MongoDB previously offered a related capability called Client-Side Field-Level Encryption (CSFLE), which allowed customers to process some encrypted data. But it suffered from several shortcomings, ...

We can also restrict potential matches with other search criteria using the restrictWithinMatch field, which uses standard MongoDB query filter syntax.

There are several features for expanding query capabilities in the new MongoDB 4.4 release. The highlight is a new "Union" operator geared for more complex queries.