From: | Guillaume Lelarge <guillaume(at)lelarge(dot)info> |
---|---|
To: | pgsql-general(at)postgresql(dot)org |
Cc: | Jeff Amiel <becauseimjeff(at)yahoo(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Subject: | Re: empty string causes planner to avoid index. Makes me sad. |
Date: | 2009-11-27 22:50:54 |
Message-ID: | 200911272350.54263.guillaume@lelarge.info |
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Lists: | pgsql-general |
Le vendredi 27 novembre 2009 à 23:32:14, Jeff Amiel a écrit :
> --- On Fri, 11/27/09, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > You didn't show us any evidence of that, either. Both
> > of your test
> > cases are using the index.
>
> Ok...third try. The cost when passing in an empty string is SIGNIFICANTLY
> higher than when not. Wouldn't seem that the planner is using the index
> effectively.
>
> Aggregate (cost=231884.57..231884.57 rows=1 width=0)
>
> versus
>
> Aggregate (cost=34.83..34.83 rows=1 width=0)
>
But in the first example (the empty string one), it fetched 170468 rows from
the index, and in the second one (the 17-spaces string), it fetched only 18
rows. It seems quite normal that the first one is costier then the second one.
--
Guillaume.
http://www.postgresqlfr.org
http://dalibo.com
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