> -----Ursprüngliche Nachricht-----
> Von: Tom Lane [mailto:[EMAIL PROTECTED]
> Gesendet: Montag, 5. Dezember 2005 16:12
> An: Markus Wollny
> Cc: [email protected]
> Betreff: Re: AW: AW: [PERFORM] Queries taking ages in PG 8.1,
> have been much faster in PG<=8.0
>
> "Markus Wollny" <[EMAIL PROTECTED]> writes:
> >> Could we see the pg_stats row for answer.session_id in
> both 8.0 and
> >> 8.1?
>
> > Here you are:
>
> > 8.1:
> > Correlation -0.0736492
>
> > 8.0.3:
> > Correlation -0.237136
>
> Interesting --- if the 8.1 database is a dump and restore of
> the 8.0, you'd expect the physical ordering to be similar.
I dumped the data from my 8.0.1 cluster on 2005-11-18 00:23 using pg_dumpall
with no further options; the dump was passed through iconv to clear up some
UTF-8 encoding issues, then restored into a fresh 8.1 cluster where it went
productive; I used the very same dump to restore the 8.0.3 cluster. So there is
a difference between the two datasets, an additional 230.328 rows in the
answers-table.
> Why is 8.1 showing a significantly lower correlation? That
> has considerable impact on the estimated cost of an indexscan
> (plain not bitmap), and so it might explain why 8.1 is
> mistakenly avoiding the indexscan ...
I just ran a vacuum analyze on the table, just to make sure that the stats are
up to date (forgot that on the previous run, thanks to pg_autovacuum...), and
the current correlation on the 8.1 installation is now calculated as -0.158921.
That's still more than twice the value as for the 8.0-db. I don't know whether
that is significant, though.
Kind regards
Markus
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