Always one row with group by / agregate

dip=> create table t ( x int , y int );
CREATE
dip=> select x,min(y) from t group by x;
x|min
-+---
 |
(1 row)


select min(y) should return one row with a null value, but it's
expected. I thought that the first part of the job was to process
the group by and then the select on each group, but it doesn't
seem to be the case.

Alain