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From: David Rowley <dgrowleyml@gmail.com>
Date: Tue, 27 Aug 2024 23:03:00 +1200
Message-ID: <CAApHDvoQRoKHNfTaw+Sn=bZGKVmn8W-5yBPGP7+98mWJz5A9uQ@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: nikhil raj <nikhilraj474@gmail.com>, 
	"pgsql-generallists.postgresql.org" <pgsql-general@lists.postgresql.org>, 
	NIKITA PATEL <patelnikita1411@gmail.com>, Patel Khushbu <patelkhushbu2067@gmail.com>
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On Tue, 27 Aug 2024 at 14:03, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Yeah, I got that same result by bisecting.  It seems like it's
> somehow related to the cast to information_schema.sql_identifier:
> we are able to get rid of that normally but seem to fail to do so
> in this query.

In case it saves you a bit of time, I stripped as much of the
unrelated stuff out as I could and got:

create table t (a name, b int);
explain select * from (select a::varchar,b from (select distinct a,b
from t) st) t right join t t2 on t.b=t2.b where t.a='test';

getting rid of the cast or swapping to INNER JOIN rather than RIGHT
JOIN means that qual_is_pushdown_safe() gets a Var rather than a
PlaceHolderVar.

David