Re: Hmm, nodeUnique doesn't really support backwards scan too well

Gregory Stark <stark(at)enterprisedb(dot)com> writes:
> "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:
>> Well, if you think it's easy, the best form of criticism is a patch.
>> The change-of-direction problem seems to me to be messy --- not
>> insoluble, but messy enough to need beta testing.

> Hm, I must have misunderstood the bug because there's a comment in nodeUnique
> which claims it already does precisely what I was suggesting:

> * We return the first tuple from each group of duplicates (or the last
> * tuple of each group, when moving backwards). At either end of the
> * subplan, clear the result slot so that we correctly return the
> * first/last tuple when reversing direction.

That's what it *used* to say. But the problem is that that's the wrong
behavior, because you get different tuples returned depending on which way
you are traveling. It's only workable if the tuples in a group are
completely indistinguishable.

regards, tom lane