Database Design

You are a senior database architect who designs schemas that scale and writes queries that stay fast under load.

Use this skill when

• Designing a new database schema or modifying an existing one
• Optimizing slow queries using EXPLAIN ANALYZE
• Creating or reviewing database migrations
• Choosing between normalization and denormalization
• Setting up connection pooling or ORM configuration
• Debugging N+1 queries, deadlocks, or performance issues
• Deciding between PostgreSQL and SQLite for a project

Schema Design Principles

Normalization to 3NF

1NF: Every column holds atomic values. No arrays, no comma-separated lists. 2NF: Every non-key column depends on the entire primary key (relevant for composite keys). 3NF: No transitive dependencies. If A -> B -> C, then C should be in B's table, not A's.

-- Bad: transitive dependency (city -> state)
CREATE TABLE customers (
    id SERIAL PRIMARY KEY,
    name TEXT NOT NULL,
    city TEXT,
    state TEXT    -- determined by city, not by customer
);

-- Good: normalize to 3NF
CREATE TABLE cities (
    id SERIAL PRIMARY KEY,
    name TEXT NOT NULL,
    state TEXT NOT NULL,
    UNIQUE (name, state)
);
CREATE TABLE customers (
    id SERIAL PRIMARY KEY,
    name TEXT NOT NULL,
    city_id INT REFERENCES cities(id)
);

When to Denormalize

Denormalize when:

• Read:write ratio exceeds 100:1 and JOINs are the bottleneck
• Reporting queries scan millions of rows across 5+ tables
• You need sub-millisecond reads (caching at DB level)

Patterns:

• Materialized views: precomputed JOINs, refreshed on schedule
• Computed columns: store full_name alongside first_name + last_name
• Counter caches: store comment_count on posts, update via trigger

-- Materialized view for dashboard stats
CREATE MATERIALIZED VIEW order_stats AS
SELECT
    date_trunc('day', created_at) AS day,
    COUNT(*) AS order_count,
    SUM(total) AS revenue
FROM orders
GROUP BY 1;

-- Refresh on schedule (or after batch inserts)
REFRESH MATERIALIZED VIEW CONCURRENTLY order_stats;

CONCURRENTLY requires a unique index on the view. Without it, reads are blocked during refresh.

Indexing Strategy

B-tree (default, 90% of cases)

-- Single column: equality and range queries
CREATE INDEX idx_users_email ON users (email);

-- Composite: leftmost prefix rule applies
CREATE INDEX idx_orders_user_date ON orders (user_id, created_at DESC);
-- This index serves: WHERE user_id = X, WHERE user_id = X AND created_at > Y
-- It does NOT serve: WHERE created_at > Y (without user_id)

-- Covering index: includes columns to avoid table lookup
CREATE INDEX idx_orders_covering ON orders (user_id, created_at DESC)
    INCLUDE (total, status);
-- Now: SELECT total, status FROM orders WHERE user_id = 1 ORDER BY created_at DESC
-- is an index-only scan. No heap fetch.

Partial Index (filter out noise)

-- Only index active users (skip 80% of rows)
CREATE INDEX idx_users_active_email ON users (email) WHERE is_active = true;

-- Only index unprocessed jobs
CREATE INDEX idx_jobs_pending ON jobs (created_at) WHERE status = 'pending';

-- Example: Query using the partial index
SELECT * FROM users WHERE email = 'user@example.com' AND is_active = true;
-- Uses idx_users_active_email (fast, small index)

SELECT * FROM users WHERE email = 'user@example.com' AND is_active = false;
-- Cannot use partial index, falls back to seq scan or full email index

Partial indexes are smaller, faster, and should be your first optimization before adding hardware.

GIN Index (PostgreSQL full-text, JSONB, arrays)

-- JSONB containment queries
CREATE INDEX idx_products_tags ON products USING GIN (tags);
-- SELECT * FROM products WHERE tags @> '["electronics"]';

-- Full-text search
CREATE INDEX idx_articles_search ON articles USING GIN (to_tsvector('english', title || ' ' || body));
-- SELECT * FROM articles WHERE to_tsvector('english', title || ' ' || body) @@ to_tsquery('database & design');

SQLite Indexing

SQLite uses B-tree exclusively. Same composite index rules apply. Key difference: SQLite has no concurrent index creation -- CREATE INDEX locks the entire database.

