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Day 04 Advanced Topics

Indexes and Performance

Create single, compound, and text indexes. Use explain() to analyze query plans.

~1 hour Hands-on Precision AI Academy

Today’s Objective

Create single, compound, and text indexes. Use explain() to analyze query plans.

Without an index, MongoDB scans every document to find matches. With an index, it jumps directly to matching documents. On a million-document collection, the difference is seconds vs milliseconds.

Creating Indexes
CREATING INDEXES
// Single-field index
db.users.createIndex({ email: 1 })

// Unique index
db.users.createIndex({ email: 1 }, { unique: true })

// Compound index
db.products.createIndex({ category: 1, price: -1 })

// Text index for full-text search
db.products.createIndex({ name: 'text', description: 'text' })

// Sparse index (only index documents that have the field)
db.users.createIndex({ phone: 1 }, { sparse: true })

// List indexes
db.users.getIndexes()

// Drop an index
db.users.dropIndex('email_1')
explain(): Understand Query Plans
EXPLAIN(): UNDERSTAND QUERY PLANS
// Without explain
db.users.find({ email: '[email protected]' })

// With explain
db.users.find({ email: '[email protected]' }).explain('executionStats')

// Key fields to check:
// executionStats.nReturned → documents returned
// executionStats.totalDocsExamined → documents scanned
// If nReturned << totalDocsExamined → you need an index
// executionStats.executionTimeMillis → time in ms
Index selectivity matters. An index on status where 90% of documents have status='active' is nearly useless — it still scans most documents. Index fields with high cardinality (many unique values): email, username, ID.
📝 Day 4 ExerciseAnalyze Query Performance
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Supporting Resources

Go deeper with these references.

MongoDB Docs
Official MongoDB Manual Complete reference documentation for all MongoDB features and operations.
→
MongoDB University
Free MongoDB Courses Official free courses on MongoDB fundamentals, aggregation, and performance.
→
GitHub
MongoDB Driver Examples Official Node.js and Python driver examples on GitHub.
→

Day 4 Checkpoint

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Atlas and Mongoose
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