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Re: JDBC question for PG 8.3.9

From: Craig Ringer <craig(at)postnewspapers(dot)com(dot)au>
To: Dave Crooke <dcrooke(at)gmail(dot)com>
Cc: pgsql-performance <pgsql-performance(at)postgresql(dot)org>
Subject: Re: JDBC question for PG 8.3.9
Date: 2010-04-14 23:10:28
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
On 15/04/10 04:49, Dave Crooke wrote:
> Hi foilks
> I am using PG 8.3 from Java. I am considering a performance tweak which
> will involve holding about 150 java.sql.PreparedStatment objects open
> against a single PGSQL connection. Is this safe?
> I know that MySQL does not support prepared statements /per se/, and so
> their implementation of PreparedStatement is nothing more than some
> client-side convenience code that knows how to escape and format
> constants for you. Is this the case for PG, or does the PG JDBC driver
> do the real thing?

Pg supports real server-side prepared statements, as does the JDBC driver.

IIRC (and I can't say this with 100% certainty without checking the 
sources or a good look at TFM) the PostgreSQL JDBC driver initially does 
only a client-side prepare. However, if the PreparedStatement is re-used 
more than a certain number of times (five by default?) it switches to 
server-side prepared statements.

This has actually caused a bunch of performance complaints on the jdbc 
list, because the query plan may change at that switch-over point, since 
with a server-side prepared statement Pg no longer has a specific value 
for each parameter and may pick a more generic plan.

Again only IIRC there's a configurable threshold for prepared statement 
switch-over. I thought all this was in the PgJDBC documentation and/or 
javadoc - if it's not, it needs to be.

Craig Ringer

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