How to Create Index in MongoDB: Syntax and Examples

To create an index in MongoDB, use the db.collection.createIndex() method specifying the fields to index and their sort order. For example, db.users.createIndex({ name: 1 }) creates an ascending index on the name field.

Syntax

The basic syntax to create an index in MongoDB is:

• db.collection.createIndex(keys, options)

Here, keys is an object specifying the fields to index and their sort order (1 for ascending, -1 for descending). options is optional and can specify index type, uniqueness, and more.

mongodb

db.collection.createIndex({ field1: 1, field2: -1 }, { unique: true })

Example

This example creates an ascending index on the username field in the users collection. It helps queries that filter or sort by username run faster.

mongodb

use mydatabase

db.users.createIndex({ username: 1 })

Output

{
  "createdCollectionAutomatically" : false,
  "numIndexesBefore" : 1,
  "numIndexesAfter" : 2,
  "ok" : 1
}

Common Pitfalls

Common mistakes when creating indexes include:

• Not specifying the correct sort order (1 or -1) for fields.
• Creating duplicate indexes that already exist, which wastes space.
• Forgetting to set unique: true when a unique constraint is needed.
• Creating too many indexes, which can slow down writes.

Always check existing indexes with db.collection.getIndexes() before creating new ones.

mongodb

/* Wrong: missing sort order */
db.users.createIndex({ username: "asc" })

/* Right: use 1 for ascending */
db.users.createIndex({ username: 1 })

Quick Reference

Command	Description
db.collection.createIndex({ field: 1 })	Create ascending index on field
db.collection.createIndex({ field: -1 })	Create descending index on field
db.collection.createIndex({ field: 1 }, { unique: true })	Create unique index
db.collection.getIndexes()	List all indexes on collection
db.collection.dropIndex(indexName)	Remove an index by name

Key Takeaways

Use db.collection.createIndex() with field and sort order to create indexes.

Specify 1 for ascending and -1 for descending index order.

Check existing indexes before creating new ones to avoid duplicates.

Use unique: true option to enforce unique values in an index.

Avoid creating too many indexes to keep write performance optimal.