>>>>> "Joe" == Joe Wildish <joe-postgresql....@elusive.cx> writes:
Joe> Agreed. My assumption was that we would record in the data Joe> dictionary the behaviour (or “polarity") of each aggregate Joe> function with respect to the various operators. Column in Joe> pg_aggregate? I don’t know how we’d record it exactly. I haven't looked at the background of this, but if what you want to know is whether the aggregate function has the semantics of min() or max() (and if so, which) then the place to look is pg_aggregate.aggsortop. (For a given aggregate foo(x), the presence of an operator oid in aggsortop means something like "foo(x) is equivalent to (select x from ... order by x using OP limit 1)", and the planner will replace the aggregate by the applicable subquery if it thinks it'd be faster.) As for operators, you can only make assumptions about their meaning if the operator is a member of some opfamily that assigns it some semantics. For example, the planner can assume that WHERE x=y AND x=1 implies that y=1 (assuming x and y are of appropriate types) not because it assumes that "=" is the name of a transitive operator, but because the operators actually selected for (x=1) and (x=y) are both "equality" members of the same btree operator family. Likewise proving that (a>2) implies (a>1) requires knowing that > is a btree comparison op. -- Andrew (irc:RhodiumToad)
