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From: Tom Lane <[email protected]>
To: Tomas Vondra <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Subject: Re: Problems with estimating OR conditions, IS NULL on LEFT JOINs
Date: Fri, 23 Jun 2023 20:08:34 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>

Tomas Vondra <[email protected]> writes:
> The problem is that the selectivity for "IS NULL" is estimated using the
> table-level statistics. But the LEFT JOIN entirely breaks the idea that
> the null_frac has anything to do with NULLs in the join result.

Right.

> I wonder how to improve this, say by adjusting the IS NULL selectivity
> when we know to operate on the outer side of the join. We're able to
> do this for antijoins, so maybe we could do that here, somehow?

This mess is part of the long-term plan around the work I've been doing
on outer-join-aware Vars.  We now have infrastructure that can let
the estimator routines see "oh, this Var isn't directly from a scan
of its table, it's been passed through a potentially-nulling outer
join --- and I can see which one".  I don't have more than vague ideas
about what happens next, but that is clearly an essential step on the
road to doing better.

			regards, tom lane

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