enable_hashagg

Definition PG 18 manual

Enables or disables the query planner's use of hashed aggregation plan types. The default is on.

How it works

enable_hashagg controls whether the planner may choose hashed aggregation, which groups rows in hash tables instead of sorted streams.

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.

The switch changes which candidate paths the planner may cost; it does not make the chosen method faster by itself. Its user context permits session- or transaction-local changes; newly performed or newly planned work sees the value.

Operational considerations

Treating enable_hashagg 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.

The switch changes which candidate paths the planner may cost; it does not make the chosen method faster by itself.

Workload guidance

OLAP: Benchmark representative analytical plans with and without enable_hashagg. 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 hashed aggregation, which groups rows in hash tables instead of sorted streams explains a regression, then repair statistics, indexes, estimates, or query shape instead of leaving a cluster-wide method ban.

SMALL: Do not use enable_hashagg 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.