Database Design skill

Database Design

Schema Design Patterns

Normalization Quick Reference

Denormalization Patterns

-- Materialized counter cache (avoid COUNT queries)
ALTER TABLE posts ADD COLUMN comments_count INT DEFAULT 0;

-- Trigger to maintain it
CREATE FUNCTION update_comments_count() RETURNS TRIGGER AS $$
BEGIN
  IF TG_OP = 'INSERT' THEN
    UPDATE posts SET comments_count = comments_count + 1 WHERE id = NEW.post_id;
  ELSIF TG_OP = 'DELETE' THEN
    UPDATE posts SET comments_count = comments_count - 1 WHERE id = OLD.post_id;
  END IF;
  RETURN NULL;
END; $$ LANGUAGE plpgsql;

Indexing Strategies

Composite index rule: Left-to-right prefix matching. Index on (a, b, c) serves queries on (a), (a, b), (a, b, c) — not (b, c).

Query Optimization

-- Always start here
EXPLAIN (ANALYZE, BUFFERS, FORMAT TEXT) SELECT ...;

Key indicators in query plans:

• Seq Scan on large tables → missing index
• Nested Loop with high row counts → consider Hash Join via better stats
• Rows Removed by Filter ≫ Actual Rows → index not selective enough
• High Buffers: shared read → data not cached, check shared_buffers

N+1 Detection and Fixes

// BAD: N+1 with Prisma
const users = await prisma.user.findMany();
for (const u of users) {
  const posts = await prisma.post.findMany({ where: { authorId: u.id } }); // N queries
}

// GOOD: Eager load
const users = await prisma.user.findMany({ include: { posts: true } });

// GOOD: Drizzle with explicit join
const result = await db.select().from(users).leftJoin(posts, eq(users.id, posts.authorId));

Migration Workflow

Zero-Downtime Checklist

1. Add nullable column (safe, no lock)
2. Backfill data in batches (UPDATE ... WHERE id BETWEEN $1 AND $2)
3. Add NOT NULL constraint using ALTER TABLE ... ADD CONSTRAINT ... NOT VALID then VALIDATE CONSTRAINT
4. Deploy app code using new column
5. Drop old column after confirmation period

# Migration file naming: YYYYMMDDHHMMSS_description.sql
20260101120000_add_users_role.up.sql
20260101120000_add_users_role.down.sql

Dangerous operations (take ACCESS EXCLUSIVE lock):

• ALTER TABLE ... ADD COLUMN ... DEFAULT (PG < 11)
• ALTER TABLE ... ALTER COLUMN TYPE
• CREATE INDEX without CONCURRENTLY

Always use CREATE INDEX CONCURRENTLY in production.

PostgreSQL Power Features

-- JSONB: query nested data
SELECT * FROM events WHERE payload->>'type' = 'click' AND (payload->'meta'->>'duration')::int > 500;

-- CTE for readability
WITH active_users AS (
  SELECT id FROM users WHERE last_login > NOW() - INTERVAL '30 days'
)
SELECT p.* FROM posts p JOIN active_users u ON p.author_id = u.id;

-- Window function: running total
SELECT date, revenue, SUM(revenue) OVER (ORDER BY date ROWS UNBOUNDED PRECEDING) AS running_total
FROM daily_sales;

-- Table partitioning (range)
CREATE TABLE events (id BIGINT, created_at TIMESTAMPTZ, data JSONB)
  PARTITION BY RANGE (created_at);
CREATE TABLE events_2026_q1 PARTITION OF events
  FOR VALUES FROM ('2026-01-01') TO ('2026-04-01');

Connection Pooling

Use PgBouncer in transaction mode for serverless/high-connection environments:

# pgbouncer.ini
[databases]
mydb = host=127.0.0.1 dbname=mydb
[pgbouncer]
pool_mode = transaction
max_client_conn = 1000
default_pool_size = 20

Rule of thumb: default_pool_size ≈ 2-3× CPU cores of your database server.

Backup Strategy

# Automated daily backup
pg_dump -Fc --no-owner mydb | zstd > "backup_$(date +%Y%m%d).dump.zst"
# Restore
zstd -d backup_20260101.dump.zst | pg_restore -d mydb --no-owner

References

See references/ for index tuning guides, migration templates, and ORM comparison matrices.