Currently is not possible to use as shard key a prefix of a multikey index.

For this reason, even if we have as shard key {a:1} and we have already the index {a:1, b:1} that is considered multikey ONLY because the field b is an array, we must have an index on {a:1} or a non multikey index where a is a prefix.

Allowing to use {a:1, b:1} for sharding will allow to save RAM, space and write overhead.