Covered Queries in MongoDB

Covered Query: All the fields in the query are part of an index and all fields returned in the query are in the same index.

MongoDB matches the query conditions and returns the result using the same index without actually looking inside documents. Since indexes are present in RAM, fetching data from indexes is much faster as compared to fetching data by scanning documents.

Using Covered Queries
Consider the following document in customers collection:

   "_id": ObjectId("67667989hu9uh665"),
   "contact": "987654321"
   "gender": "M",
   "name": "Dilip",
   "user_name": "dksingh"

We should  create a compound index for customers collection on fields gender and user_name using following query:

>db. customers.ensureIndex({gender:1,user_name:1})

Here now, this index will cover the following query:

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

It would fetch the required data from indexed data which is very fast. Since our index does not include _id field it means MongoDB did not go into database documents, we have explicitly excluded _id from the result set of our query as MongoDB by default returns _id field in every query.

See the following query here we did not create an index of gender so when we execute this query, MongoDB  will scan this document in the database.

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

In the end, remember that an index cannot cover a query if:

Any of the indexed fields is an array OR indexed fields is a subdocument

1 comment:

  1. Good Explanation, Its help me to under stand Index and use of query with it..


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