Re: WIP Join Removal

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: List pgsql-patches <pgsql-patches(at)postgresql(dot)org>
Subject: Re: WIP Join Removal
Date: 2008-09-02 17:13:40
Message-ID: 1220375620.4371.479.camel@ebony.2ndQuadrant
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On Tue, 2008-09-02 at 12:28 -0400, Tom Lane wrote:
> Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> > As discussed on 26 June, "Join Removal/Vertical Partitioning", here's a
> > patch to remove joins in certain circumstances.
> Some points not made in the thread so far:

Various comments accepted and agreed.

> > + * XXX Seems not worth searching partial indexes because those
> > + * are only usable with a appropriate restriction, so the
> > + * only way they could ever be usable is if there was a
> > + * restriction that exactly matched the index predicate,
> > + * which is possible but generally unlikely.
> I haven't thought this through entirely, but wouldn't a partial index be
> okay if it's marked predOK? You might be right that the case is
> unlikely, but if it's only one extra line to support it ...

As of now, its it is predOK then that implies there was a qual on the
checkrel so can't remove the join. So no need to check.

> > + if (removable &&
> > + joinrel->cheapest_total_path < keeprel->cheapest_total_path)
> This test on cheapest_total_path seems pretty wacko

and stuff below it definitely is, from upthread discussion.

Hang fire on more comments - its a short patch but needs a few thwacks
from the cluestick yet before next review.

Simon Riggs
PostgreSQL Training, Services and Support

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