Re: 8.1 count(*) distinct: IndexScan/SeqScan - Mailing list pgsql-performance

From Kyle Cordes
Subject Re: 8.1 count(*) distinct: IndexScan/SeqScan
Date
Msg-id 438681E0.6070006@kylecordes.com
Whole thread Raw
In response to Re: 8.1 count(*) distinct: IndexScan/SeqScan  (Tom Lane <tgl@sss.pgh.pa.us>)
Responses Re: 8.1 count(*) distinct: IndexScan/SeqScan  ("Steinar H. Gunderson" <sgunderson@bigfoot.com>)
List pgsql-performance
Tom Lane wrote:

>What "same result"?  You only ran it up to 2K rows, not 2M.  In any
>case, EXPLAIN without ANALYZE is pretty poor ammunition for complaining
>that the planner made the wrong choice.  I ran the same
>

Hello, sorry to jump in mid-stream, but this reminded me of something.

I have hit cases where I have a query for which there is a somewhat
"obvious" (to a human...) query plan that should make it possible to get
a query answer pretty quickly.  Yet the query "never" finishes (or
rather, after hours of waiting I finally kill it).  I assume this is
because of a sub-optimal query plan.  But, it appears that an EXPLAIN
ANALYZE runs the actual query, so it takes as long as the actual query.

In such a case, how can I go about tracking down the issue, up to an
including a complaint about the query planner?   :-)

(Overall, I'm pretty pleased with the PG query planner; it often gets
better results than another, popular commercial DBMS we use here....
that is just a general impression, not the result of setting up the same
schema in each for a comparison.)

Kyle Cordes
www.kylecordes.com



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