According to mongoDB document overview of index,If your application queries a collection on a particular field or set of fields, then an index on the queried field or a compound index on the set of fields can prevent the query from scanning the whole collection to find and return the query results. For more information about indexes click here.
What is a Covered Query
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:
We should create a compound index for customers collection on fields gender and user_name using following query:
Here now, this index will cover the following query:
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 result set of our query as MongoDB by default returns _id field in every query.
See following query here we did not create index of gender so when we execute this query, mongoDB will scan the this document in database.
At 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