Type of Indexes
We have seen how indexes help with query performance and how we can create, drop, and list indexes in the collection. MongoDB supports different types of indexes, such as single key, multikey, and compound indexes. Each of these indexes has different advantages that you will need to know before deciding which type is suitable for your collection. Let's start with a brief overview of default indexes.
Default Indexes
As seen in the previous chapters, each document in a collection has a primary key (namely, the _id field) and is indexed by default. MongoDB uses this index to maintain the uniqueness of the _id field, and it is available on all the collections.
Single-Key Indexes
An index created using a single field from a collection is called a single-key index. You used a single-key index earlier in this chapter. The syntax is as follows:
db.collection.createIndex({ field1: type}, {options})
Compound Indexes
Single-key indexes are preferable...