-- SQLite: use WITHOUT ROWID for narrow lookup tables
CREATE TABLE sessions (
    token TEXT PRIMARY KEY,
    user_id INTEGER NOT NULL,
    expires_at INTEGER NOT NULL
) WITHOUT ROWID;

EXPLAIN ANALYZE: Reading Query Plans

EXPLAIN (ANALYZE, BUFFERS, FORMAT TEXT)
SELECT u.name, COUNT(o.id) as order_count
FROM users u
LEFT JOIN orders o ON o.user_id = u.id
WHERE u.created_at > '2025-01-01'
GROUP BY u.id;

What to look for:

• Seq Scan on large tables: Add an index. If the table has <1000 rows, Seq Scan is fine.
• Nested Loop with inner Seq Scan: The inner table needs an index on the join column.
• Hash Join: Normal for large table joins. Worry if Batches > 1 (means spilling to disk -- increase work_mem).
• Sort with external merge: Sort is spilling to disk. Add an index that matches the ORDER BY, or increase work_mem.
• Rows: estimated vs actual: If wildly off (10x+), run ANALYZE tablename to update statistics.
• Buffers: shared hit vs read: read means disk I/O. If read >> hit, your working set exceeds shared_buffers.

Migrations

Up/Down Pattern

-- 001_create_users.up.sql
CREATE TABLE users (
    id SERIAL PRIMARY KEY,
    email TEXT NOT NULL UNIQUE,
    password_hash TEXT NOT NULL,
    created_at TIMESTAMPTZ NOT NULL DEFAULT now()
);

-- 001_create_users.down.sql
DROP TABLE users;

Safe Migration Rules (zero-downtime)

1. Never rename a column directly. Add new, backfill, deploy code that reads both, drop old.
2. Never add NOT NULL without a default. Existing rows will fail. Add column nullable, backfill, then set NOT NULL.
3. Never drop a column that's still read by running code. Deploy code that stops reading it first.
4. Create indexes concurrently in PostgreSQL:

CREATE INDEX CONCURRENTLY idx_users_email ON users (email);
-- This does NOT lock the table. Regular CREATE INDEX does.

5. Wrap DDL in transactions (PostgreSQL supports transactional DDL, MySQL does NOT).

Tools: golang-migrate, Prisma Migrate, Drizzle Kit, Alembic (Python).

GORM (Go ORM)

// Define models with tags
type User struct {
    ID        uint      `gorm:"primaryKey"`
    Email     string    `gorm:"uniqueIndex;not null"`
    Name      string    `gorm:"size:100;not null"`
    Orders    []Order   `gorm:"foreignKey:UserID"`
    CreatedAt time.Time `gorm:"autoCreateTime"`
    UpdatedAt time.Time `gorm:"autoUpdateTime"`
}

type Order struct {
    ID        uint      `gorm:"primaryKey"`
    UserID    uint      `gorm:"index;not null"`
    Total     float64   `gorm:"type:decimal(10,2);not null"`
    Status    string    `gorm:"type:varchar(20);default:'pending'"`
    CreatedAt time.Time `gorm:"autoCreateTime"`
}

// Auto-migrate (development only, use migrations in production)
db.AutoMigrate(&User{}, &Order{})

// Queries with preloading (avoid N+1)
var users []User
db.Preload("Orders").Where("email LIKE ?", "%@example.com").Find(&users)

// Raw SQL for complex queries
var results []struct {
    Name       string
    OrderCount int64
}
db.Raw(`
    SELECT users.name, COUNT(orders.id) as order_count
    FROM users
    LEFT JOIN orders ON orders.user_id = users.id
    GROUP BY users.id
`).Scan(&results)

// Transactions
err := db.Transaction(func(tx *gorm.DB) error {
    if err := tx.Create(&user).Error; err != nil {
        return err
    }
    if err := tx.Create(&order).Error; err != nil {
        return err
    }
    return nil
})

GORM best practices:

• Use gorm:"index" on all foreign keys (GORM doesn't auto-index FKs).
• Use Preload() or Joins() to avoid N+1 queries.
• Never use AutoMigrate in production. Generate SQL and review before running.
• Use db.Raw() for complex queries GORM can't express efficiently.
• Add gorm:"-" to fields that shouldn't be persisted.

Tools: golang-migrate, Prisma Migrate, Drizzle Kit, Alembic (Python).

Connection Pooling

Why

Each PostgreSQL connection costs ~10MB RAM. 100 connections = 1GB. Serverless functions can spawn hundreds.

PgBouncer (external pooler)

[pgbouncer]
pool_mode = transaction    ; Release connection after each transaction
max_client_conn = 1000     ; Accept up to 1000 app connections
default_pool_size = 20     ; Only 20 actual PostgreSQL connections

transaction mode: connection released after COMMIT. Best for web apps. Breaks: prepared statements, LISTEN/NOTIFY, session variables. session mode: connection held for entire client session. Use only if you need prepared statements.

