Proper Implementation of Hashed Shard Key In MongoDB

Question

I have a collection that is currently indexed/queried by the built-in "_id" (ObjectId). I don't want to shard on this key since it is sequential (date-prefixed). The documentation for Mongo 2.4 says that I can shard on a hash of this key, which sounds great. Like so:

sh.shardCollection( "records.active", { _id: "hashed" } )

Question: do I have to first create the hashed index on the active collection with:

db.active.ensureIndex({ _id: "hashed" })

Or is that not necessary? I don't want to waste space with more indexing than is necessary.

Related question: if I do create a hashed index with ensureIndex({ _id: "hashed"}) can I drop the default "id" index? Will Mongo know to take queries on the _id field, hash them and run them against the hashed index?

Thanks...

Answer 1

Both the _id index and the hashed _id index will be needed. In MongoDB 2.4 you do not have to explicitly call db.active.ensureIndex({ _id: "hashed" }) before sharding your collection, but if you don't the sh.shardCollection( "records.active", { _id: "hashed" } ) will create the hashed index for you.

The _id index is required for replication.

To shard a collection in MongoDB you have to have an index on the shard key. This has not changed in MongoDB 2.4 and the hashed _id index will be required for sharding to work.