MongoDB Documentation, Release 3.0.4 Text Indexes Hashed Indexes 8.1.2 Index Properties Unique Indexes Sparse Indexes You can combine the sparse index option with the unique index option to reject documents that have duplicate values for a field but ignore documents that do not have the indexed key. The sparse (page 505) property of an index ensures that the index only contain entries for documents that have the indexed field. The index skips documents that do not have the indexed field. The unique (page 504) property for an index causes MongoDB to reject duplicate values for the indexed field. Other than the unique constraint, unique indexes are functionally interchangeable with other MongoDB indexes. To support hash based sharding (page 674), MongoDB provides a hashed index (page 502) type, which indexes the hash of the value of a field. These indexes have a more random distribution of values along their range, but only support equality matches and cannot support range-based queries. See Text Indexes (page 500) for more information on text indexes and search. MongoDB provides a text index type that supports searching for string content in a collection. These text indexes do not store language-specific stop words (e.g. “the”, “a”, “or”) and stem the words in a collection to only store root words. Chapter 8. Indexes 484