enable_parallel_hash

Definition PG 18 manual

Enables or disables the query planner's use of hash-join plan types with parallel hash. Has no effect if hash-join plans are not also enabled. The default is on.

How it works

enable_parallel_hash controls whether the planner may choose parallel hash, in which workers cooperate to build and probe a shared hash table.

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.

It has no effect when enable_hashjoin is off and also depends on a parallel-safe plan and available workers. Its user context permits session- or transaction-local changes; newly performed or newly planned work sees the value.

Operational considerations

Treating enable_parallel_hash 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.

It has no effect when enable_hashjoin is off and also depends on a parallel-safe plan and available workers.

Workload guidance

OLAP: Benchmark representative analytical plans with and without enable_parallel_hash. 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 parallel hash, in which workers cooperate to build and probe a shared hash table explains a regression, then repair statistics, indexes, estimates, or query shape instead of leaving a cluster-wide method ban.

SMALL: Do not use enable_parallel_hash 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.