Date: Tue, 29 Mar 2022 16:16:41 -0700
From: Andres Freund <andres@anarazel.de>
To: David Rowley <dgrowleyml@gmail.com>
Cc: Robert Haas <robertmhaas@gmail.com>,
	Julien Rouhaud <rjuju123@gmail.com>,
	Magnus Hagander <magnus@hagander.net>,
	Justin Pryzby <pryzby@telsasoft.com>,
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Add parameter jit_warn_above_fraction
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Hi,

On 2022-03-30 09:05:44 +1300, David Rowley wrote:
> I really believe that the main problem here is that JIT only enables
> when the *total* plan cost reaches a certain threshold.

Yes, that is/was a clear design mistake. It wasn't quite as bad back when it
was written - partitioning blows the problem up by an order of magnitude or
three. . The costing really needs to at least multiply the number of
to-be-compiled expressions with some cost to decide whether the cost of JITing
is worth it, rather than making "flat" decision.


> I did propose a patch to address this in [1]. It does need more work
> and I do plan to come back to it for v16.

FWIW, that doesn't seem quite right - won't it stop JITing e.g. on the inner
side of a nested loop, just because it's cheap, even though that's where the
bulk of the benefits comes from?

Greetings,

Andres Freund