Node.js MongoDB Indexes and Query Performance

What is MongoDB Indexes and Query Performance in Node.js?

An index is a sorted structure that lets MongoDB find matching documents without scanning the whole collection. Without one, every query reads every document — fine at a thousand records, fatal at a million.

Why MongoDB Indexes and Query Performance matters

Slow API endpoints are usually missing indexes rather than slow code. A query that scans 400,000 documents takes seconds; the same query on an index takes milliseconds.

MongoDB Indexes and Query Performance example

// explain shows what actually happened
const plan = await Job.find({ city: 'Pune', status: 'active' })
  .sort({ createdAt: -1 })
  .explain('executionStats');

console.log(plan.executionStats.totalDocsExamined);   // want ≈ nReturned
console.log(plan.queryPlanner.winningPlan.stage);     // COLLSCAN is bad, IXSCAN is good

// a compound index matching filter + sort
jobSchema.index({ city: 1, status: 1, createdAt: -1 });

Key points to remember

• Compound index order matters: equality fields first, then sort fields.
• COLLSCAN in an explain plan means no usable index exists.
• Indexes speed reads and slow writes — index deliberately, not exhaustively.
• Build indexes in the background on a live production collection.

Index types

Common mistakes with MongoDB Indexes and Query Performance

• A case-insensitive regular expression without a leading anchor, which cannot use an index.
• Sorting on a field absent from the index used for filtering, forcing an in-memory sort.