3.33. MongoDB overlay index query

3.33.1. Use override index query ¶

To test the override index query, use the following users collection:

{
   "_id": ObjectId("53402597d852426020000002"),
   "contact": "987654321",
   "dob": "01-01-1991",
   "gender": "M",
   "name": "Tom Benzamin",
   "user_name": "tombenzamin"
}

We create a federated index in the users collection with the fields gender and user_name:

>db.users.createIndex({gender:1,user_name:1})

注： Versions prior to 5.0 can use the db.collection.ensureIndex() , but ensureIndex() Has been removed since version 5.0, using the createIndex() Instead.

The index now overrides the following query:

>db.users.find({gender:"M"},{user_name:1,_id:0})

In other words, for the above query, MongoDB will not look in the database file. Instead, it extracts data from the index, which is a very fast data query.

Since the_ id field is not included in our index,_ id is returned by default in the query, so we can exclude it from the query result set of MongoDB.

If_ id is not excluded from the following example, the query will not be overwritten:

>db.users.find({gender:"M"},{user_name:1})

Finally, you cannot use an override index query if it is the following query:

• All index fields are an array