database-design

Database Design

Schema Design Principles

Normalization Guidelines

-- 1NF: Atomic values, no repeating groups
-- 2NF: No partial dependencies on composite keys
-- 3NF: No transitive dependencies

-- Users table (normalized)
CREATE TABLE users (
  id SERIAL PRIMARY KEY,
  email VARCHAR(255) UNIQUE NOT NULL,
  created_at TIMESTAMPTZ DEFAULT NOW()
);

-- Addresses table (separate entity)
CREATE TABLE addresses (
  id SERIAL PRIMARY KEY,
  user_id INTEGER REFERENCES users(id) ON DELETE CASCADE,
  street VARCHAR(255),
  city VARCHAR(100),
  country VARCHAR(100),
  is_primary BOOLEAN DEFAULT false
);

Denormalization for Performance

-- When read performance matters more than write consistency
CREATE TABLE order_summaries (
  id SERIAL PRIMARY KEY,
  order_id INTEGER REFERENCES orders(id),
  customer_name VARCHAR(255),  -- Denormalized from customers
  total_amount DECIMAL(10,2),
  item_count INTEGER,
  last_updated TIMESTAMPTZ DEFAULT NOW()
);

Index Design

Common Index Patterns

-- B-tree (default) for equality and range queries
CREATE INDEX idx_users_email ON users(email);

-- Composite index (order matters!)
CREATE INDEX idx_orders_user_date ON orders(user_id, created_at DESC);

-- Partial index for specific conditions
CREATE INDEX idx_active_users ON users(email) WHERE deleted_at IS NULL;

-- GIN index for array/JSONB columns
CREATE INDEX idx_posts_tags ON posts USING GIN(tags);

-- Covering index (includes additional columns)
CREATE INDEX idx_orders_covering ON orders(user_id) INCLUDE (total, status);

Index Analysis

-- Check index usage
SELECT
  schemaname, tablename, indexname,
  idx_scan, idx_tup_read, idx_tup_fetch
FROM pg_stat_user_indexes
ORDER BY idx_scan DESC;

-- Find missing indexes
SELECT
  relname, seq_scan, seq_tup_read,
  idx_scan, idx_tup_fetch
FROM pg_stat_user_tables
WHERE seq_scan > idx_scan
ORDER BY seq_tup_read DESC;

Migration Patterns

Safe Migration Template

-- Always use transactions
BEGIN;

-- Add column with default (non-blocking in PG 11+)
ALTER TABLE users ADD COLUMN status VARCHAR(20) DEFAULT 'active';

-- Create index concurrently (doesn't lock table)
CREATE INDEX CONCURRENTLY idx_users_status ON users(status);

-- Backfill data in batches
UPDATE users SET status = 'active' WHERE status IS NULL AND id BETWEEN 1 AND 10000;

COMMIT;