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Drop a Hashed Shard Key Index

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Starting in MongoDB 6.0.12 (and 5.0.22), you can drop the index for a hashed shard key.

This can speed up data insertion for collections sharded with a hashed shard key. It can also speed up data ingestion when using mongosync.

Dropping an unnecessary index can speed up CRUD operations. Each CRUD operation has to update all the indexes related to a document. Removing one index can increase the speed of all CRUD operations.

When you drop a hashed shard key index, the server disables balancing for that collection and excludes the collection from future balancing rounds. In order to include the collection in balancing rounds once again, you must recreate the shard key index.

1

Run the following command to stop the balancer:

sh.stopBalancer()

You can only run sh.stopBalancer() on mongos. sh.stopBalancer() produces an error if run on mongod.

2

Starting in MongoDB 6.0.3, you can run an aggregation using the $shardedDataDistribution stage to confirm no orphaned documents remain:

db.aggregate([
{ $shardedDataDistribution: {} },
{ $match: { "ns": "<database>.<collection>" } }
])

$shardedDataDistribution has output similar to the following:

[
{
"ns": "test.names",
"shards": [
{
"shardName": "shard-1",
"numOrphanedDocs": 0,
"numOwnedDocuments": 6,
"ownedSizeBytes": 366,
"orphanedSizeBytes": 0
},
{
"shardName": "shard-2",
"numOrphanedDocs": 0,
"numOwnedDocuments": 6,
"ownedSizeBytes": 366,
"orphanedSizeBytes": 0
}
]
}
]

Ensure that "numOrphanedDocs" is 0 for each shard in the cluster.

3

Run the following command to drop the index:

db.collection.dropIndex("<index name>")
4

Run the following command to restart the balancer on the cluster:

sh.startBalancer()
← Convert a Replica Set to a Sharded Cluster