From: Andreas Seltenreich <andreas+pg@gate450.dyndns.org>
To: Guido Neitzer <guido.neitzer@pharmaline.de>
Cc: pgsql-performance@postgresql.org
Subject: Re: Index not used with prepared statement
References: <33A6D7B7-FDF3-456E-9154-402E9C5BC5D9@pharmaline.de>
Date: Sun, 11 Sep 2005 11:03:23 +0200
In-Reply-To: <33A6D7B7-FDF3-456E-9154-402E9C5BC5D9@pharmaline.de> (Guido
	Neitzer's message of "Sun, 11 Sep 2005 10:29:09 +0200")
User-Agent: Gnus/5.110004 (No Gnus v0.4) Emacs/21.4 (gnu/linux)
Message-ID: <87vf18x9gk.fsf@gate450.dyndns.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii

Guido Neitzer schrob:

> I have a performance problem with prepared statements (JDBC prepared  
> statement).
>
> This query:
>
> PreparedStatement st = conn.prepareStatement("SELECT id FROM  
> dga_dienstleister WHERE plz like '45257'");
>
> does use an index.
>
> This query:
>
>         String plz = "45257";
>         PreparedStatement st = conn.prepareStatement("SELECT id FROM  
> dga_dienstleister WHERE plz like ?");
>         st.setString(1, plz);
>
> does NOT use an index.
>
> As it should in the end result in absolutely the same statement, the  
> index should be used all the time.

I'm not perfectly sure, but since the index could only be used with a
subset of all possible parameters (the pattern for like has to be
left-anchored), I could imagine the planner has to avoid the index in
order to produce an universal plan (the thing behind a prepared
statement).

Is there a reason you are using the like operator at all? IMO using
the =-operator instead in your example should produce an "index-using
prepared statement".

HTH
Andreas
--