counting distinct rows on more than one column

Hi,

on 7.0.3 want to COUNT
 SELECT DISTINCT a,b FROM t;

I can't find a solution because any combination with
count with more than one column gives syntax errors.

One solution would be to set a view:

CREATE VIEW v AS SELECT DISTINCT a,b FROM t;

and then

SELECT count(a) FROM v

but views don't support distinct in v7.0.3

Ok I could use a temporary table but my select distinct tends to give
large results.

Any clues?

Dirk