I'm looking for a simple way to get the reachestimate of a union of targeting specs.
It looks like the flexible spec (https://developers.facebook.com/docs/marketing-api/targeting-specs/v2.3#flexible) can't provide a union of different specs. The flexible targeting spec allows:
1. "AND" different specs (and not "OR").
2. "OR" different segments (interests, behaviors...) in one targeting spec.
The second option provides a union, which is larger then the real union of the two targeting specs. For example:
audience_1 = (interest_1 INTERSECTION behavior_1)
audience_2 = (interest_2 INTERSECTION behavior_2)
Then if I build a new spec with the two interests & two behaviors then it will give me:
(interest_1 UNION interest_2) UNION (behavior_1 UNION behavior_2)
which is a larger group than:
(interest_1 INTERSECTION behavior_1) UNION (interest_2 INTERSECTION behavior_2)
The first option provides an intersection between targeting specs, not the union, and in case I want the union of more than 2 audiences it becomes more complicated, it requires using the inclusion-exclusion principle (https://en.wikipedia.org/wiki/Inclusion%E2%80%93exclusion_principle) which allow finding the union using intersections, but that's not as simple as I hoped it to be, and requires many API calls to get sizes of all the subsets of audiences in all sizes.