GROUP BY DISTINCT

When combining multiple grouping items, such as rollups and cubes, the
resulting flattened grouping sets can contain duplicate items. The
standard provides for this by allowing GROUP BY DISTINCT to deduplicate
them prior to doing the actual work.

For example:
GROUP BY ROLLUP (a,b), ROLLUP (a,c)

expands to the sets:
(a,b,c), (a,b), (a,b), (a,c), (a), (a), (a,c), (a), ()

but:
GROUP BY DISTINCT ROLLUP (a,b), ROLLUP (a,c)

expands to just the sets:
(a,b,c), (a,b), (a,c), (a), ()

Attached is a patch to implement this for PostgreSQL.
--
Vik Fearing