// A vector with size equal to the number of elements in the index key pattern. Each element in
// the vector is an ordered set of positions (starting at 0) into the corresponding indexed
// field that represent what prefixes of the indexed field cause the index to be multikey.
//
// For example, with the index {'a.b': 1, 'a.c': 1} where the paths "a" and "a.b" cause the
// index to be multikey, we'd have a std::vector<std::set<size_t>>{{0U, 1U}, {0U}}.
using MultikeyPaths = std::vector<std::set<size_t>>;