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Shard Keys

Shard a Collection

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  • Shard Key Fields and Values
  • Missing Shard Key Fields
  • Change a Document's Shard Key Value
  • Change a Collection's Shard Key

To shard a collection, you must specify the full namespace of the collection that you want to shard and the shard key. You can use the mongosh method sh.shardCollection() to shard a collection:

sh.shardCollection(<namespace>, <key>) // Optional parameters omitted
namespace
Specify the full namespace of the collection that you want to shard ("<database>.<collection>").
key

Specify a document { <shard key field1>: <1|"hashed">, ... } where

For more information on the sharding method, see sh.shardCollection().

Shard Key Fields and Values

Missing Shard Key Fields

Starting in version 4.4, documents in sharded collections can be missing the shard key fields. A missing shard key falls into the same range as a null-valued shard key. See Missing Shard Key Fields.

In version 4.2 and earlier, shard key fields must exist in every document to be able to shard a sharded collection. To set missing shard key fields, see Set Missing Shard Key Fields.

Change a Document's Shard Key Value

Starting in MongoDB 4.2, you can update a document's shard key value unless the shard key field is the immutable _id field. In MongoDB 4.2 and earlier, a document's shard key field value is immutable.

For details on updating the shard key value, see Change a Document's Shard Key Value.

Change a Collection's Shard Key

Starting in MongoDB 5.0, you can reshard a collection by changing a document'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, the choice of shard key cannot be changed after sharding.

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Shard Keys