Re: Some questions about JIT optimization

On Mon, 26 Jan 2026 at 16:11, ywgrit <yw987194828(at)gmail(dot)com> wrote:
>
> Presentation: https://anarazel.de/talks/2018-06-01-pgcon-state-of-jit/state-of-jit.pdf. It mentions “Future things to JIT: Aggregate & Hashjoin hash computation.” I'm not entirely clear where this optimization specifically manifests. I tested tpch-100G. Performance improvements for q5/11 were minimal because expression execution constitutes a very small proportion in these two queries, leaving little room for JIT optimization. Both queries share common hot functions: `ExecParallelScanHashBucket` and `ExecParallelHashJoin`. However, I don't see how these hot functions could be optimized via JIT. I'd appreciate hearing everyone's thoughts.

I imagine Andres is talking about the computation of the hash value
itself. The talk does predate [1] (from PG18), so I expect that item
is no longer relevant.

In [1], you can see the Hash Join hash value was obtained via
ExecHashGetHashValue(), which evaluated each hash key independently
(perhaps resulting in inefficient incremental tuple deformation)
before calling the hash function on the resulting value. [1] moved all
that into the expression evaluation code so that all hash keys are
evaluated in the same expression. That allows the hash function call
to be inlined when jitted. It also allowed the "keep_nulls" run-time
check to be jitted away so that there's less overhead of effectively
having to continually check which join type is being processed.

David

[1] https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=adf97c156