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Re: Basic question about indexes/explain

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Mario Weilguni <mweilguni(at)sime(dot)com>
Cc: Terry Yapt <pgsql(at)technovell(dot)com>,pgsql-performance(at)postgresql(dot)org
Subject: Re: Basic question about indexes/explain
Date: 2002-10-26 14:24:52
Message-ID: 27301.1035642292@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
Mario Weilguni <mweilguni(at)sime(dot)com> writes:
> Everything is explained in the manual, check
> http://developer.postgresql.org/docs/postgres/indexes.html

In particular note the comments at the bottom of
http://developer.postgresql.org/docs/postgres/performance-tips.html:

"It is worth noting that EXPLAIN results should not be extrapolated to
situations other than the one you are actually testing; for example,
results on a toy-sized table can't be assumed to apply to large
tables. The planner's cost estimates are not linear and so it may well
choose a different plan for a larger or smaller table. An extreme
example is that on a table that only occupies one disk page, you'll
nearly always get a sequential scan plan whether indexes are available
or not. The planner realizes that it's going to take one disk page read
to process the table in any case, so there's no value in expending
additional page reads to look at an index."

			regards, tom lane

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