From: | Alvaro Herrera <alvherre(at)commandprompt(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Phil Frost <indigo(at)bitglue(dot)com>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: optimizing constant quals within outer joins |
Date: | 2006-07-02 00:48:21 |
Message-ID: | 20060702004821.GH8098@surnet.cl |
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Lists: | pgsql-hackers |
Tom Lane wrote:
> Alvaro Herrera <alvherre(at)commandprompt(dot)com> writes:
> > I note that the rowcount is not altered by the one-time filter. Is this
> > an issue? I imagine the problem is not being able to estimate the
> > number of rows that pass the filter.
>
> That's intentional. The filter is either going to pass all or none of
> the rows, not some fraction of them. It clearly isn't very reasonable
> to guess that it will pass none of them (except if the qual is actually
> constant FALSE).
>
> > I also wonder whether it wouldn't be better in this case to apply each
> > filter to each arm of the merge join.
>
> Uh, why? For the most part, I'd think the higher you can put the filter
> in the plan tree, the better.
Huh, sorry, I had misunderstood the meaning of a _one_-time filter :-)
--
Alvaro Herrera http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support
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