MongoDB Indexing Best Practices

Indexes in MongoDB

Like SQL, MongoDB indexes speed up queries. Without them, MongoDB scans every document (a "collection scan").

Creating Indexes

db.users.createIndex({ email: 1 });          // ascending
db.users.createIndex({ age: -1 });           // descending
db.users.createIndex({ name: 1, age: -1 });  // compound
db.users.createIndex({ email: 1 }, { unique: true });

Check Performance with explain()

db.users.find({ email: "x@y.com" }).explain("executionStats");
// Look for IXSCAN (good) vs COLLSCAN (bad)

Best Practices

• Index fields used in queries, sorts, and joins.
• Put equality fields before range fields in compound indexes.
• Use unique for emails/usernames.
• Don't over-index — writes slow down.

Text Indexes for Search

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

FAQs

What is the _id index?

MongoDB auto-creates a unique index on _id for every collection. More in our MongoDB section.

How do I see existing indexes?

Run db.collection.getIndexes().