Database Fundamentals

Indexes

• Query-based: Are indexes created for frequently queried columns?
• Composite indexes: Are multi-column queries covered?
• Not over-indexed: Are there unnecessary indexes slowing writes?
• Unique constraints: Are unique fields enforced at DB level?

Queries

• No N+1: Are related records fetched in bulk?
• Select specific fields: Are we avoiding SELECT *?
• Pagination: Do list queries limit results?
• Parameterized: Are all queries parameterized (no string concatenation)?

Migrations

• Reversible: Can this migration be rolled back?
• No data loss: Will existing data survive the migration?
• Tested: Has this been tested against production-like data?
• Incremental: Are large changes broken into smaller migrations?

Common Mistakes (Anti-Patterns)

1. The N+1 Query Problem

❌ // 1 query for users + N queries for posts
   const users = await User.findAll();
   for (const user of users) {
     user.posts = await Post.findAll({ where: { userId: user.id } });
   }

✅ // 1 query with JOIN
   const users = await User.findAll({
     include: [{ model: Post }]
   });

   // Or 2 queries with IN clause
   const users = await User.findAll();
   const userIds = users.map(u => u.id);
   const posts = await Post.findAll({ where: { userId: userIds } });

2. Missing Indexes

❌ // Queried frequently, but no index
   SELECT * FROM orders WHERE user_id = ?
   SELECT * FROM products WHERE category = ? AND status = 'active'

✅ CREATE INDEX idx_orders_user_id ON orders(user_id);
   CREATE INDEX idx_products_category_status ON products(category, status);

3. SELECT * Everywhere

❌ SELECT * FROM users; // Returns 50 columns

✅ SELECT id, name, email FROM users; // Only what's needed

4. String Concatenation (SQL Injection)

❌ db.query(`SELECT * FROM users WHERE email = '${email}'`);

✅ db.query('SELECT * FROM users WHERE email = ?', [email]);

5. Destructive Migrations

❌ -- Can't be rolled back
   DROP TABLE users;
   ALTER TABLE orders DROP COLUMN status;

✅ -- Add new, migrate data, then drop old (in separate migrations)
   -- Migration 1: Add new column
   ALTER TABLE orders ADD COLUMN status_new VARCHAR(20);
   -- Migration 2: Copy data
   UPDATE orders SET status_new = status;
   -- Migration 3: Drop old (after verification)
   ALTER TABLE orders DROP COLUMN status;

Socratic Questions

Ask the junior these questions instead of giving answers:

1. Schema: "Why did you choose this data type?"
2. Relationships: "What happens if this related record is deleted?"
3. Indexes: "Which columns are queried together? Are they indexed?"
4. N+1: "How many queries does this operation execute?"
5. Migration: "What happens if we need to roll this back?"

Normalization Quick Reference

When to Denormalize

• Read-heavy workloads with rare writes
• Calculated aggregates (e.g., order totals)
• Caching frequently accessed derived data

Index Strategy

-- Single column (most common)
CREATE INDEX idx_users_email ON users(email);

-- Composite (for multi-column queries)
-- Order matters! Most selective first
CREATE INDEX idx_orders_user_date ON orders(user_id, created_at);

-- Partial (for filtered queries)
CREATE INDEX idx_active_users ON users(email) WHERE active = true;

-- Unique (enforces constraint)
CREATE UNIQUE INDEX idx_users_email_unique ON users(email);

Index Rules of Thumb

1. Index columns in WHERE clauses
2. Index columns in JOIN conditions
3. Index columns in ORDER BY (if used with WHERE)
4. Don't over-index write-heavy tables
5. Consider composite indexes for multi-column queries

Query Optimization Checklist

1. Use EXPLAIN to analyze query plan
2. Avoid SELECT * - specify columns
3. Use LIMIT for pagination
4. Add indexes for WHERE/JOIN columns
5. Use WHERE instead of HAVING when possible
6. Avoid functions on indexed columns in WHERE
7. Use EXISTS instead of IN for large subqueries

Red Flags to Call Out

ORM Best Practices

// Eager loading (avoid N+1)
const users = await User.findAll({
  include: [{ model: Post, attributes: ['id', 'title'] }]
});

// Select specific fields
const users = await User.findAll({
  attributes: ['id', 'name', 'email']
});

// Pagination
const users = await User.findAll({
  limit: 20,
  offset: (page - 1) * 20
});

// Raw queries for complex operations
const results = await sequelize.query(
  'SELECT ... complex query ...',
  { type: QueryTypes.SELECT }
);