Database Design

Database Design

Design efficient, maintainable database schemas with safe migration strategies.

Design Process

┌─────────────────────────────────────────────────────────────────┐
│                    DATABASE DESIGN PROCESS                       │
├─────────────────────────────────────────────────────────────────┤
│                                                                 │
│  1. REQUIREMENTS      2. MODELING         3. SCHEMA             │
│  ┌─────────────┐     ┌─────────────┐     ┌─────────────┐       │
│  │ Entities    │  →  │ ER diagram  │  →  │ Tables &    │       │
│  │ Attributes  │     │ Relations   │     │ Columns     │       │
│  │ Constraints │     │ Cardinality │     │ Constraints │       │
│  └─────────────┘     └─────────────┘     └─────────────┘       │
│                                                                 │
│  4. INDEXES          5. MIGRATION        6. REVIEW              │
│  ┌─────────────┐     ┌─────────────┐     ┌─────────────┐       │
│  │ Query       │  →  │ Safe changes│  →  │ Performance │       │
│  │ patterns    │     │ Rollback    │     │ Consistency │       │
│  │ Performance │     │ Zero-down   │     │ Integrity   │       │
│  └─────────────┘     └─────────────┘     └─────────────┘       │
│                                                                 │
└─────────────────────────────────────────────────────────────────┘

Core Principles

1. Data Integrity First

Enforce constraints at the database level, not just application.

-- ✅ Database enforces integrity
CREATE TABLE orders (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  user_id UUID NOT NULL REFERENCES users(id),
  total DECIMAL(10,2) NOT NULL CHECK (total >= 0),
  status VARCHAR(20) NOT NULL DEFAULT 'pending'
);

-- ❌ Relying only on application validation
CREATE TABLE orders (
  id INTEGER,
  user_id INTEGER,  -- no FK
  total TEXT        -- wrong type, no constraint
);

2. Normalize by Default, Denormalize with Purpose

Start with 3NF. Denormalize only when you have measured performance needs.

✅ Normalize: Eliminate redundancy, maintain consistency
✅ Denormalize: Reduce joins for read-heavy queries (with justification)
❌ Premature optimization without query patterns

3. Explicit Over Implicit

Clear naming, explicit constraints, documented decisions.

-- ✅ Explicit
user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE
created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()

-- ❌ Implicit
user INTEGER  -- What does this reference?
created TEXT  -- When? Timezone?

4. Plan for Evolution

Schema will change. Design for safe migrations.

✅ Additive changes preferred (add columns, tables)
✅ Nullable new columns (can deploy before backfill)
✅ Soft deletes for recoverable data
❌ Breaking changes without migration plan

Quick Decisions

Primary Key Strategy

Recommendation: UUID v7 or ULID for new projects. Auto-increment for simple apps.

Relationship Patterns

One-to-One:   user ←→ profile       → FK + UNIQUE on child
One-to-Many:  user ←→ orders        → FK on "many" side
Many-to-Many: users ←→ roles        → Junction table
Self-ref:     employee ←→ manager   → FK to same table
Polymorphic:  comments on any entity → See references/schema-patterns.md

When to Denormalize

Rule: Measure first. Denormalize with documentation.

Soft Delete vs Hard Delete

-- Soft delete pattern
deleted_at TIMESTAMPTZ DEFAULT NULL
-- Query non-deleted: WHERE deleted_at IS NULL
-- Create partial index for performance
CREATE INDEX idx_users_active ON users(email) WHERE deleted_at IS NULL;

Standard Schema Patterns

Timestamps

Always include, always use timezone:

created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
-- Add trigger or ORM hook for updated_at

Audit Columns

created_by UUID REFERENCES users(id),
updated_by UUID REFERENCES users(id),
created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW()

Soft Delete

deleted_at TIMESTAMPTZ DEFAULT NULL,
deleted_by UUID REFERENCES users(id)

Versioning

version INTEGER NOT NULL DEFAULT 1
-- Increment on update, use for optimistic locking

Index Design Checklist

□ Primary key (automatic)
□ Foreign keys (add manually in most DBs)
□ Unique constraints
□ Columns in WHERE clauses (high selectivity)
□ Columns in ORDER BY
□ Columns in JOIN conditions
□ Composite indexes for multi-column queries

⚠️ Avoid:
□ Indexing low-cardinality columns alone (status, boolean)
□ Over-indexing (slows writes)
□ Indexes that duplicate existing coverage

Index Types Quick Reference

Migration Safety Checklist

Before any production migration:

