Thread: Index not used with prepared statement

Hi.

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 have to set the
protocolVersion=2 and use the JDBC2 driver to get it working (but
then the statements are created like in the first query, so no
solution, only a workaround).

I'm not sure whether this is a bug (I think it is) or a problem of
understanding.

Known problem?

I have tried PG 8.0.1, 8.0.3, 8.1beta with the JDBC-drivers

- postgresql-8.0-312.jdbc2.jar --> okay with protocolVersion=2 in the
URL
- postgresql-8.0-312.jdbc3.jar --> not okay whatever I do

I'm on Mac OS X, if that matters.

cug

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
--

On 11.09.2005, at 11:03 Uhr, Andreas Seltenreich wrote:

> 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).

Hmm. Now I get it. So I have to look that my framework doesn't
produce a preparedStatement, instead build a complete statement
string. Weird.

> 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".

Yes, you are right, but then I can't pass anything like '45%' to the
query. It will just return nothing.

I use the "like" because I build the queries on the fly and add a %
at the end where necessary.

And, to be clear: this is a minimal example, most of my queries are
generated by a framework. This was an example to test the behaviour.

Okay, I had problems with the understanding of prepared statements on
the client and the server side. What I thought was, that I get a
preparedStatement by JDBC which also inserts the values into the
string and this is executed on the server side.

cug