Tom,
We were looking at the explain results and noticed that it was converting
the 'wynn%' into fname >= 'wynn' and fname < 'wyno'
So we modified our function to mimic this transformation, where we have
two variables that contain the 'wynn' and 'wyno':
Fname >= @var1 and fname < @var2
This seems to pick up the index ok, but is this a valid and reliable
solution? Or is there another/better way to do this?
Thanks for the explanation, it really helps.
John
-----Original Message-----
From: Tom Lane [mailto:tgl@sss.pgh.pa.us]
Sent: Friday, April 27, 2007 5:57 PM
To: Listmail
Cc: John Cole; 'pgsql-general@postgresql.org'
Subject: Re: [GENERAL] Query in function not using index...
Listmail <lists@peufeu.com> writes:
>> Any idea why using a variable v_streetName instead of a string 'wynn%'
>> behaves differently?
> Yeah.
> 'wynn%' doesn't start with % so LIKE uses index.
> But Postgres doesn't know that you know that the contents of this
> variable never starts with '%'...
Even more to the point: the transformation of LIKE 'pattern' into an
index range is a plan-time optimization, so if the planner hasn't got a
constant pattern to work with, it'll never happen. (Yeah, this should
be improved someday.)
If you need to depend on LIKE indexing with non-constant patterns,
you'll have to use EXECUTE to force the query to be replanned each time.
regards, tom lane
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