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Data Bases > Pgsql Performance > Re: Understandi...
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Re: Understanding histograms

by stark@[EMAIL PROTECTED] (Gregory Stark) Apr 30, 2008 at 08:53 PM

"Tom Lane" <tgl@[EMAIL PROTECTED]
> writes:

> Right.  As a matter of policy we never estimate less than one matching
> row; and I've seriously considered pu****ng that up to at least two rows
> except when we see that the query condition matches a unique constraint.
> You can get really bad join plans from overly-small estimates.

This is something that needs some serious thought though. In the case of
partitioned tables I've seen someone get badly messed up plans because
they
had a couple hundred partitions each of which estimated to return 1 row.
In
fact of course they all returned 0 rows except the correct partition.
(This
was in a join so no constraint exclusion)

-- 
  Gregory Stark
  EnterpriseDB          http://www.enterprisedb.com
  Ask me about EnterpriseDB's 24x7 Postgres sup****t!

-- 
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 8 Posts in Topic:
Understanding histograms
len@[EMAIL PROTECTED] (L  2008-04-29 21:56:32 
Re: Understanding histograms
tgl@[EMAIL PROTECTED] (T  2008-04-30 01:19:51 
Re: Understanding histograms
lenshap@[EMAIL PROTECTED]  2008-04-29 23:32:18 
Re: Understanding histograms
tgl@[EMAIL PROTECTED] (T  2008-04-30 10:43:11 
Re: Understanding histograms
pgsql@[EMAIL PROTECTED]   2008-04-30 15:47:02 
Re: Understanding histograms
tgl@[EMAIL PROTECTED] (T  2008-04-30 19:17:44 
Re: Understanding histograms
stark@[EMAIL PROTECTED]   2008-04-30 20:53:44 
Re: Understanding histograms
tgl@[EMAIL PROTECTED] (T  2008-05-01 00:41:07 

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tan12V112 Mon Oct 13 2:18:41 CDT 2008.