How do you optimize MongoDB performance?

db.users.find({ email: "john@example.com" }).explain("executionStats");

// Check these metrics:
// - totalDocsExamined vs nReturned
// - executionTimeMillis
// - stage: COLLSCAN (bad) vs IXSCAN (good)

// Bad - fetches all fields
db.users.find({ status: "active" });

// Good - only needed fields
db.users.find({ status: "active" }, { name: 1, email: 1 });

// Index covers the query entirely
db.users.createIndex({ email: 1, name: 1 });
db.users.find({ email: "john@example.com" }, { name: 1, _id: 0 });

// Compound index order matters
// Supports: { a: 1 }, { a: 1, b: 1 }
// Does NOT support: { b: 1 }
db.collection.createIndex({ a: 1, b: 1 });

// ESR Rule: Equality, Sort, Range
db.orders.createIndex({ status: 1, createdAt: -1, total: 1 });

// Bad - unbounded array
{ comments: [/* thousands of comments */] }

// Good - reference or bucket pattern
{ recentComments: [/* last 10 */], commentCount: 5000 }

// Instead of counting on every read
{ viewCount: 15234 }  // Increment on write