Re: Performance improvement for joins where outer side is unique

From: David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Performance improvement for joins where outer side is unique
Date: 2017-04-07 01:59:40
Message-ID: CAKJS1f-tMfBFJPSP6d07zNgpRZuKXs9D6BJ3GjTdAneMRwOmHg@mail.gmail.com
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On 7 April 2017 at 13:41, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> David Rowley <david(dot)rowley(at)2ndquadrant(dot)com> writes:
>> On 7 April 2017 at 11:47, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> What I'm on about is that you can't do the early advance to the
>>> next outer tuple unless you're sure that all the quals that were
>>> relevant to the uniqueness proof have been checked for the current
>>> inner tuple. That affects all three join types not only merge.
>
>> Well, look at the join code and you'll see this only happens after the
>> joinqual is evaulated. I didn't make a special effort here. I just
>> borrowed the location that JOIN_SEMI was already using.
>
> Right, and that's exactly the point: some of the conditions you're
> depending on might have ended up in the otherqual not the joinqual.
>
> We'd discussed rearranging the executor logic enough to deal with
> such situations and agreed that it seemed too messy; but that means
> that the optimization needs to take care not to use otherqual
> (ie pushed-down) conditions in the uniqueness proofs.
>
>> Oh yeah. I get it, but that's why we ignore !can_join clauses
>
> can_join seems to me to be not particularly relevant ... there's
> nothing that prevents that from getting set for pushed-down clauses.
>
> It's possible that the case I'm worried about is unreachable in
> practice because all the conditions that could be of interest would?
> be strict and therefore would have forced join strength reduction.
> But I'm not comfortable with assuming that.

Okay, well how about we protect against that by not using such quals
as unique proofs? We'd just need to ignore anything that's
outerjoin_delayed?

If we're struggling to think of a case that this will affect, then we
shouldn't be too worried about any missed optimisations.

A patch which does this is attached.

--
David Rowley http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services

Attachment Content-Type Size
unique_joins_2017-04-07b.patch application/octet-stream 95.7 KB

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