performance of functions - or rather lack of it

We are building a postgresql based backend database for our 'hosting
provisioning' system. In a vain attempt to add some, what I thought, simple
performance tweaks, I thought I would try putting some of the larger and
more straighforward queries into functions. For everything else the same,
the functions are on the whole slower. Should they be ?

The whole thing is being driver through perl DBI. This may be contributory.

Anyhow, the original query:

SELECT COUNT(mb.instance) FROM domain dm, mail ms, mailbox mb WHERE
dm.enabled = true and dm.existent = true and dm.available = true AND
ms.enabled = true and ms.existent = true and ms.available = true AND
mb.enabled = true and mb.existent = true and mb.available = true AND
dm.parent = ms.parent AND
mb.parent = ms.serviceid AND
dm.instance = $q_domain AND
mb.instance = $q_local_part;

where $q_XXX are quoted perl scalars. The function is then:

CREATE FUNCTION mail_is_mailbox(text, text) RETURNS int4 AS '
SELECT COUNT(mb.instance) FROM domain dm, mail ms, mailbox mb WHERE
dm.enabled = true and dm.existent = true and dm.available = true AND
ms.enabled = true and ms.existent = true and ms.available = true AND
mb.enabled = true and mb.existent = true and mb.available = true AND
dm.parent = ms.parent AND
mb.parent = ms.serviceid AND
dm.instance = $2 AND
mb.instance = $1;'
LANGUAGE 'sql';

SELECT mail_is_mailbox($q_local_part, $q_domain);

Running both these 1000 times from a remote (same subnet 100BaseTX) client
with the same query results in time for the function typically 20 - 25% more
than the bare query. 22 vs 16 seconds for example.

I would have thought that not sending the long SQL across the wire 1000
times would have saved some time even without any potential query
optimisations by pre-parsing the SQL ?

rgds,
--
Peter Galbavy
Knowledge Matters Ltd.
http://www.knowledge.com/