sqlite_protobuf

SQLite's virtual table API (https://www.sqlite.org/vtab.html) makes it possible to access other data structures through the query engine. You don't need to know much if anything about how the database engine executes queries, you only need to implement the callbacks it needs to do its job. A few years ago I wrote an extension to let me search through serialized Protobufs which were stored as blobs in a regular database. https://github.com/rgov/sqlite_protobuf