DISTINCT ON: speak now or forever hold your peace

If I don't hear loud hollers very soon, I'm going to eliminate the
DISTINCT ON "feature" for 7.0.  As previously discussed, this feature
is not standard SQL and has no clear semantic interpretation.

I hadn't been planning to touch DISTINCT before 7.0, but changed my
mind when I noticed this little gem:

create table foo1 (f1 int, f2 int, f3 int);
insert into foo1 values(1,2,3);
insert into foo1 values(1,2,null);
insert into foo1 values(1,null,2);
insert into foo1 values(1,2,4);
insert into foo1 values(1,2,4);

select * from foo1;f1 | f2 | f3
----+----+---- 1 |  2 |  3 1 |  2 | 1 |    |  2 1 |  2 |  4 1 |  2 |  4
(5 rows)

select distinct * from foo1;f1 | f2 | f3
----+----+---- 1 |  2 |  3 1 |  2 |  4 1 |  2 |
(3 rows)

Didn't know that (NULL, 2) == (2, NULL), did you?  The problem here
is that nodeUnique is doing a bitwise comparison of the tuple contents
(which is bad enough --- not all datatypes think equality is bitwise),
and it's neglecting to include the null-field bitmap in what it
compares.  Rather than just band-aid the null-field problem, I'm
going to fix it right.  As long as I have to touch it, I'll deal
with DISTINCT ON too.
        regards, tom lane