Re: empty string causes planner to avoid index. Makes me sad. - Mailing list pgsql-general

Le vendredi 27 novembre 2009 à 23:32:14, Jeff Amiel a écrit :
> --- On Fri, 11/27/09, Tom Lane <tgl@sss.pgh.pa.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.
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