Re: Does Type Have = Operator?

On Wed, May 11, 2016 at 9:54 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:

> On Tue, May 10, 2016 at 9:16 PM, David G. Johnston
> <david(dot)g(dot)johnston(at)gmail(dot)com> wrote:
> > Brute force: you'd have to query pg_amop and note the absence of a row
> with
> > a btree (maybe hash too...) family strategy 3 (1 for hash) [equality]
> where
> > the left and right types are the same and match the type in question.
>
> The core system uses this kind of thing to find equality operators in
> a number of cases.
>
> We often assume that the operator which implements equality for the
> type's default btree operator class is the canonical one for some
> purpose. Ditto for the default hash operator class.
>

​Yeah, the user-facing documentation covers it pretty deeply if not in one
central location.

But apparently the core system also uses the fact that "=", if present, is
an equality operator and, less so, that no other operator is expected​

​to be used for equality.

I suspect that such an expectation is not enforced though - e.g., someone
could define "==" to mean equality ​if they so choose (the lesser
property). Its hard to imagine defining "=" to mean something different in
logic, though, without intentionally trying to be cryptic.

David J.