The ideal shard key allows MongoDB to distribute documents evenly throughout the cluster while facilitating common query patterns. A suboptimal shard key can lead to uneven data distribution and the following problems:
To address these issues, MongoDB allows you to change your shard key:
Starting in MongoDB 5.0, you can reshard a collection by changing a collection's shard key.
Starting in MongoDB 4.4, you can refine a shard key by adding a suffix field or fields to the existing shard key.
In MongoDB 4.2 and earlier, a document's shard key is immutable.
For more information on common performance and scaling issues and advice on how to fix them, read Troubleshoot Shard Keys.