MongoDB — Indexing Fundamentals

🍃 MongoDB 7.0+ 🟢 Chapter 26 of 50 📂 Phase 06: Indexing & Performance Optimization 📅 2026 Edition

Understand B-tree indexes, single field indexes, and how indexes transform slow COLLSCAN scans into fast IXSCAN lookups.

1What is an Index & Why Do You Need It?

Without an index, MongoDB must scan every single document in a collection (called a COLLSCAN). An index is a specialized B-tree data structure that stores a sorted list of field values, allowing MongoDB to perform fast logarithmic lookups (called an IXSCAN).

Creating an Index

// Create ascending index on email field
db.users.createIndex({ email: 1 });

// List all indexes on collection
db.users.getIndexes();

// Drop an index
db.users.dropIndex("email_1");

2The Cost of Indexing

⚠️ Index Trade-offs:

Faster Reads: Reduces query execution time from seconds to milliseconds.
Slower Writes: Every insertOne() or deleteOne() must update both the document AND all associated indexes.
RAM Memory Usage: Indexes must fit entirely inside WiredTiger RAM cache for optimal speed.

Written by Our Compiler Technical Editorial Team
Reviewed for accuracy & tested on MongoDB 7.0+ Standards · Last updated August 2026