How to create single field and compound indexes in MongoDB

Creating indexes with createIndex

Use createIndex to add an index to any collection. The value 1 specifies ascending order and -1 specifies descending. For single-field equality queries the direction is irrelevant. For sorting and range queries, direction determines whether MongoDB can satisfy the sort using the index or must sort in memory after scanning.

// Single-field unique index — prevents duplicate emails
await db.collection('users').createIndex({ email: 1 }, { unique: true })

// Compound index — field order follows the ESR rule:
// E = Equality fields first, S = Sort fields second, R = Range fields last
await db.collection('orders').createIndex({
  userId: 1,       // E: exact match on userId
  status: 1,       // S: sort output by status
  createdAt: -1    // R: date range queries, most recent first
})

// Sparse index — skip documents where the field is absent
await db.collection('users').createIndex(
  { phoneNumber: 1 },
  { sparse: true }  // documents without phoneNumber are excluded from index
)

// Index a nested field via dot notation
await db.collection('users').createIndex({ 'address.city': 1 })

// List all indexes on a collection
const indexes = await db.collection('orders').indexes()
console.log(indexes)

Why the ESR rule matters

Placing a Range field before the Sort field forces MongoDB to sort results in working memory after the index scan — an in-memory sort stage. This is expensive on large result sets and defeats the purpose of the index covering the sort. Equality first, Sort second, Range last eliminates the in-memory sort.