Hashed Indexes

Hashed indexes maintain entries with hashes of the values of theindexed field.

Hashed indexes support shardingusing hashed shard keys. Hashed based sharding uses a hashed index of a field as the shardkey to partition data across your sharded cluster.

Using a hashed shard key to shard a collection results in a more randomdistribution of data. SeeDeploy Sharded Cluster using Hashed Sharding for moredetails.

Hashing Function

Hashed indexes use a hashing function to compute the hash of the valueof the index field. [1] The hashing function collapsesembedded documents and computes the hash for the entire value but doesnot support multi-key (i.e. arrays) indexes.

Tip

MongoDB automatically computes the hashes when resolving queries usinghashed indexes. Applications do not need to compute hashes.

Create a Hashed Index

To create a hashed index, specifyhashed as the value of the index key, as in the followingexample:

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

Considerations

MongoDB supports hashed indexes of any single field. The hashingfunction collapses embedded documents and computes the hash for the entirevalue, but does not support multi-key (i.e. arrays) indexes.

You may not create compound indexes that have hashed index fieldsor specify a unique constraint on a hashed index; however, you cancreate both a hashed index and an ascending/descending (i.e.non-hashed) index on the same field: MongoDB will use the scalar indexfor range queries.

253 Limit

Warning

MongoDB hashed indexes truncate floating point numbers to 64-bit integersbefore hashing. For example, a hashed index would store the samevalue for a field that held a value of 2.3, 2.2, and 2.9.To prevent collisions, do not use a hashed index for floatingpoint numbers that cannot be reliably converted to 64-bitintegers (and then back to floating point). MongoDB hashed indexes donot support floating point values larger than 2⁵³.

To see what the hashed value would be for a key, seeconvertShardKeyToHashed().

PowerPC and 263

For hashed indexes, MongoDB 4.2 ensuresthat the hashed value for the floating point value 2⁶³ onPowerPC is consistent with other platforms.

Although hashed indexes on a field that maycontain floating point values greater than 2⁵³ is anunsupported configuration, clients may still insert documents where theindexed field has the value 2⁶³.

To list all hashed indexes for allcollections in your deployment, you can use the followingoperation in the mongo shell:

db.adminCommand("listDatabases").databases.forEach(function(d){
   let mdb = db.getSiblingDB(d.name);
   mdb.getCollectionInfos({ type: "collection" }).forEach(function(c){
      let currentCollection = mdb.getCollection(c.name);
      currentCollection.getIndexes().forEach(function(idx){
        let idxValues = Object.values(Object.assign({}, idx.key));
 
        if (idxValues.includes("hashed")) {
          print("Hashed index: " + idx.name + " on " + idx.ns);
          printjson(idx);
        };
      });
   });
});

To check if the indexed field contains the value 2⁶³, run thefollowing operation for the collection and the indexed field:

• If the indexed field type is a scalar and never a document:

// substitute the actual collection name for <collection>
// substitute the actual indexed field name for <indexfield>
 
db.<collection>.find( { <indexfield>: Math.pow(2,63) } );

• If the indexed field type is a document (or a scalar), you can run:

// substitute the actual collection name for <collection>
// substitute the actual indexed field name for <indexfield>
 
db.<collection>.find({
    $where: function() {
        function findVal(obj, val) {
            if (obj === val)
                return true;
 
            for (const child in obj) {
                if (findVal(obj[child], val)) {
                    return true;
                }
            }
            return false;
        }
        return findVal(this.<indexfield>, Math.pow(2, 63));
    }
})