[GENERAL] oracle's first_value function for postgres?

[Ben]

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?

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