constraint_exclusion

constraint_exclusion — Enables the planner to use constraints to optimize queries. Observed in PG9.0–19 Beta 3; its last measured boot default is partition 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 to use constraints to optimize queries.”

Identity

Type , Valueenum: 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 , Valuepartition, on, off: — for non-enum types
Category , ValueQuery Tuning / Other Planner Options: Upstream classification
Latest boot value , Valuepartition: partition

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	`partition`	—	partition

How it works

constraint_exclusion lets the planner compare query predicates with CHECK constraints and omit relations whose constraints prove that they cannot match. partition limits that work to traditional inheritance children and UNION ALL arms; it is separate from declarative partition pruning.

The proof is attempted at planning time, so enabling it more broadly increases planning work even when no relation can be excluded. It relies on constraints that are visible and logically contradictory to the query conditions.

For declaratively partitioned tables, enable_partition_pruning is the primary control. constraint_exclusion remains useful for inheritance-based partitioning and carefully constructed constraint-backed UNION ALL views. 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 constraint_exclusion 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 constraint_exclusion 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 constraint_exclusion 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 constraint_exclusion 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.

enable_partition_pruning · from_collapse_limit · join_collapse_limit · default_statistics_target