Re: Removing redundant grouping columns

I wrote:
> Richard Guo <guofenglinux(at)gmail(dot)com> writes:
>> While we are here, I wonder if we can do the same trick for
>> distinctClause, to cope with cases like
>> select distinct a.x, b.y from a, b where a.x = b.y;

> We do that already, no?

Oh, wait, I see what you mean: we are smart in code paths that rely
on distinct_pathkeys, but not in the hash-based code paths. Right,
that can be fixed the same way. 0001 attached is the same as before,
0002 adds similar logic for the distinctClause.

The plan change in expected/pg_trgm.out is surprising at first
glance, but I believe it's correct: the item that is being
dropped is a parameterless STABLE function, so its value is not
supposed to change for the duration of the scan.

regards, tom lane