I've pretty much finished the project I got a bee in my bonnet about
last week, which is to teach SELECT DISTINCT how to (optionally) use
hashing for grouping in the same way that GROUP BY has been able to do
for awhile.
There are still two places in the system that hard-wire the use of
sorting for duplicate elimination:
* Set operations (UNION/INTERSECT/EXCEPT)
* Aggregate functions with DISTINCT
I'm thinking of trying to fix set operations before I leave this topic,
but I'm not sure it's worth the trouble to change DISTINCT aggregates.
They'd be a lot more work (since there's no executor infrastructure
in place that could be used) and the return on investment seems low.
Comments?
regards, tom lane