From: | Wim Ceulemans <wim(dot)ceulemans(at)nice(dot)be> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Joseph Shraibman <jks(at)selectacast(dot)net>, Simon Hardingham <simon(at)netxtra(dot)net>, pgsql-general(at)postgresql(dot)org |
Subject: | Re: query optimiser changes 6.5->7.0 |
Date: | 2000-06-02 09:06:04 |
Message-ID: | 393778FC.ADE5346B@nice.be |
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Lists: | pgsql-general |
Tom Lane wrote:
>
> Joseph Shraibman <jks(at)selectacast(dot)net> writes:
> > OK this may seem like a stupid question, but isn't index scan always
> > better except for the pathalogical simple case where the work to be done
> > is trivial anyway?
>
> No. If it were, the optimizer would be a whole lot simpler ;-)
>
> In practice an indexscan only wins if it will visit a relatively
> small percentage of the tuples in the table. The $64 questions
> are how small is small enough, and how can the optimizer guess
> how many tuples will be hit in advance of doing the query...
>
Isn't there a way to tell the optimizer to use an index scan if one
knows this is going to be the best.
I have seen lots of posts concerning people who are trying to force the
optimizer to use an index scan by rephrasing their SQL statements. Isn't
there a possibility to provide some syntax (non SQL compliant I know) to
force the optimizer to do an index scan?
Regards
Wim
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