MongoDB Documentation, Release 3.0.4 For a single-field index and sort operations, the sort order (i.e. ascending or descending) of the index key does not matter because MongoDB can traverse the index in either direction. See Single Field Indexes (page 487) and Sort with a Single Field Index (page 548) for more information on single-field indexes. MongoDB also supports user-defined indexes on multiple fields, i.e. compound indexes (page 489). The order of fields listed in a compound index has significance. For instance, if a compound index consists of { userid: 1, score: -1 } , the index sorts first by userid and then, within each userid value, sorts by score . For compound indexes and sort operations, the sort order (i.e. ascending or descending) of the index keys can deter- mine whether the index can support a sort operation. See Sort Order (page 490) for more information on the impact of index order on results in compound indexes. See Compound Indexes (page 489) and Sort on Multiple Fields (page 548) for more information on compound indexes. MongoDB uses multikey indexes (page 491) to index the content stored in arrays. If you index a field that holds an array value, MongoDB creates separate index entries for every element of the array. These multikey indexes (page 491) allow queries to select documents that contain arrays by matching on element or elements of the arrays. MongoDB automatically determines whether to create a multikey index if the indexed field contains an array value; you do not need to explicitly specify the multikey type. See Multikey Indexes (page 491) and Multikey Index Bounds (page 511) for more information on multikey indexes. To support efficient queries of geospatial coordinate data, MongoDB provides two special indexes: 2d indexes (page 497) that uses planar geometry when returning results and 2sphere indexes (page 496) that use spherical ge- ometry to return results. See 2d Index Internals (page 499) for a high level introduction to geospatial indexes. 483 8.1. Index Introduction Multikey Index Geospatial Index Compound Index