distinct values and count over window function

Hello, this is my first post to this particular list (I've also posted
to performance list a while ago to report a bug).

I generate complex dynamic queries and paginate them using "LIMIT 50". I
have a need for implementing some statistics for the results as well and
I'd like to know if I can avoid extra queries or how to be able to do
that without much performance hit.

The statistics calculation that bothers me most is a distinct one. For
example, suppose you have a query that would return more than 50
distinct values while only the first 50 ones are brought by the
paginated query.

When the user asks for its statistics it should show all counts by
distinct value including those not present in the current results page
for certain columns.

I noticed that there is a plan for PG 9.2 to include a query_to_json
function that would indeed help me to get this done when used altogether
with a window function.

Our application is currently running PG 9.1 (was rolled back from 9.2
due to a performance bug that seems to be now fixed but not released
yet). Is there anyway I could avoid running another query to get this
kind of statistics?

The queries can become really complex and take almost 5s to complete in
some cases. Which would mean that performing 2 queries with those same
conditions would take about 10s... I could perform them in different
times but then the user would have to wait for 5s just to get the
statistics calculation... This is what I'd like to avoid.

This application is a clustered web application, so I'd rather avoid any
client library support for paginating results using cursors or something
like that and stick with OFFSET and LIMIT.

Any help is appreciated!

Thanks,
Rodrigo.