Re: Prepared statement vs. parameterized

On Fri, 27 Oct 2006, Bucky Jordan wrote:

> I've only used the PG JDBC driver for pretty standard things in the
> past, and on pretty small db's. However, the current project I'm working
> on will be using rather large data sets and likely make use of some more
> advanced features. So, I have a few questions related to JDBC
> parameterized statements and their relation to prepared statements on
> the PG backend.
>
> It appears that parameterized statements are always prepared- is this
> correct? If so, why (JDBC spec, performance..)?

They are always prepared (using the V3 protocol), but there are two
methods of prepared statement execution.  A statement can be prepared with
an explicit name (that is good for the rest of the connection) or on the
"unnamed" statement (that is good until reused).  Statements prepared on
the unnamed statement use the bound parameter values for planning purposes
so you should get performance equivalent to a non-parameterized query.
Statements prepared with explicit names do use generic planning
placeholders and can get bad query plans if you have uneven data
distributions.  When a PreparedStatement object is first created it will
use the unnamed statement for its first several uses.  Once the driver
detects that it is being reused again and again it will switch over to a
named statement.  This execution count is kept in the PreparedStatement
object and no pooling is done, so you have to look at your
PreparedStatement object lifetime to determine if it's being reused or
created again.  The switchover point is configured (or disabled) via a URL
parameter prepareThreshold[1] or may be set on each individual connection
or statement by using pg specific java code.

Kris Jurka

[1]
http://jdbc.postgresql.org/documentation/81/connect.html#connection-parameters