On Mon, Jul 31, 2017 at 5:18 PM, Alvaro Herrera <alvhe...@2ndquadrant.com> wrote:
> Tom Lane wrote: > > Alvaro Herrera <alvhe...@2ndquadrant.com> writes: > > > ... However, when you create an index, you can > > > indicate which operator class to use, and it may not be the default > one. > > > If a different one is chosen at index creation time, then a query using > > > COUNT(distinct) will do the wrong thing, because DISTINCT will select > > > an equality type using the type's default operator class, not the > > > equality that belongs to the operator class used to create the index. > > > > > That's wrong: DISTINCT should use the equality operator that > corresponds > > > to the index' operator class instead, not the default one. > > > > Uh, what? Surely the semantics of count(distinct x) *must not* vary > > depending on what indexes happen to be available. > > Err ... > > > I think what you meant to say is that the planner may only choose an > > optimization of this sort when the index's opclass matches the one > > DISTINCT will use, ie the default for the data type. I understand the problem. I am currently researching how to resolve it. Best Regards, Mark Rofail
