The 3.1.1 version of partial indexes decides if a given query is compatible with a given partial index by checking if the query's root predicate describes a subset of the documents that the partial index document filter describes. However, this does not allow $or predicates to be indexed by partial indexes, as $or predicates are indexed by a union of scans on separate indexes.

This would allow the following query to be answerable by a union of scans on the following two indexes:

• query: find({$or: [{a: 1, f: true}, {b: 1}]})
• index #1: key pattern {a: 1}, partial index document filter {f: true}
• index #2: key pattern {b: 1}