The db.collection.reIndex() drops all indexes on a collection and recreates them. This operation may be expensive for collections that have a large amount of data and/or a large number of indexes.
Call this method, which takes no arguments, on a collection object. For example:
Normally, MongoDB compacts indexes during routine updates. For most users, the db.collection.reIndex() is unnecessary. However, it may be worth running if the collection size has changed significantly or if the indexes are consuming a disproportionate amount of disk space.
db.collection.reIndex() will rebuild indexes in the background if the index was originally specified with this option. However, db.collection.reIndex() will rebuild the _id index in the foreground, which takes the database’s write lock.
Changed in version 2.6: Reindexing operations will error if the index entry for an indexed field exceeds the Maximum Index Key Length. Reindexing operations occur as part of compact and repairDatabase commands as well as the db.collection.reIndex() method.
Because these operations drop all the indexes from a collection and then recreate them sequentially, the error from the Maximum Index Key Length prevents these operations from rebuilding any remaining indexes for the collection and, in the case of the repairDatabase command, from continuing with the remainder of the process.
Index Creation for more information on the behavior of indexing operations in MongoDB.