From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Greg Smith <greg(at)2ndquadrant(dot)com> |
Cc: | Karl Larsson <karl(dot)larsson47(at)gmail(dot)com>, pgsql-performance(at)postgresql(dot)org |
Subject: | Re: seq scan instead of index scan |
Date: | 2009-12-18 06:27:11 |
Message-ID: | 29683.1261117631@sss.pgh.pa.us |
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Lists: | pgsql-performance |
Greg Smith <greg(at)2ndquadrant(dot)com> writes:
> Karl Larsson wrote:
>> When I make a subquery Postgres don't care about my indexes and makes
>> a seq scan instead of a index scan. Why?
> Data set is just too small for it to matter. Watch what happens if I
> continue from what you posted with much bigger tables:
> ...
> There's the index scan on both tables that you were expecting.
And if you go much past that, it's likely to switch *away* from
indexscans again (eg, to a hash join, which has no use for ordered
input). This is not wrong. Indexes have their place but they are not
the solution for every query.
regards, tom lane
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