-Original Message-
From: Tom Lane [mailto:[EMAIL PROTECTED]
Sent: Wed 12/14/2005 9:36 PM
To: Gregory S. Williamson
Cc: pgsql-performance@postgresql.org; PostGIS Users Discussion
Subject: Re: [PERFORM] [postgis-users] Is my query planner failing
me,
or vice versa?
Gregory S. Williamson [EMAIL PROTECTED] writes:
Forgive the cross-posting, but I found myself wondering if might not
be some way future way of telling the planner that a given table
(column ?) has a high likelyhood of being TOASTed.
What would you expect the planner to do with the information, exactly?
We could certainly cause ANALYZE to record some estimate of this, but
I'm not too clear on what happens after that...
regards, tom lane
-Original Message-
From: [EMAIL PROTECTED] [mailto:postgis-users-
[EMAIL PROTECTED] On Behalf Of Gregory S. Williamson
Sent: 15 December 2005 12:03
To: Tom Lane
Cc: pgsql-performance@postgresql.org; PostGIS Users Discussion
Subject: RE: [PERFORM] [postgis-users] Is my query planner failing me,or
vice versa?
Well, what does the random_page_cost do internally ?
I don't think I'd expect postgres to be able to *do* anything in
particular, any more than I would expect it to do something about slow
disk I/O or having limited cache. But it might be useful to the EXPLAIN
ANALYZE in estimating costs of retrieving such data.
Admittedly, this is not as clear as wanting a sequential scan in
preference to indexed reads when there are either very few rows or a huge
number, but it strikes me as useful to me the DBA to have this factoid
thrust in front of me when considering why a given query is slower than I
might like. Perhaps an added time based on this factor and the
random_page_cost value, since lots of TOAST data and a high access time
would indicate to my (ignorant!) mind that retrieval would be slower,
especially over large data sets.
Forgive my ignorance ... obviously I am but a humble user. grin.
G
As I understood from the original discussions with Markus/Tom, the problem
was that the optimizer didn't consider the value of the VacAttrStats
stawidth value when calculating the cost of a sequential scan. I don't know
if this is still the case though - Tom will probably have a rough idea
already whereas I would need to spend some time sifting through the source.
However, I do know that the PostGIS statistics collector does store the
average detoasted geometry size in stawidth during ANALYZE so the value is
there if it can be used.
Kind regards,
Mark.
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