Mark Volpe <volpe.mark@epamail.epa.gov> writes:
> Is it possible to use an aggregate on a view
> that contains an aggregate?
Not at present. Views are implemented by a rewriter that tries
to transform your query into another SQL query, and since aggregates
of aggregates are not possible, it doesn't work. In current sources
I actually get an error from your second example:
regression=# SELECT sum(total) FROM y;
ERROR: Aggregate function calls may not be nested
For the same sort of reason, GROUPed views don't play nice with an
outer query that specifies different grouping (ie, has a GROUP clause
of its own, or perhaps an aggregate).
What we need in order to fix this is subselects in FROM clauses;
if the rewriter could transform your query into
SELECT sum(total) FROM (SELECT n, count(*) AS total FROM x GROUP BY n)
then everything would just work (and the rewriter would get a lot
simpler, too ;-)). We don't have subselects in FROM yet, but I hope
to see them in 7.1, or 7.2 at the latest.
regards, tom lane