enable_gathermerge

enable_gathermerge — Enables the planner’s use of gather merge plans. Observed in PG10–19 Beta 3; its last measured boot default is on in PG19 Beta 3, with user context. This is a beta-snapshot fact and can change before PostgreSQL 19 GA.

Note

Fact — official short description: “Enables the planner’s use of gather merge plans.”

Identity

Type , Valuebool: Upstream pg_settings type
Context , Valueuser: Settable by an ordinary user
Unit , Value—: Raw unit
Range , Value—: Raw limits in the last observed version
Enum values , Value—: — for non-enum types
Category , ValueQuery Tuning / Planner Method Configuration: Upstream classification
Latest boot value , Valueon: on

Lifecycle

Fact	Value
First observed	PG10
Present in	PG10–19 Beta 3
Removed in	No
Introduction commit	`355d3993c53e` — Add a Gather Merge executor node.
Commit date	2017-03-09
Discussion	thread 1

Default history

Measured PG9.0–19 Beta 3 boot defaults
Versions	Raw `boot_val`	Unit	Human value
PG10–19 Beta 3	`on`	—	on

How it works

enable_gathermerge controls whether the planner may choose Gather Merge, which preserves the order produced by parallel workers while merging their 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.

Tuning advice

Tip

Advice. These are workload-specific starting points and must be validated with measurements.

Workload	Guidance
OLTP	Keep the upstream default for routine OLTP. Use a transaction- or session-local change to diagnose whether choosing Gather Merge, which preserves the order produced by parallel workers while merging their streams explains a regression, then repair statistics, indexes, estimates, or query shape instead of leaving a cluster-wide method ban.
OLAP	Benchmark representative analytical plans with and without enable_gathermerge. Judge planning time, memory, spill, and execution time together; a win for one report is not evidence for a global setting.
Small nodes	Do not use enable_gathermerge 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.

Pigsty

Values use the fixed 8-vCPU, 32-GiB, 100-GiB SSD fixture and render the current Pigsty templates for PG19 Beta 3; this does not assert current Pigsty support for that historical or beta release.

Template	Effective value	Versus upstream boot	Source expression
OLTP	Unmodified	—	—
OLAP	Unmodified	—	—
CRIT	Unmodified	—	—
TINY	Unmodified	—	—

Caution

Advice — pending human review. Fact from the current Pigsty template projection: OLTP: PG10–19 Beta 3 unmodified; OLAP: PG10–19 Beta 3 unmodified; CRIT: PG10–19 Beta 3 unmodified; TINY: PG10–19 Beta 3 unmodified. No Pigsty-specific rationale is inferred from an absent override.

Common pitfalls

Treating enable_gathermerge 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.

max_parallel_workers_per_gather · enable_sort · enable_incremental_sort · parallel_leader_participation