MongoDB — Single Field & Compound Indexes

🍃 MongoDB 7.0+ 🟢 Chapter 27 of 50 📂 Phase 06: Indexing & Performance Optimization 📅 2026 Edition

Master Compound Indexes, the Prefix Rule, and the ESR (Equality, Sort, Range) rule for optimal index ordering.

1The ESR Rule for Compound Index Field Ordering

When designing a compound index spanning multiple fields, arrange fields using the ESR Rule:

E (Equality): Place fields tested for exact match (e.g. status: "active") FIRST.
S (Sort): Place fields used in .sort() SECOND.
R (Range): Place fields tested for ranges (e.g. price: { $gt: 100 }) LAST.

ESR Compound Index Example

// Query: Find products in "electronics", sort by createdAt desc, price > 50
db.products.createIndex({ category: 1, createdAt: -1, price: 1 });

2Understanding Covered Queries

A Covered Query occurs when an index contains all fields requested by a query filter and projection. MongoDB reads results directly from the index in RAM without fetching documents from disk!

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