Indexing & Schema Design

Optimizing queries and designing effective document schemas

Index Types

typescript

// Index Examples
// Single field index
await users.createIndex({ email: 1 }); // 1 = ascending

// Compound index (order matters!)
await orders.createIndex({ userId: 1, createdAt: -1 });

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

// Text index for full-text search
await posts.createIndex({ title: 'text', content: 'text' });
await posts.find({ $text: { $search: 'mongodb indexing' } });

// TTL index — auto-delete after time
await sessions.createIndex({ createdAt: 1 }, { expireAfterSeconds: 86400 });

// Partial index — index only matching documents
await orders.createIndex(
  { customerId: 1 },
  { partialFilterExpression: { status: 'pending' } }
);

// Check query performance
const plan = await users
  .find({ email: 'alice@ex.com' })
  .explain('executionStats');
// Look for: IXSCAN (good) vs COLLSCAN (bad)

Schema Design Patterns

Embed: data accessed together (user + address). Faster reads, atomic writes.
Reference: independent entities or large/growing arrays. More flexible, requires $lookup.
Subset pattern: embed frequently-accessed subset, full data in separate collection.
Bucket pattern: group time-series or IoT data into fixed-size documents.

Transactions

typescript

// Multi-Document Transactions
const session = client.startSession();
try {
  session.startTransaction();
  await accounts.updateOne({ _id: from }, { $inc: { balance: -amount } }, { session });
  await accounts.updateOne({ _id: to }, { $inc: { balance: amount } }, { session });
  await session.commitTransaction();
} catch (err) {
  await session.abortTransaction();
  throw err;
} finally {
  session.endSession();
}

💬 When should you embed vs reference?

Embed when: data is always accessed together, doesn't grow unboundedly, and you need atomic updates. Reference when: data is shared across documents, arrays grow large, or you need independent access. Rule of thumb: if you always need it together, embed it.