Hash grouping, aggregates

So one of the items on the TODO list is "Add hash for evaluating GROUP BY
aggregates (Tom)"

I'm finding this would benefit a lot of my queries. Most of the time seems to
be going into sorts for group by clauses. I don't know how long it would take
to build a hash of course, but I suspect it would be less than the sort.

Is this something a beginner could figure out? I'm thinking I need a normal
Hash node that builds exactly the same kind of hash as a join, then a HashScan
node that picks all the rows out of the hash.

The neat thing is that hash aggregates would allow grouping on data types that
have = operators but no useful < operator.

(Incidentally, I'm fond of "nested loop", I remember when I was a beginner SQL
programmer looking at plans and it was intuitively obvious what it meant. I
suspect for a beginner looking at "nestloop" it might not be quite so
obvious.)

--
greg