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.

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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 }: