Re: Optimizer + bind variables

On Wed, Nov 04, 2009 at 07:43:16AM +0800, Craig Ringer wrote:
- David Kerr wrote:
- > Does/is it possible for the PG optimizer come up with differnet plans when
- > you're using bind variables vs when you send static values?
-
- Yes, if the bind variable form causes your DB access driver to use a
- server-side prepared statement. Pg can't use its statistics to improve
- its query planning if it doesn't have a value for a parameter when it's
- building the query plan.

hmm, that's a little unclear to me.

let's assume that the application is using prepare:

Assuming the database hasn't changed, would:
PREPARE bla1 as SELECT * from users where username = '$1';
explain execute bla1

give the same output as
explain select * from users where username = 'dave';

?

- Whether a server-side prepared statement is used or not depends on how
- you're connecting to the database - ie your DB access driver and
- version. If you're using JDBC, I *think* the JDBC driver does parameter
- placement client-side unless you're using a JDBC prepared statement and
- the JDBC prepared statement is re-used several times, at which point it
- sets up a server-side prepared statement. AFAIK otherwise it uses
- client-side (or Pg protocol level) parameter placement.
that's interesting, i'll need to find out which mine are using, probably
a mix of both.

- > if it's possible for the plan to be different how can i generate an
- > xplan for the bind version?
-
- xplan = explain? If so:
yeah, sorry.

- Use PREPARE to prepare a statement with the params, then use:
-
- EXPLAIN EXECUTE prepared_statement_name(params);
-
- eg:
-
- x=> PREPARE blah AS SELECT * FROM generate_series(1,100);
- PREPARE
- x=> EXPLAIN EXECUTE blah;
- QUERY PLAN
- ------------------------------------------------------------------------
- Function Scan on generate_series (cost=0.00..12.50 rows=1000 width=4)
- (1 row)

great thanks!

Dave