MongoDB Documentation, Release 3.0.4 MongoDB versions prior to 2.0, however, support only the {v:0} format. If you need to roll back MongoDB to a version prior to 2.0, you must drop and re-create your indexes. To build pre-2.0 indexes, use the dropIndexes() and createIndex() methods. You cannot simply reindex the collection. When you reindex on versions that only support {v:0} indexes, the v fields in the index definition still hold values of 1 , even though the indexes would now use the {v:0} format. If you were to upgrade again to version 2.0 or later, these indexes would not work. Example Suppose you rolled back from MongoDB 2.0 to MongoDB 1.8, and suppose you had the following index on the items collection: { "v" : 1, "key" : { "name" : 1 }, "ns" : "mydb.items", "name" : "name_1" } The v field tells you the index is a {v:1} index, which is incompatible with version 1.8. To drop the index, issue the following command: db.items.dropIndex( { name : 1 } ) db.foo.createIndex( { name : 1 } , { v : 0 } ) To recreate the index as a {v:0} index, issue the following command: See also: Index Performance Enhancements (page 910). 8.3.2 Index Management Tutorials Instructions for managing indexes and assessing index performance and use. Remove Indexes (page 524) Drop an index from a collection. Modify an Index (page 525) Modify an existing index. Rebuild Indexes (page 526) In a single operation, drop all indexes on a collection and then rebuild them. Manage In-Progress Index Creation (page 527) Check the status of indexing progress, or terminate an ongoing in- dex build. Return a List of All Indexes (page 527) Obtain a list of all indexes on a collection or of all indexes on all collections in a database. Measure Index Use (page 528) Study query operations and observe index use for your database. Remove Indexes To remove an index from a collection use the dropIndex() method and the following procedure. If you simply need to rebuild indexes you can use the process described in the Rebuild Indexes (page 526) document. Indexing Tutorials (page 517) and Index Concepts (page 486) for more information about indexes and indexing oper- ations in MongoDB. See also: 524 Chapter 8. Indexes