Обсуждение: Appending "LIMIT" to query drastically decreases performance

Can anyone explain the following odd behavior?
I have a query that completes in about 90 ms. If I append LIMIT to the
very end, eg. "LIMIT 500" the evaluation time increases to about 800 ms.
How can performance get *worse* by giving the database the option to
stop the evaluation earlier (when it reaches the output 500 rows)?

I have pasted both queries together with output from explain analyze here:
    http://pastebin.com/m3c0d1896

On Fri, 30 Nov 2007, cluster wrote:
> Can anyone explain the following odd behavior?
> I have a query that completes in about 90 ms. If I append LIMIT to the
> very end, eg. "LIMIT 500" the evaluation time increases to about 800 ms.
> How can performance get *worse* by giving the database the option to
> stop the evaluation earlier (when it reaches the output 500 rows)?

The planner doesn't always get it right. Simple.

Have you done a "VACUUM FULL ANALYSE" recently?

Matthew

--
It is better to keep your mouth closed and let people think you are a fool
than to open it and remove all doubt.                  -- Mark Twain

In response to Matthew <matthew@flymine.org>:

> On Fri, 30 Nov 2007, cluster wrote:
> > Can anyone explain the following odd behavior?
> > I have a query that completes in about 90 ms. If I append LIMIT to the
> > very end, eg. "LIMIT 500" the evaluation time increases to about 800 ms.
> > How can performance get *worse* by giving the database the option to
> > stop the evaluation earlier (when it reaches the output 500 rows)?
>
> The planner doesn't always get it right. Simple.
>
> Have you done a "VACUUM FULL ANALYSE" recently?

I don't know about the "FULL" part ... but certainly an ANALYZE.

Please post EXPLAIN ANALYZE output for the two queries.

--
Bill Moran
Collaborative Fusion Inc.
http://people.collaborativefusion.com/~wmoran/

wmoran@collaborativefusion.com
Phone: 412-422-3463x4023

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> Please post EXPLAIN ANALYZE output for the two queries.

As I wrote in my first post, I pasted this together with the two queries
at pastebin.com:
   http://pastebin.com/m3c0d1896