The most common and simple type of index is the single field index. An example of a single field and key index is the index on ObjectId (_id), which is generated by default in every MongoDB collection. The ObjectId index is also unique, preventing a second document from having the same ObjectId in a collection.

An index on a single field, based on the mongo_book database that we used throughout the previous chapters, is defined like this:

> db.books.createIndex( { price: 1 } )

Here, we create an index on the field name in ascending order of index creation. For a descending order, the same index would be created like this:

> db.books.createIndex( { price: -1 } )

The ordering for index creation is important if we expect our queries to favor values on the first documents stored in our index. However, due to the extremely efficient time complexity that indexes have, this will not be a consideration for the most common use cases.

An index can be used for exact match queries or range queries on a field value. In the former case, the search can stop as soon as our pointer reaches the value after O(log n) time.

In range queries, due to the fact that we are storing values in order in our B-tree index, once we find the border value of our range query in a node of our B-tree, we will know that all of the values in its children will be part of our result set, allowing us to conclude our search.

An example of this is shown as follows: