Tom Lane wrote:
> Alvaro Herrera <alvherre@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.
Um, yeah, absolutely.
--
Álvaro Herrera https://www.2ndQuadrant.com/
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