All Your GUCs in a Row: jit_expressions and jit_tuple_deforming

An explanation of two PostgreSQL JIT-related GUCs, jit_expressions and jit_tuple_deforming, both boolean, user-context, and on by default since Postgres 11. Expression compilation replaces interpreted evaluation of WHERE clauses, target lists, and aggregate transitions with native code; tuple deforming generates a specialized function for converting packed tuple bytes into datum arrays. The two are asymmetric: turning off jit_tuple_deforming still shows a JIT block minus deform functions, but turning off jit_expressions removes the JIT block entirely, since deforming code is only ever generated while compiling expressions. This asymmetry is exploited for bug isolation: disabling jit, then selectively disabling jit_tuple_deforming, narrows a suspected JIT bug to either the deform compiler or the expression compiler without a server restart. These settings are explicitly framed as debugging tools, not performance tuning knobs.

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