Jan Urbański <j.urbanski@students.mimuw.edu.pl> writes:
> OK, there might have been a mental shortcut there. "Can't be compared"
> was supposed to mean "can't decide whether one value of that type is
> bigger than another". Doing DISTINCT without an equality operator is
> nonsense. Doing it without a comparision operator is only very slow.
Well, you're still missing my point, which is how do you decide which
operator is "equality"? It was already pointed out upthread that
ignoring the type's operators and using bitwise comparison is a pretty
sucky alternative. The only infrastructure in Postgres that can
identify which operators have which semantics is index opclasses.
I see two possible TODO items in this discussion. One is that type
"point" is sorely lacking in opclass support. The other is that it
might be interesting to support DISTINCT in cases where only a hash
opclass, not a btree opclass, is available --- which would lead to
a hash-aggregation-like implementation instead of sort-and-uniq.
The value as far as type point is concerned is that you'd not have to
invent some arbitrary linear sort ordering for points.
The idea of supporting DISTINCT with neither type of opclass available
seems to me to be indefensible on *both* semantics and performance
grounds.
regards, tom lane
