Ability to provide more than one PartitionKey per collection
There are times when documents need to be queried by a secondary key field and the PartitionKey is not known.
In order to avoid cross partition queries, the document needs to be stored twice - once with the primary PartitionKey and again with the secondary key field set as the PartitionKey.
It'd be great if a secondary PartitionKey could be specified to allow for efficient querying without requiring saving the document multiple times.
It is not possible to physically store data in a single partition with two partition keys. Physical partitions are functionally different computers.
Services like DynamoDB which offer global secondary indexes do this by storing the data twice.
To do this in Cosmos DB store the data in different collections and use Change Feed to keep them in sync. This provides the same functionality and provides additional flexibility in what data is shared between collections allowing for higher efficiency than simply copying all properties.