Sharded collections require an index that supports the shard key. The index can be an index on the shard key or a compound index where the shard key is a prefix of the index.
If the collection is empty,
sh.shardCollection()creates the index on the shard key if such an index does not already exists.If the collection is not empty, you must create the index first before using
sh.shardCollection().If you reshard a collection using
sh.reshardCollection(), you do not need to create the index on the new shard key beforehand. The resharding operation builds the required indexes automatically. To learn more, see the Reshard a Collection procedure.
You cannot drop or hide an index if it is the only non-hidden index that supports the shard key.
Starting in MongoDB 7.0.3, 6.0.12, and 5.0.22, you can drop the index for a hashed shard key. For details, see Drop a Hashed Shard Key Index.
Unique Indexes
MongoDB can enforce a uniqueness constraint on a ranged shard key index. Using a unique index on the shard key enforces uniqueness on the entire key combination and not individual components of the shard key.
For a ranged sharded collection, only an index on the shard key, a compound index where the shard key is a prefix, or the default _id index can be unique. You cannot specify a unique constraint on a hashed index.
To learn about the restrictions that apply to unique indexes on sharded collections, how to enforce uniqueness on the shard key values, and how to maintain uniqueness on a field that is not your shard key, see Maintain Uniqueness in Sharded Clusters.