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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.
Changed in version 4.2.2.
For MongoDB 4.2.2 and later,
an exclusive (W) lock on the collection and blocks other operations on
the collection until it completes.
For more information on locking in MongoDB, see FAQ: Concurrency.