What are Indexes in MongoDB?

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In MondoDB, Indexes are used to execute query efficiently. Without indexes, MongoDB must perform a collection scan, i.e. scan every document in a collection, to select those documents that match the query statement. If an appropriate index exists for a query, MongoDB can use the index to limit the number of documents it must inspect.

In MongoDB, indexes are data structures that improve the speed of data retrieval operations on a collection. They store a small amount of data for each indexed field and a reference to the location of the corresponding documents. By creating indexes on fields that are frequently used in queries, MongoDB can quickly locate and return the documents that match the query criteria.

Key points about indexes in MongoDB:

  1. Improving Query Performance: Indexes significantly speed up the process of querying data. When a query is executed, MongoDB can use indexes to quickly locate the relevant documents, reducing the time needed for data retrieval.
  2. Default Index on _id Field: MongoDB automatically creates an index on the _id field for every collection. This ensures fast retrieval based on the document’s primary key.
  3. Compound Indexes: MongoDB allows the creation of compound indexes that include multiple fields. This can be beneficial for queries that involve multiple criteria.
  4. Index Types: MongoDB supports various index types, including single-field indexes, compound indexes, multikey indexes (for arrays), text indexes, and geospatial indexes.
  5. Index Creation: Indexes can be created using the createIndex method in the MongoDB shell or by using the createIndex function in a programming language’s MongoDB driver.
  6. Index Use and Query Optimization: While indexes improve read performance, they can slightly impact write performance as the indexes need to be updated whenever data is added, modified, or removed. Therefore, it’s essential to carefully consider which fields to index based on the query patterns of your application.

Example of creating an index in MongoDB:

db.collection.createIndex({ fieldName: 1 });

This example creates a single-field index on the specified field in ascending order (1 for ascending, -1 for descending).