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Date: Mon, 31 Mar 2025 09:34:54 +0300
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Subject: Re: Memoize ANTI and SEMI JOIN inner
To: David Rowley <dgrowleyml@gmail.com>, Andrei Lepikhov <lepihov@gmail.com>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
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From: Alena Rybakina <a.rybakina@postgrespro.ru>
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On 31.03.2025 06:33, David Rowley wrote:
> On Mon, 31 Mar 2025 at 16:21, Alena Rybakina <a.rybakina@postgrespro.ru> wrote:
>> However, is it necessary to check that extra->inner_unique must be false for SEMI/ANTI joins here, or am I missing something? It looks a little confusing at this point.
> If it is necessary, I don't see the reason for it. It was me that
> worked on unique joins and I see no reason why a SEMI or ANTI join
> couldn't be marked as unique. The reason they're not today is that the
> only point of the unique join optimisation is so that during
> execution, the join nodes could skip to the next outer tuple after
> matching the current outer to an inner.  If the join is unique, then
> there are no more join partners to find for the current outer after
> matching it up once. With SEMI and ANTI joins, we skip to the next
> outer tuple after finding a match anyway, so there's no point in going
> to the trouble of setting the inner_unique flag.
>
> I can't say definitively that we won't find a reason in the future
> that we should set inner_unique for SEMI/ANTI joins, so I don't follow
> the need for the Assert.
>
> Maybe you're seeing something that I'm not. What do you think will
> break if both flags are true?
>
Actually, I was mainly confused by the code itself - the check seemed to 
contradict the explanation. It looked like we were enforcing that
inner_unique must be false for SEMI/ANTI joins, even though it's not 
actually important for those join types.
That’s why I originally proposed either adding an Assert or removing 
this flag from check altogether, just to make it more explicit.

So, I agree with your explanation — by the definition of SEMI and ANTI 
joins, there's no need to set inner_unique, and also no need to assert 
against it.
These joins skip to the next outer tuple once they find a match (or fail 
to find one, in the case of ANTI).

I updated the diff, where I left changes only in the code comment.

-- 
Regards,
Alena Rybakina
Postgres Professional

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