Re: BUG #17871: JIT during postgresql_fdw remote_estimates EXPLAIN have very negatively effect on planning time

Hi,

On 2023-03-28 16:05:00 +1300, David Rowley wrote:
> On Tue, 28 Mar 2023 at 10:38, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > I was wondering if we could skip doing even that much when
> > EXEC_FLAG_EXPLAIN_ONLY is set.  Without a test case, though,
> > I'm not very sure where the jitter is getting reached.
> 
> It seems to be coming from the qual initialisation in nodeSeqscan.c:
> ie. ExecInitSeqScan -> ExecInitQual -> ExecReadyExpr ->
> jit_compile_expr
> 
> I guess that means just add a check for EXEC_FLAG_EXPLAIN_ONLY in
> jit_compile_expr, or only set the es_jit_flags bits in
> standard_ExecutorStart() when not in EXEC_FLAG_EXPLAIN_ONLY. I'm not
> really sure which of those is better, but the attached does the former
> and seems to fix the issue.

I'm a bit worried about either of these making EXPLAIN less informative. Right
now we can see whether and how much would be JITed without ANALYZE. After this
not anymore.

Greetings,

Andres Freund