Indexing Fundamentals

Without indexes, MongoDB must scan every document in a collection to satisfy a query — a collection scan. Indexes dramatically speed up queries by letting MongoDB jump directly to matching documents, much like a book's index.

1. The Default _id Index

Every collection automatically gets a unique index on _id — you never need to create this one manually.

2. Single Field Indexes

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

// Create a unique index (prevents duplicate emails)
db.users.createIndex({ email: 1 }, { unique: true })

// View all indexes on a collection
db.users.getIndexes()

3. Compound Indexes

Compound indexes cover queries filtering or sorting on multiple fields — but field order matters.

db.orders.createIndex({ customerId: 1, orderDate: -1 })

// Efficient — uses the compound index
db.orders.find({ customerId: ObjectId("...") }).sort({ orderDate: -1 })

// Less efficient — can't use the index for orderDate alone
db.orders.find({ orderDate: { $gt: ISODate("2026-01-01") } })

4. Multikey Indexes

When you index a field that holds an array, MongoDB automatically creates a multikey index — indexing each array element individually.

db.products.createIndex({ tags: 1 })

// Matches any product whose tags array contains "electronics"
db.products.find({ tags: "electronics" })

5. Analyzing Query Performance

db.orders.find({ customerId: ObjectId("...") }).explain("executionStats")

6. Indexing Fundamentals Checklist

• ✅ Understand the default _id index
• ✅ Create single field and unique indexes
• ✅ Apply the ESR rule for compound indexes
• ✅ Recognize multikey indexes on array fields
• ✅ Use explain() to verify index usage