Querying with two or more :query:`$in` expressions (e.g. { a: { $in:
[ "a", "b", "c" ] }, b: { $in: [ "b", "c" ] } } ) can hit a
combinatorial limit if the query uses a compound index on these fields
(e.g. { a: 1, b: 1 } ). Specifically, if the number of combinations
(i.e. the product of the number of distinct elements in the
respective arrays) is equal to or greater than 4000000, MongoDB will
throw an exception of "combinatorial limit of $in partitioning of
result set exceeded".