Question

I am building a webapp using Codeigniter (PHP) and MongoDB.
I am creating indexes and have one question.

If I am querying on three fields (_id, status, type) and want to
create an index do I need to include _id when ensuring the index like this:

db.comments.ensureIndex({_id: 1, status : 1, type : 1});

or will this due?

db.comments.ensureIndex({status : 1, type : 1});

Answer 1

You don’t need to implicitly create index on the _id field, it’s done automatically. See the mongo documentation:

The _id Index

For all collections except capped collections, an index is automatically created for the _id field. This index is special and cannot be deleted. The _id index enforces uniqueness for its keys (except for some situations with sharding).