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From: Richard Guo <guofenglinux@gmail.com>
Date: Wed, 28 Aug 2024 11:52:46 +0800
Message-ID: <CAMbWs49PDEbkb7_f=URpon+1tjUbi4Ewwhbf=6N=3XUURLZjWw@mail.gmail.com>
Subject: Re: Significant Execution Time Difference Between PG13.14 and PG16.4
 for Query on information_schema Tables.
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: David Rowley <dgrowleyml@gmail.com>, nikhil raj <nikhilraj474@gmail.com>, 
	pgsql-hackers@lists.postgresql.org
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On Wed, Aug 28, 2024 at 11:30=E2=80=AFAM Richard Guo <guofenglinux@gmail.co=
m> wrote:
> On Wed, Aug 28, 2024 at 5:52=E2=80=AFAM Tom Lane <tgl@sss.pgh.pa.us> wrot=
e:
> > I realized that actually we do have the mechanism for making that
> > work: we could apply add_nulling_relids to the expression, if it
> > meets those same conditions.
>
> I think this should work, as long as we apply add_nulling_relids only
> to Vars/PHVs that belong to the subquery in this case, because only
> those Vars/PHVs would be nulled by the outer joins contained in the
> nullingrels.

To be more concrete, I know theoretically it is the whole expression
that is nullable by the outer joins, not its individual vars.  But in
this case if the contained vars (that belong to the subquery) become
NULL, the whole expression would be NULL too, because it does not
contain any non-strict constructs.  That's why I think this approach
should work.

Thanks
Richard