From: Tom Lane <tgl@sss.pgh.pa.us>
To: Tomas Vondra <tomas@vondra.me>
cc: pgsql-hackers@lists.postgresql.org
Subject: Re: Improve hash join's handling of tuples with null join keys
In-reply-to: <d8b96e34-9d4a-4704-8cc7-d86ab06fbe92@vondra.me>
References: <3061845.1746486714@sss.pgh.pa.us>
 <d8b96e34-9d4a-4704-8cc7-d86ab06fbe92@vondra.me>
Comments: In-reply-to Tomas Vondra <tomas@vondra.me>
	message dated "Tue, 06 May 2025 02:12:09 +0200"
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Date: Mon, 05 May 2025 20:58:31 -0400
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Tomas Vondra <tomas@vondra.me> writes:
> My personal experience is that the growEnabled heuristics is overly
> sensitive, and probably does not trigger very often.

Yeah, it would be good to make it not quite all-or-nothing.

> But more importantly, wasn't the issue discussed in [1] about parallel
> hash joins?

I'm not clear on that either; it seemed that the OP was able to
trigger it in some non-parallel cases too.  But we don't have a
reproducer so I can't say for sure.  Building a reproducer would
be a useful exercise for testing this.  There might well be some
parallel-specific misbehavior that would be worth ameliorating
independently of this work, in case of a lot of non-null duplicate
keys.

>> This passes check-world, and I've extended a couple of existing test
>> cases to ensure that the new code paths are exercised.  I've not done
>> any real performance testing, though.

> Are you planning to? If not, I can try to collect some numbers, but I
> can't promise that before pgconf.dev.

If you have time after the conference, please feel free.

> BTW do you consider this to be a bugfix for PG18? Or would it have to
> wait for PG19 at this point?

This has been like this forever I suspect --- certainly for as long
as we've had PHJ, and probably longer.  So I'm seeing it as new work
for v19, not something we'd attempt to back-patch.

			regards, tom lane