Re: WIP Join Removal

On Tue, 2008-09-02 at 13:20 +0300, Heikki Linnakangas wrote:
> Simon Riggs wrote:
> > It turns out that a join like this
> >
> > select a.col2
> > from a left outer join b on a.col1 = b.col1
> > where b.col2 = 1;
> >
> > can be cheaper if we don't remove the join, when there is an index on
> > a.col1 and b.col2, because the presence of b allows the values returned
> > from b to be used for an index scan on a.
>
> Umm, you *can't* remove that join.

Yes, you can. The presence or absence of rows in b is not important to
the result of the query because of the "left outer join".

I spent nearly a whole day going down that deadend also.

> Because of the condition "b.col2 =
> 1", which implies that "b.col1 IS NOT NULL",

No it doesn't, but as above, it is irrelevant anyway.

> that's actually equal to:

> select a.col2
> from a inner join b on a.col1 = b.col1
> where b.col2 = 1;

--
 Simon Riggs           www.2ndQuadrant.com
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