Explain the covered query in MongoDB.

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A query is called covered query if satisfies the following two conditions:

  • The fields used in the query are part of an index used in the query.
  • The fields returned in the results are in the same index.

A covered query in MongoDB refers to a query where all the fields used in the query are covered by the index, and the query execution can be satisfied entirely using the index without having to examine the actual documents in the collection. This can result in significant performance improvements, as MongoDB can fulfill the query requirements by only accessing the index rather than retrieving and scanning the entire documents.

Key points about covered queries in MongoDB:

  1. Index Coverage:
    • All the fields involved in the query (both query conditions and projected fields) must be part of the index.
    • If an index covers all the fields required by the query, MongoDB can fulfill the query by only accessing the index.
  2. Projection:
    • Covered queries often involve projection, where only specific fields from the documents are returned in the query results.
    • The projected fields should be part of the index for the query to be considered covered.
  3. Performance Benefits:
    • Covered queries can lead to improved performance because MongoDB can avoid fetching and scanning the actual documents in the collection.
  4. Index Types:
    • Covered queries are more likely to be efficient when using compound indexes, which include multiple fields.

Here’s a simple example to illustrate a covered query:

Let’s consider a collection of documents representing books:

{
"_id": ObjectId("5f43b654c1e1eb0c9d4797a1"),
"title": "MongoDB Basics",
"author": "John Doe",
"category": "Database",
"published_year": 2020
}

Suppose you have an index on the fields {title: 1, published_year: 1}. If you perform a query like:

db.books.find({ title: "MongoDB Basics" }, { _id: 0, published_year: 1 })

This query can be considered covered if the index covers the query conditions (title) and the projected fields (published_year).

In summary, covered queries in MongoDB leverage indexes to satisfy both query conditions and projected fields, resulting in more efficient query execution.