Refining a collection's shard key allows for a more fine-grained data distribution and can address situations where the existing key has led to jumbo chunks due to insufficient cardinality.
Warning
Do not modify the range or hashed type for any of the current shard key fields. It causes data inconsistencies. For example, do not modify a shard key from { customer_id: 1 } to { customer_id: "hashed", order_id: 1 }.
Note
Starting in MongoDB 5.0, you can also reshard your collection by providing a new shard key for the collection. To learn whether you should reshard your collection or refine your shard key, see Change a Shard Key.
To refine a collection's shard key, use the refineCollectionShardKey command. The refineCollectionShardKey adds a suffix field or fields to the existing key to create the new shard key.
For example, you may have an existing orders collection in a test database with the shard key { customer_id: 1 }. You can use the refineCollectionShardKey command to change the shard key to the new shard key { customer_id: 1, order_id: 1 }:
db.adminCommand( { refineCollectionShardKey: "test.orders", key: { customer_id: 1, order_id: 1 } } )