database-design

Database Design

When to Load

• Trigger: Schema design, migrations, query optimization, indexing strategies, data modeling, N+1 fixes
• Skip: No database work involved in the current task

Database Design Workflow

Copy this checklist and track progress:

Database Design Progress:
- [ ] Step 1: Identify entities and relationships
- [ ] Step 2: Normalize schema (3NF minimum)
- [ ] Step 3: Evaluate denormalization needs
- [ ] Step 4: Design indexes for query patterns
- [ ] Step 5: Write and optimize critical queries
- [ ] Step 6: Plan migration strategy
- [ ] Step 7: Configure connection pooling
- [ ] Step 8: Validate against anti-patterns checklist

Schema Design Principles

Normalization Forms

1NF: Atomic values, no repeating groups
2NF: 1NF + no partial dependencies (all non-key columns depend on full PK)
3NF: 2NF + no transitive dependencies (non-key columns don't depend on other non-key columns)

-- WRONG: Unnormalized
CREATE TABLE orders (
  id SERIAL PRIMARY KEY,
  customer_name TEXT,
  customer_email TEXT,        -- duplicated across orders
  product1_name TEXT,         -- repeating groups
  product1_qty INT,
  product2_name TEXT,
  product2_qty INT
);

-- CORRECT: Normalized to 3NF
CREATE TABLE customers (
  id SERIAL PRIMARY KEY,
  name TEXT NOT NULL,
  email TEXT UNIQUE NOT NULL
);

CREATE TABLE orders (
  id SERIAL PRIMARY KEY,
  customer_id INT REFERENCES customers(id),
  created_at TIMESTAMPTZ DEFAULT NOW()
);

CREATE TABLE order_items (
  id SERIAL PRIMARY KEY,
  order_id INT REFERENCES orders(id),
  product_id INT REFERENCES products(id),
  quantity INT NOT NULL CHECK (quantity > 0)
);

When to Denormalize

Denormalize only when you have measured proof of performance issues:

-- Acceptable denormalization: precomputed counter to avoid COUNT(*)
ALTER TABLE posts ADD COLUMN comment_count INT DEFAULT 0;

-- Update via trigger or application code
CREATE FUNCTION update_comment_count() RETURNS TRIGGER AS $$
BEGIN
  IF TG_OP = 'INSERT' THEN
    UPDATE posts SET comment_count = comment_count + 1 WHERE id = NEW.post_id;
  ELSIF TG_OP = 'DELETE' THEN
    UPDATE posts SET comment_count = comment_count - 1 WHERE id = OLD.post_id;
  END IF;
  RETURN NULL;
END;
$$ LANGUAGE plpgsql;