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Re: query planner weirdness?

From: "Bob Duffey" <bobduffey68(at)gmail(dot)com>
To: "Adam Rich" <adam(dot)r(at)sbcglobal(dot)net>
Cc: "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-general(at)postgresql(dot)org
Subject: Re: query planner weirdness?
Date: 2008-06-28 05:00:43
Message-ID: 14422aad0806272200j44f4df96q21d03c265490c655@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-general
2008/6/28 Adam Rich <adam(dot)r(at)sbcglobal(dot)net>:

>
> > This is not wrong, or at least not obviously wrong.  A full-table
> > indexscan is often slower than seqscan-and-sort.  If the particular
> > case is wrong for you, you need to look at adjusting the planner's
> > cost parameters to match your environment.  But you didn't provide any
> > evidence that the chosen plan is actually worse than the alternative
> > ...
>
> I think I understand what Bob's getting at when he mentions blocking.
> The seqscan-and-sort would return the last record faster, but the
> indexscan returns the first record faster.  If you're iterating
> through the records via a cursor, the indexscan behavior would be
> more desirable.  You could get the initial rows back without waiting
> for all 130 million to be fetched and sorted.
>
> In oracle, there is a first-rows vs. all-rows query hint for this sort
> of thing.
>

Yes, that's exactly what I mean.  I've already tried your suggestion (set
enable_seqscan to off) with no luck.

Bob

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