Oracle has a very handy function called first_value, which can be used
to turn a set like this:
a 10
a 3
b 1
c 30
c 10
d 1
d 20
...into this:
a 10
b 1
c 30
d 1
Does postgres have something equivalent, or, even better, is there a
reasonable way to express this in standard SQL?
