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Hashed Indexes

Create a Hashed Index

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  • About this Task
  • Choose a Hashed Shard Key
  • Before You Begin
  • Examples
  • Create a Single-Field Hashed Index
  • Create a Compound Hashed Index
  • Learn More

To enable sharding for a collection that already contains data, you must create an index that supports the shard key. To enable sharding for an empty collection, you can instead specify the shard key index when you shard the collection.

Hashed indexes support hashed sharding, where data is distributed across shards based on the hashes of shard key values.

To create a single-field hashed index, specify hashed as the value of the index key:

db.<collection>.createIndex(
   {
      <field>: "hashed"
   }
)

To create a hashed index that contains multiple fields (a compound hashed index), specify hashed as the value of a single index key. For other index keys, specify the sort order (1 or -1):

db.<collection>.createIndex(
   {
      <field1>: "hashed",
      <field2>: "<sort order>",
      <field3>: "<sort order>",
      ...
   }
)

About this Task

Hashed indexing is ideal for shard keys with fields that change monotonically like ObjectId values or timestamps. When you use ranged sharding with a monotonically increasing shard key value, the chunk with an upper bound of MaxKey receives the majority incoming writes. This behavior restricts insert operations to a single shard, which removes the advantage of distributed writes in a sharded cluster.

For more information on choosing the best sharding approach for your application, see Hashed vs Ranged Sharding

Choose a Hashed Shard Key

Consider the following guidelines for your hashed shard key:

  • The field you choose for your hashed shard key should have a high cardinality, meaning a large number of different values.

  • If your data model does not contain a single field with high cardinality, consider creating a compound hashed index. A compound hashed index provides more unique indexed values and can increase cardinality.

  • Your shard key should support common query patterns. Range queries (like $gt and $lt) cannot use a hashed index. If your application often performs range queries on the fields included in your shard key, consider range-based sharding instead.

A hashed index can contain up to 32 fields.

Before You Begin

To implement hashed sharding, you must deploy a sharded cluster.

Examples

The following examples show you how to:

Create a Single-Field Hashed Index

Consider an orders collection that already contains data. Create a hashed index in the orders collection on the _id field:

db.orders.createIndex( { _id: "hashed" } )

The _id field increases monotonically, which makes it a good candidate for a hashed index key. Although _id values incrementally increase, when MongoDB generates a hash for individual _id values, those hashed values are unlikely to be on the same chunk.

After you create the index, you can shard the orders collection:

sh.shardCollection(
   "<database>.orders",
   { _id: "hashed" }
)

Create a Compound Hashed Index

Consider a customers collection that already contains data. Create a compound hashed index in the customers collection on the name, address, and birthday fields:

db.customers.createIndex(
   {
      "name" : 1
      "address" : "hashed",
      "birthday" : -1
   }
)

When you create a compound hashed index, you must specify hashed as the value of a single index key. For other index keys, specify the sort order (1 or -1). In the preceding index, address is the hashed field.

After you create the index, you can shard the customers collection:

sh.shardCollection(
   "<database>.customers",
   {
      "name" : 1
      "address" : "hashed",
      "birthday" : -1
   }
)

Learn More

← Hashed Indexes
Index Properties →

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