Consider adding type expression syntax to exclude or explicitly include subtypes

On queries over type Foo EdgeDB will include both Foo objects and all objects of its descendant types. EdgeQL currently lacks a way to turn off automatic inclusion of inherited objects (a-la the SQL ONLY keyword). There is a way to filter over the exact value of .__type__, but that is neither elegant, nor very efficient if spelled in a most concise way (FILTER .__type__.name = 'default::BaseType').

ONLY seems to be a good candidate for a modifier in a particular case of excluding all descendants.

By nature the ONLY operator (and its tentative EXCEPT counterpart) are type expression operators similar to the type union | and intersection &, so perhaps #1578 should be implemented first to avoid the tedium of repeating the inheritance op, e.g WITH MyType AS TYPE Foo ONLY or WITH MyType AS TYPE Foo EXCEPT Subtype. Also, perhaps EXCEPT should be - or ^ to be closer to other type operators.

[elprans]

So if ONLY is a type expression operator then “select specifically Foo” would be SELECT Foo[IS ONLY Foo], right?

Correct, although I’m thinking a postfix form would be better.

[dmgolembiowski]

Depends on where it would be placed.

ONLY is semantically a type expression operator, unlike DETACHED, which is an expression operator that manipulates scope.

This is an interesting idea — especially since it fits the EdgeDB niche where “everything is a set”.
From my perspective, [IS ONLY] could be an implementation derived from creating disjoint unions, and could even play an interesting role with the :- operator since the disjoint union of A and B boils down to A − B.