about new join syntax performance

Hello,

we are working with v7.0.2 but to upgrade to the recent version of postgres regarding to the full join syntax most demanded by our web-application.
I'd like to get some tips about how to rewrite our queries to achieve better performance.

[ from the manual 2.1.1.2. Subqueries in FROM clause ]
" More interesting cases, which can't be reduced to a plain join, arise when the subquery involves grouping or aggregation."

Take a look at this query:
SELECT * FROM (SELECT did, count(*) FROM values WHERE ... GROUP BY did HAVING count(*) > ...) AS V(did, ok) JOIN document AS D USING(did);

My question is this significantly faster than two separate queries which are used now (in v7.0.2) via PHP (libpq):
SELECT did, count(*) INTO TEMPORARY TABLE tmp FROM values WHERE ... GROUP BY did HAVING count(*) > ...;
SELECT * FROM tmp AS V, document AS D WHER V.did = D.did;

additional informations:
table document:
did serial primary key,
content text not null
total rows = 22.000 -

table values :
did serial references (document),
value text not null,
/* other columns */
total rows = 2 or 5 times of document rows, (there are 2 up to 5 references to each document)

I'd like to know whether it's worth changing the scripts to use the new join syntax.

Papp Gyozo
- pgerzson(at)freestart(dot)hu