Prisma with Connection Pooling

// Use connection_limit to control pool size
const prisma = new PrismaClient({
  datasources: {
    db: { url: process.env.DATABASE_URL + "?connection_limit=5" },
  },
});

For serverless (Vercel, AWS Lambda): use Prisma Accelerate or PgBouncer. Direct connections from Lambda will exhaust the pool.

ORM Patterns

Prisma

// Avoid N+1: use include, not sequential queries
const users = await prisma.user.findMany({
  where: { isActive: true },
  include: {
    orders: {
      where: { status: "completed" },
      orderBy: { createdAt: "desc" },
      take: 5,
    },
  },
});

Drizzle (SQL-like, type-safe)

import { eq, desc, sql } from "drizzle-orm";

const result = await db
  .select({
    name: users.name,
    orderCount: sql<number>`count(${orders.id})`.as("order_count"),
  })
  .from(users)
  .leftJoin(orders, eq(orders.userId, users.id))
  .where(eq(users.isActive, true))
  .groupBy(users.id)
  .orderBy(desc(sql`count(${orders.id})`));

SQLAlchemy (Python)

from sqlalchemy import select, func
from sqlalchemy.orm import joinedload

# Eager load to avoid N+1
stmt = (
    select(User)
    .options(joinedload(User.orders))
    .where(User.is_active == True)
)
users = session.scalars(stmt).unique().all()

Transactions & Isolation Levels

-- PostgreSQL default: READ COMMITTED
-- Each statement sees committed data at statement start.

-- REPEATABLE READ: snapshot at transaction start. Use for reports.
BEGIN ISOLATION LEVEL REPEATABLE READ;
SELECT sum(balance) FROM accounts;  -- consistent snapshot
SELECT count(*) FROM accounts WHERE balance > 1000;  -- same snapshot
COMMIT;

-- SERIALIZABLE: full serializability. Use for financial operations.
BEGIN ISOLATION LEVEL SERIALIZABLE;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
UPDATE accounts SET balance = balance + 100 WHERE id = 2;
COMMIT;
-- If concurrent transaction conflicts, one gets: ERROR: could not serialize access
-- Your code MUST retry on serialization failure.

SQLite: uses DEFERRED (default), IMMEDIATE, or EXCLUSIVE locking.

• BEGIN IMMEDIATE: grabs write lock upfront. Prevents "database is locked" errors in concurrent writes.

Anti-Patterns

N+1 Queries

# BAD: 1 query for users + N queries for orders
users = session.query(User).all()
for user in users:
    print(user.orders)  # Each triggers a SELECT

# GOOD: 1 query with JOIN
users = session.query(User).options(joinedload(User.orders)).all()

Missing Indexes on Foreign Keys

PostgreSQL does NOT auto-index foreign keys. Every FK column needs an explicit index, or JOINs and cascading deletes will Seq Scan.

-- After adding a FK, always add the index
ALTER TABLE orders ADD CONSTRAINT fk_user FOREIGN KEY (user_id) REFERENCES users(id);
CREATE INDEX idx_orders_user_id ON orders (user_id);  -- DON'T FORGET THIS

Over-Normalization

If you're joining 6 tables to render a product card, you've gone too far. Denormalize the read path, keep writes normalized. CQRS pattern: separate read models (denormalized) from write models (normalized).

Unbounded Queries

-- BAD: no LIMIT, could return millions of rows
SELECT * FROM logs WHERE level = 'error';

-- GOOD: always paginate
SELECT * FROM logs WHERE level = 'error'
ORDER BY created_at DESC
LIMIT 50 OFFSET 0;

-- BETTER: cursor-based pagination (stable, no offset scan)
SELECT * FROM logs WHERE level = 'error' AND created_at < $last_seen
ORDER BY created_at DESC
LIMIT 50;

Using TEXT for Everything

Use the right type. TIMESTAMPTZ not TEXT for dates. INTEGER not TEXT for IDs. INET not TEXT for IPs. Correct types enable index efficiency, validation, and query planner optimization.

PostgreSQL vs SQLite Decision Guide

Choose SQLite when: single-user app, embedded/mobile, CLI tools, <100 concurrent readers, <10GB database, prototyping. **Choose PostgreSQL when**: multi-user web app, concurrent writes, need JSONB/full-text/GIS, >10GB, need replication, need row-level security.

SQLite scales reads remarkably well with WAL mode:

PRAGMA journal_mode = WAL;      -- Write-ahead logging: concurrent reads during writes
PRAGMA busy_timeout = 5000;     -- Wait 5s instead of instant SQLITE_BUSY
PRAGMA synchronous = NORMAL;    -- Good enough durability for most apps
PRAGMA cache_size = -64000;     -- 64MB cache (negative = KB)