Indexes in MongoDB

Without indexes, MongoDB performs a collection scan — reading every document to find matches. On large collections this is very slow. Indexes are data structures that store a small, ordered subset of field values, allowing MongoDB to jump directly to the matching documents.

The Default _id Index

Every collection automatically has a unique index on the _id field. This index is created for you and cannot be dropped.

Creating a Single-Field Index

Use createIndex to add an index on any field:

// Ascending index on the email field
db.users.createIndex({ email: 1 })

// Descending index on createdAt — useful for "latest first" queries
db.articles.createIndex({ createdAt: -1 })

Compound Indexes

A compound index covers multiple fields. MongoDB can use the index for queries on the leading prefix of the index fields.

// Covers queries filtering by status and sorting by createdAt
db.orders.createIndex({ status: 1, createdAt: -1 })

The order of fields in a compound index matters. Place equality filters first, then range filters, and finally sort fields — this follows the ESR rule (Equality, Sort, Range).

Unique Indexes

A unique index prevents duplicate values in the indexed field across the collection:

db.users.createIndex({ email: 1 }, { unique: true })

Attempting to insert a document with a duplicate email will now raise a duplicate key error.

Sparse and Partial Indexes

A sparse index only indexes documents that contain the indexed field, saving space when the field is optional:

db.users.createIndex({ githubHandle: 1 }, { sparse: true })

A partial index applies an index only to documents matching a filter expression:

// Only index active users
db.users.createIndex(
  { email: 1 },
  { partialFilterExpression: { status: "active" } }
)

Text Indexes

Text indexes support full-text search across string fields:

db.articles.createIndex({ title: "text", body: "text" })
db.articles.find({ $text: { $search: "mongodb aggregation" } })

explain() — Understanding Query Performance

Use explain to see whether MongoDB is using an index:

db.users.find({ email: "alice@example.com" }).explain("executionStats")

Look for IXSCAN (index scan) in the winning plan. COLLSCAN (collection scan) means no index was used and you should consider adding one.

Listing and Dropping Indexes

db.users.getIndexes()          // list all indexes
db.users.dropIndex({ email: 1 }) // drop a specific index