What is refining a shard key and how to do it?
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.
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 }: