jit_tuple_deforming

Definition PG 18 manual

Determines whether tuple deforming is JIT compiled, when JIT compilation is activated (see Section 30.2). The default is on.

How it works

jit_tuple_deforming controls JIT compilation of the code that extracts attributes from PostgreSQL heap tuples after a query crosses the main JIT gate.

Disabling it leaves tuple deforming interpreted while expression JIT and other compilation can remain active. This isolates a compiler path for correctness and overhead diagnosis.

It is not an independent performance switch for every scan. Tuple layout, selected columns, row count, jit_above_cost, and total compilation time determine any effect. Its user context permits session- or transaction-local changes; newly performed or newly planned work sees the value.

Operational considerations

Leaving jit_tuple_deforming 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.

Workload guidance

OLAP: Long analytical runs can amplify jit_tuple_deforming's debug overhead and artifacts. Prefer standard EXPLAIN and statistics first, and isolate any developer experiment from normal users.

OLTP: Do not tune production OLTP with jit_tuple_deforming. Enable it only for a bounded reproduction with an owner, log/disk budget, rollback condition, and evidence-capture plan; restore the default immediately afterward.

SMALL: Keep jit_tuple_deforming at its upstream default. A small host has less spare CPU, disk, connection, and log capacity for developer instrumentation.