□ Migration is reversible (has rollback plan)
□ Tested on copy of production data
□ Estimated lock time calculated
□ Deployment can proceed if migration fails
□ New code works with old AND new schema
□ Backfill strategy for new columns

Safe Migration Patterns

Dangerous Operations

-- ⚠️ Locks table (avoid on large tables)
ALTER TABLE users ADD COLUMN name VARCHAR(255) NOT NULL DEFAULT '';
ALTER TABLE users ADD CONSTRAINT ... (without NOT VALID);
CREATE INDEX idx_users_email ON users(email);  -- non-concurrent

-- ✅ Safe alternatives
ALTER TABLE users ADD COLUMN name VARCHAR(255);  -- nullable first
ALTER TABLE users ADD CONSTRAINT ... NOT VALID;
ALTER TABLE users VALIDATE CONSTRAINT ...;  -- separate transaction
CREATE INDEX CONCURRENTLY idx_users_email ON users(email);

Anti-Patterns

❌ God Tables

-- Everything in one table
CREATE TABLE data (
  id SERIAL,
  type VARCHAR(50),
  json_blob JSONB
);
-- No type safety, impossible to query efficiently

❌ EAV (Entity-Attribute-Value)

-- Attributes as rows
CREATE TABLE attributes (
  entity_id INT,
  attribute_name VARCHAR(100),
  attribute_value TEXT
);
-- Impossible to enforce types, terrible performance

❌ Implicit Relationships

-- Magic strings instead of FKs
CREATE TABLE orders (
  user TEXT,  -- Is this user_id? username? email?
  product TEXT
);

❌ Over-denormalization

-- Copying everything everywhere
CREATE TABLE orders (
  user_name VARCHAR(255),     -- What if name changes?
  user_email VARCHAR(255),    -- Stale data guaranteed
  user_address TEXT,
  user_phone VARCHAR(50)
);

❌ Missing Constraints

-- Database allows invalid data
CREATE TABLE products (
  price DECIMAL,  -- Can be NULL, negative, anything
  quantity INT    -- Negative inventory?
);

ORM Integration

Prisma Conventions

model User {
  id        String   @id @default(uuid())
  email     String   @unique
  orders    Order[]
  createdAt DateTime @default(now()) @map("created_at")
  updatedAt DateTime @updatedAt @map("updated_at")
  
  @@map("users")
}

model Order {
  id        String   @id @default(uuid())
  userId    String   @map("user_id")
  user      User     @relation(fields: [userId], references: [id])
  total     Decimal
  status    OrderStatus @default(PENDING)
  
  @@map("orders")
  @@index([userId])
}

Drizzle Conventions

export const users = pgTable('users', {
  id: uuid('id').primaryKey().defaultRandom(),
  email: varchar('email', { length: 255 }).notNull().unique(),
  createdAt: timestamp('created_at').notNull().defaultNow(),
  updatedAt: timestamp('updated_at').notNull().defaultNow(),
});

export const orders = pgTable('orders', {
  id: uuid('id').primaryKey().defaultRandom(),
  userId: uuid('user_id').notNull().references(() => users.id),
  total: decimal('total', { precision: 10, scale: 2 }).notNull(),
  status: varchar('status', { length: 20 }).notNull().default('pending'),
}, (table) => ({
  userIdIdx: index('orders_user_id_idx').on(table.userId),
}));

Design Deliverables

When designing a new schema, produce:

1. Entity list - All entities with key attributes
2. ER diagram - Relationships and cardinality (Mermaid or similar)
3. Schema DDL - CREATE TABLE statements
4. Index plan - Expected queries and supporting indexes
5. Migration plan - If modifying existing schema

Example ER Diagram (Mermaid)

erDiagram
    users ||--o{ orders : places
    users ||--o{ addresses : has
    orders ||--|{ order_items : contains
    products ||--o{ order_items : "ordered in"
    
    users {
        uuid id PK
        string email UK
        string name
        timestamp created_at
    }
    
    orders {
        uuid id PK
        uuid user_id FK
        decimal total
        enum status
        timestamp created_at
    }

References:

• references/schema-patterns.md — Common schema patterns (users, multi-tenant, polymorphic, etc.)
• references/migration-strategies.md — Safe migration techniques, rollback procedures, zero-downtime patterns
• references/index-design.md — When and how to add indexes, query analysis
• references/postgresql.md — PostgreSQL-specific features, extensions, optimization
• references/prisma-patterns.md — Prisma schema design, migrations, best practices
• references/query-optimization.md — Identifying and fixing slow queries, EXPLAIN analysis