Magnus reported a similar problem with path names. I looked at his
statistics and found that even at 100 buckets, his LIKE 'f:/.../%" query
would never span more than one bucket, and because all the path names
were unique, there were no most common values.
In the case where the LIKE hits only one bucket, and there are no most
common values, how is the optimzier supposed to estimate the number of
rows, especially for cases where the values in the buckets are unevenly
distributed.
---------------------------------------------------------------------------
Robert Treat wrote:
> After some extensive discussion on irc, berkus, myself and a few others
> think we have uncovered a possible bug, or at the least some odd
> behavior in > 8.1.1. It centers around my recent post to performance
> http://archives.postgresql.org/pgsql-performance/2006-01/msg00248.php
> and how I could not seem to get some of the estimates to become
> reasonable even after bumping up my stats target to 400 which caused
> every row to be analyzed. If you look at the left join and hash join
> estimates of the third query you'll note they seem to always get
> estimated to 1 for no reason that we could come up with.
>
> Someone else on irc seemed to have a similar problem to this, so we are
> wondering if there is some problem here. So the question really is if
> someone can deduce the behavior from looking at what was provided in the
> email? If not and you have questions let me know, otherwise I can send a
> chopped up test database which can reproduce the query issues off list
> should someone want to walk through the pg code to investigate. TIA
>
>
> Robert Treat
> --
> Build A Brighter Lamp :: Linux Apache {middleware} PostgreSQL
>
>
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> TIP 2: Don't 'kill -9' the postmaster
>
--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 359-1001
+ If your life is a hard drive, | 13 Roberts Road
+ Christ can be your backup. | Newtown Square, Pennsylvania 19073
