From: Tom Lane <tgl@sss.pgh.pa.us>
To: David Rowley <dgrowleyml@gmail.com>
cc: ma lz <ma100@hotmail.com>,
        "pgsql-general@postgresql.org" <pgsql-general@postgresql.org>
Subject: Re: Why not do distinct before SetOp
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Comments: In-reply-to David Rowley <dgrowleyml@gmail.com>
	message dated "Wed, 06 Nov 2024 11:11:51 +1300"
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Date: Tue, 05 Nov 2024 18:09:41 -0500
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David Rowley <dgrowleyml@gmail.com> writes:
> On Tue, 5 Nov 2024 at 04:18, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> A different idea that occurred to me while looking at this is:
>> why have we got all this machinery to add and check a flag
>> column, rather than arranging things so that the two input
>> relations are "outer" and "inner" children of the SetOp?

> I've no idea why it's not like that. The current design is quite
> strange and feels dated. It might be worth making that change as even
> if we gave joins better support for IS NOT DISTINCT FROM and made
> INTERSECT use INNER JOIN instead and EXCEPT use anti join, we'd still
> need nodeSetOp.c for INTERSECT ALL and EXCEPT ALL.

Yeah.  We'd still need it, and besides which it seems like a fairly
small project, unlike the other thing which could take multiple
years to get to an acceptable state.

Of course, I might be overestimating the performance benefit we'd get.
But I'm tempted to give it a try.

			regards, tom lane