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List:       postgresql-general
Subject:    Re: [HACKERS] [PERFORM] temporary indexes
From:       Lukas Smith <smith () pooteeweet ! org>
Date:       2006-02-28 23:02:55
Message-ID: 4404D69F.7000407 () pooteeweet ! org
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Kevin Grittner wrote:

> I rewrote the query to use IN predicates rather than EXISTS predicates,
> and the cost estimates look like this:
> 
> EXISTS, no index:  1.6 billion
> EXISTS, with index:  0.023 billion
> IN, no index:  13.7 billion
> IN, with index:  10.6 billion
> 
> At least for the two EXISTS cases, the estimates were roughly accurate.
>  These plans were run against the data after the fix, but analyze has
> not been run since then, so the estimates should be comparable with the
> earlier post.
> 
> I'm not used to using the IN construct this way, so maybe someone can
> spot something horribly stupid in how I tried to use it.

I will have a look at your queries tomorrow. Some general advice (rdbms 
agnostic) on when to use IN and when to use EXISTS taken from "SQL 
performance tuning":

- if the inner table has few rows and the outer has many then IN is 
preferred
- if however you have a restrictive expression on the outer query you 
should preferr EXISTS
- use NOT EXISTS instead of NOT IN (break out early)

regards,
Lukas

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