join_collapse_limit

join_collapse_limit — Sets the FROM-list size beyond which JOIN constructs are not flattened. Observed in PG9.0–19 Beta 3; its last measured boot default is 8 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: “Sets the FROM-list size beyond which JOIN constructs are not flattened.”

Identity

Type , Valueinteger: Upstream pg_settings type
Context , Valueuser: Settable by an ordinary user
Unit , Value—: Raw unit
Range , Value1 – 2147483647: Raw limits in the last observed version
Enum values , Value—: — for non-enum types
Category , ValueQuery Tuning / Other Planner Options: Upstream classification
Latest boot value , Value8: 8

Lifecycle

Fact	Value
First observed	PG9.0 (research boundary)
Present in	PG9.0–19 Beta 3
Removed in	No
Introduction commit	Not asserted: predates the PG9.0 research boundary
Commit date	—
Discussion	—

Default history

Measured PG9.0–19 Beta 3 boot defaults
Versions	Raw `boot_val`	Unit	Human value
PG9.0–19 Beta 3	`8`	—	8

How it works

join_collapse_limit controls when explicit JOIN constructs, except FULL JOIN, are flattened into a reorderable FROM list. A value of 1 preserves the written explicit join order.

Flattening enlarges the set of join orders the planner can explore, improving opportunities at the cost of planning CPU and memory. Outer-join semantics still constrain legal reorderings.

The exposed item count interacts with from_collapse_limit and geqo_threshold. Using 1 as a manual join-order tool transfers responsibility to SQL authors and is not a general plan-stability guarantee. 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 join_collapse_limit at its upstream default unless representative plans show a repeatable, workload-wide problem. Test a local override first and include planning latency as well as execution latency.
OLAP	Analytical SQL can make join_collapse_limit more visible because joins, cursors, or recursion are larger. Benchmark the full statement family and inspect estimates rather than copying a single successful value.
Small nodes	On a small host, avoid increasing planning search or memory pressure through join_collapse_limit without a measured benefit. Prefer query-local structure or a scoped role setting over a cluster-wide override.

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: PG9.0–19 Beta 3 unmodified; OLAP: PG9.0–19 Beta 3 unmodified; CRIT: PG9.0–19 Beta 3 unmodified; TINY: PG9.0–19 Beta 3 unmodified. No Pigsty-specific rationale is inferred from an absent override.

Common pitfalls

Treating join_collapse_limit as an executor resource limit rather than a planning assumption or policy.
Testing only one parameter set or one data distribution.
Expecting an already cached plan to be rewritten automatically.
Using a global override to hide stale statistics or fragile SQL structure.

from_collapse_limit · geqo_threshold · geqo · enable_hashjoin · enable_nestloop