Sharded collections require an index that supports the shard key. The index can be an index on the shard key or a compound index where the shard key is a prefix of the index.
If the collection is empty,
sh.shardCollection()creates the index on the shard key if such an index does not already exists.If the collection is not empty, you must create the index first before using
sh.shardCollection().
You cannot drop or hide an index if it is the only non-hidden index that supports the shard key.
Starting in MongoDB 7.0.3, 6.0.12, and 5.0.22, you can drop the index for a hashed shard key. For details, see Drop a Hashed Shard Key Index.
Unique Indexes
MongoDB can enforce a uniqueness constraint on a ranged shard key index. Through the use of a unique index on the shard key, MongoDB enforces uniqueness on the entire key combination and not individual components of the shard key.
For a ranged sharded collection, only the following indexes can be unique:
The index on the shard key
A compound index where the shard key is a prefix
The default
_idindex; however, the_idindex only enforces the uniqueness constraint per shard if the_idfield is not the shard key.
Important
Sharded clusters do not enforce the uniqueness constraint on _id fields
across the cluster when the _id field is not the shard key.
If the _id field is the not the shard key the uniqueness constraint only
applies to the shard that stores the document. This means that two or more
documents can have the same _id value, provided they occur on different
shards.
For example, consider a sharded collection with shard key {x:
1} that spans two shards A and B. Because the _id key is
not the shard key, the collection could have a document
with _id value 1 in shard A and another document with
_id value 1 in shard B.
In cases where the _id field is not the shard key, MongoDB expects
applications to enforce the uniqueness of _id values across the shards.
The unique index constraints mean that:
For a to-be-sharded collection, you cannot shard the collection if the collection has multiple unique indexes unless the shard key is the prefix for all the unique indexes.
For an already-sharded collection, you cannot create unique indexes on other fields unless the shard key is included as the prefix.
A unique index stores a null value for a document missing the indexed field; that is a missing index field is treated as another instance of a
nullindex key value. For more information, see Missing Document Field in a Unique Single-Field Index.
To enforce uniqueness on the shard key values, pass the unique
parameter as true to the sh.shardCollection() method:
If the collection is empty,
sh.shardCollection()creates the unique index on the shard key if such an index does not already exist.If the collection is not empty, you must create the index first before using
sh.shardCollection().
Although you can have a unique compound index where the shard
key is a prefix, if using unique
parameter, the collection must have a unique index that is on the shard
key.
You cannot specify a unique constraint on a hashed index.
To maintain uniqueness on a field that is not your shard key, see Unique Constraints on Arbitrary Fields.