enable_self_join_elimination

enable_self_join_elimination — Enables removal of unique self-joins. Observed in PG18–19 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 removal of unique self-joins.”

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	PG18
Present in	PG18–19 Beta 3
Removed in	No
Introduction commit	`fc069a3a6319` — Implement Self-Join Elimination
Commit date	2025-02-13
Discussion	thread 1

Default history

Measured PG9.0–19 Beta 3 boot defaults
Versions	Raw `boot_val`	Unit	Human value
PG18–19 Beta 3	`on`	—	on

How it works

enable_self_join_elimination controls whether the planner may choose removing provably redundant self-joins on plain tables when uniqueness and predicates preserve semantics.

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 optimization is limited to plain tables and requires proofs that removing the duplicate relation preserves results. 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 PG18 default on. If a regression is suspected, compare on and off in one session and inspect whether uniqueness proofs and predicates allow a redundant plain-table self-join to be removed; do not disable it cluster-wide to mask bad estimates.
OLAP	Self-join elimination can reduce scans and join work in generated analytical SQL. Validate plan shape and result equivalence on representative statements, remembering that the optimization is limited to plain tables and provable uniqueness.
Small nodes	Keep on unless a reproducible PG18 planner regression is isolated. The switch does not remove arbitrary self-joins, so query and index design remain necessary when the proof conditions are absent.

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: PG18–19 Beta 3 unmodified; OLAP: PG18–19 Beta 3 unmodified; CRIT: PG18–19 Beta 3 unmodified; TINY: PG18–19 Beta 3 unmodified. No Pigsty-specific rationale is inferred from an absent override.

Common pitfalls

Treating enable_self_join_elimination 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 optimization is limited to plain tables and requires proofs that removing the duplicate relation preserves results.

join_collapse_limit · from_collapse_limit · enable_nestloop · enable_hashjoin