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Re: Indexes and statistics

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "David Witham" <davidw(at)unidial(dot)com(dot)au>
Cc: pgsql-sql(at)postgresql(dot)org
Subject: Re: Indexes and statistics
Date: 2004-02-18 05:10:05
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-sql
"David Witham" <davidw(at)unidial(dot)com(dot)au> writes:
> One of the customers is quite large (8.3% of the records):

Hmm.  Unless your rows are quite wide, a random sampling of 8.3% of the
table would be expected to visit every page of the table, probably
several times.  So the planner's cost estimates do not seem out of line
to me; an indexscan *should* be slow.  The first question to ask is why
the deviation from reality.  Are the rows for that customer ID likely to
be physically concentrated into a limited number of physical pages?
Do you have so much RAM that the whole table got swapped in, eliminating
the extra I/O that the planner is expecting?

			regards, tom lane

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