Shard Keys

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 _id index; however, the _id index only enforces the uniqueness constraint per shard if the _id field is not the shard key or the prefix of the shard key.

  Important

  Uniqueness and the _id Index

  If the _id field is not the shard key or the prefix of the shard key, _id index only enforces the uniqueness constraint per shard and not across 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 part of 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.

  If the _id field is not the shard key nor the prefix of the shard key, MongoDB expects applications to enforce the uniqueness of the _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 other unique indexes.

• For an already-sharded collection, you cannot create unique indexes on other fields.

• 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 null index 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.

Chunk Range and Missing Shard Key Fields

Missing shard key fields fall within the same chunk range as shard keys with null values. For example, if the shard key is on the fields { x: 1, y: 1 }, then:

Read/Write Operations and Missing Shard Key Fields

To target documents with missing shard key fields, you can use the { $exists: false } filter condition on the shard key fields. For example, if the shard key is on the fields { x: 1, y: 1 }, you can find the documents with missing shard key fields by running this query:

db.shardedcollection.find( { $or: [ { x: { $exists: false } }, { y: { $exists: false } } ] } )

If you specify a null equality match filter condition (e.g. { x: null }), the filter matches both those documents with missing shard key fields and those with shard key fields set to null.

Some write operations, such as a write with an upsert specification, require an equality match on the shard key. In those cases, to target a document that is missing the shard key, include another filter condition in addition to the null equality match. For example:

{ _id: <value>, <shardkeyfield>: null } // _id of the document missing shard key