MongoDB Documentation, Release 3.0.4 8.1.1 Index Types Default _id Single Field MongoDB provides a number of different index types to support specific types of data and queries. All MongoDB collections have an index on the _id field that exists by default. If applications do not specify a value for _id the driver or the mongod will create an _id field with an ObjectId value. The _id index is unique and prevents clients from inserting two documents with the same value for the _id field. In addition to the MongoDB-defined _id index, MongoDB supports the creation of user-defined ascend- ing/descending indexes on a single field of a document (page 487). Chapter 8. Indexes 482