Seven NoSQL Databases in a Week

The default _id index

MongoDB creates the default _id index when creating a document. The _id index prevents users from inserting two documents with the same _id value. You cannot drop an index on an _id field.

The following syntax is used to create an index in MongoDB:

>db.collection.createIndex(<key and index type specification>, <options>);

The preceding method creates an index only if an index with the same specification does not exist. MongoDB indexes use the B-tree data structure.

The following are the different types of indexes:

• Single field: In addition to the _id field index, MongoDB allows the creation of an index on any single field in ascending or descending order. For a single field index, the order of the index does not matter ...