Horton

Re: Do not scan index in right table if condition for left join evaluates to false using columns in left table

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
Cc: Илья Жарков <izharkov1243@gmail.com>, pgsql-hackers@lists.postgresql.org
Date: 2024-12-07T23:13:15Z
Lists: pgsql-hackers

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  1. Rewrite maybe_reread_subscription() comment

Andres Freund <andres@anarazel.de> writes:
> On 2024-12-07 17:06:52 -0500, Tom Lane wrote:
>> One could imagine that we split up the join filter conditions into
>> "depends on RHS" and "doesn't depend on RHS" subsets, and make the
>> nestloop plan node evaluate the latter set only once per LHS row,
>> and then skip the inner-side scan when that condition fails.

> As I wrote in my other email, I'm also somewhat dubious it's worth having
> explicit code for this in nodeNestloop.c.

Yeah.  Your idea of pushing the "doesn't depend on RHS" subset into
a one-time Result filter atop the RHS is interesting though.  Then we
don't need any new executor machinery, but we pay for that with a more
complex planner patch.  Not sure how hard that would be.

			regards, tom lane