Compound indexes are a generalization of single-key indexes, allowing for multiple fields to be included in the same index. They are useful when we expect our queries to span multiple fields in our documents and also for consolidating our indexes when we start having too many of them in our collection.
Compound indexes can have as many as 31 fields. They cannot have a hashed index type.
A compound index is declared in a similar way to single indexes, by defining the fields we want to index and the order of indexing:
> db.books.createIndex({"name": 1, "isbn": 1})