Michael Nolan <htfoot@gmail.com> writes:
> It also appears you cannot group on a column of type xid.
You can in 8.4 and up. Previous versions only know how to GROUP BY
sortable columns, which requires a btree opclass, which xid doesn't
have and really can't have because it doesn't have a linear ordering.
There is a hash opclass for it, though, so in versions that know how to
GROUP BY using hashing, it'll work.
> Would adding a <> operator enable that?
No, it's pretty irrelevant ...
regards, tom lane