jit_expressions

jit_expressions — Allow JIT compilation of expressions. Observed in PG11–19 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: “Allow JIT compilation of expressions.”

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 , ValueDeveloper Options: Upstream classification
Latest boot value , Valueon: on

Lifecycle

Fact	Value
First observed	PG11
Present in	PG11–19 Beta 3
Removed in	No
Introduction commit	`2a0faed9d702` — Add expression compilation support to LLVM JIT provider.
Commit date	2018-03-20
Discussion	thread 1

Default history

Measured PG9.0–19 Beta 3 boot defaults
Versions	Raw `boot_val`	Unit	Human value
PG11–19 Beta 3	`on`	—	on

How it works

jit_expressions controls whether expression evaluation is JIT compiled after the query has already crossed the top-level JIT cost gate. It does not activate JIT by itself.

Disabling it leaves expressions interpreted while other eligible JIT work can still occur. This isolates expression-code generation for testing correctness or compilation overhead.

The setting is a developer diagnostic rather than the normal way to tune JIT; jit_above_cost and the top-level jit switch govern production selection. 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	Do not tune production OLTP with jit_expressions. Enable it only for a bounded reproduction with an owner, log/disk budget, rollback condition, and evidence-capture plan; restore the default immediately afterward.
OLAP	Long analytical runs can amplify jit_expressions’s debug overhead and artifacts. Prefer standard EXPLAIN and statistics first, and isolate any developer experiment from normal users.
Small nodes	Keep jit_expressions at its upstream default. A small host has less spare CPU, disk, connection, and log capacity for developer instrumentation.

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: PG11–19 Beta 3 unmodified; OLAP: PG11–19 Beta 3 unmodified; CRIT: PG11–19 Beta 3 unmodified; TINY: PG11–19 Beta 3 unmodified. No Pigsty-specific rationale is inferred from an absent override.

Common pitfalls

Leaving jit_expressions enabled after the bounded diagnostic or recovery task.
Running the experiment on the only copy of production data.
Underestimating log, core-file, temporary-file, WAL, CPU, or connection-slot amplification.
Treating a server that merely starts or completes a query as proof that data and behavior are correct.

jit · jit_above_cost · jit_tuple_deforming · jit_inline_above_cost · jit_optimize_above_cost