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") } })
ESR Rule: When designing compound indexes, order fields as Equality → Sort → Range for optimal performance.
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")| Explain Field | Meaning |
|---|---|
COLLSCAN | Full collection scan — no index used (bad for large collections) |
IXSCAN | Index scan used — efficient |
nReturned | Number of documents returned |
totalDocsExamined | Documents scanned — should be close to nReturned |
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
Key Takeaway: Indexes are the single biggest lever for query performance in MongoDB. Always index fields used in filters, sorts, and joins — but avoid over-indexing, since every index adds write overhead.
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