may change the best method, so verify with EXPLAIN (ANALYZE, BUFFERS) and keep the change scoped to the affected workload if it is still needed.