# Database Patterns

> SQL/database best practices — query optimization, N+1 prevention, proper indexing, connection pooling, migration safety, Prisma/Drizzle patterns, avoiding SELECT *, transactions

- Skill: `nadav011/database-patterns` (Agent Skill)
- Install (CLI): `npx skillmds@latest add nadav011/database-patterns`
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- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Data & Analytics
- Author: Nadav011 (https://skillmd.com/u/nadav011)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/nadav011/database-patterns

---

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# Database Patterns

---

## Never Use SELECT *

Selecting all columns retrieves data you don't need, prevents index-only scans, and breaks code when the schema changes.

```sql
-- ❌ WRONG — fetches all columns including blobs, secrets, unused fields
SELECT * FROM users WHERE id = $1;

-- ✅ CORRECT — explicit columns; lighter payloads, index-covered scans possible
SELECT id, email, name, created_at FROM users WHERE id = $1;
```

```typescript
// ❌ WRONG — Prisma: no field selection
const user = await prisma.user.findUnique({ where: { id } });

// ✅ CORRECT — Prisma: explicit select
const user = await prisma.user.findUnique({
  where: { id },
  select: { id: true, email: true, name: true, createdAt: true },
});
```

---

## N+1 Query Prevention

N+1 is the most common database performance bug: one query to fetch a list, then one more query per item to fetch related data.

```typescript
// ❌ WRONG — 1 query for posts + N queries for each post's author
const posts = await prisma.post.findMany();
for (const post of posts) {
  const author = await prisma.user.findUnique({ where: { id: post.authorId } });
  // renders post with author
}

// ✅ CORRECT — Prisma include: 2 queries total (or 1 with JOIN)
const posts = await prisma.post.findMany({
  include: { author: { select: { id: true, name: true } } },
});
```

```typescript
// ✅ CORRECT — Drizzle: explicit join
const posts = await db
  .select({
    id: postsTable.id,
    title: postsTable.title,
    authorName: usersTable.name,
  })
  .from(postsTable)
  .innerJoin(usersTable, eq(postsTable.authorId, usersTable.id));
```

**Detection:** In development, log all queries. Any route that generates more than 3 queries for a single page load likely has an N+1.

---

## Indexing Strategy

An unindexed query on a large table causes a full sequential scan. Add indexes on every column used in WHERE, JOIN ON, or ORDER BY.

```sql
-- ❌ WRONG — filtering by email with no index: full table scan
SELECT id, name FROM users WHERE email = $1;

-- ✅ CORRECT — add a unique index (also enforces uniqueness at DB level)
CREATE UNIQUE INDEX idx_users_email ON users (email);

-- ✅ CORRECT — composite index for frequent compound queries
-- Index column order: most selective first, then sort/range columns
CREATE INDEX idx_orders_user_status ON orders (user_id, status, created_at DESC);
```

**Rules:**
- Every foreign key column should have an index unless you never query from child → parent
- Partial indexes reduce index size for filtered queries: `CREATE INDEX ... WHERE status = 'active'`
- Check `EXPLAIN ANALYZE` before and after adding an index; an index that is never used wastes write overhead
- Don't index low-cardinality boolean columns alone — combine them into composite indexes

---

## Connection Pooling

Opening a new database connection per request is expensive (~50-100ms) and will exhaust the DB's connection limit under load.

```typescript
// ❌ WRONG — new connection on every request
import { Client } from 'pg';

export async function getUser(id: string) {
  const client = new Client({ connectionString: process.env.DATABASE_URL });
  await client.connect();
  const result = await client.query('SELECT id, email FROM users WHERE id = $1', [id]);
  await client.end();
  return result.rows[0];
}

// ✅ CORRECT — singleton pool, reused across all requests
import { Pool } from 'pg';

const pool = new Pool({
  connectionString: process.env.DATABASE_URL,
  max: 10,          // max connections in pool (tune for your DB plan)
  idleTimeoutMillis: 30_000,
  connectionTimeoutMillis: 5_000,
});

export async function getUser(id: string) {
  const { rows } = await pool.query(
    'SELECT id, email FROM users WHERE id = $1',
    [id],
  );
  return rows[0] ?? null;
}
```

**Serverless note:** In serverless environments (Vercel, Cloudflare Workers), use a connection pooler like PgBouncer or Neon's serverless driver — standard connection pools don't survive across cold starts.

---

## Transactions: All or Nothing

Mutations that span multiple tables must be wrapped in a transaction. A failure midway otherwise leaves data in an inconsistent state.

```typescript
// ❌ WRONG — if the second insert fails, the order exists with no items
await prisma.order.create({ data: { userId, total } });
await prisma.orderItem.createMany({ data: items.map(i => ({ orderId, ...i })) });

// ✅ CORRECT — Prisma interactive transaction
const result = await prisma.$transaction(async (tx) => {
  const order = await tx.order.create({
    data: { userId, total },
  });
  await tx.orderItem.createMany({
    data: items.map((item) => ({ orderId: order.id, ...item })),
  });
  await tx.inventory.updateMany({
    where: { productId: { in: items.map((i) => i.productId) } },
    data: { reserved: { increment: 1 } },
  });
  return order;
});
```

```sql
-- ✅ CORRECT — raw SQL transaction
BEGIN;
  INSERT INTO orders (user_id, total) VALUES ($1, $2) RETURNING id;
  INSERT INTO order_items (order_id, product_id, qty) VALUES ($3, $4, $5);
  UPDATE inventory SET reserved = reserved + 1 WHERE product_id = $4;
COMMIT;
-- Any error above automatically triggers ROLLBACK
```

**Rules:**
- Keep transactions short — long-running transactions hold locks and block other writers
- Never perform network calls (HTTP, email) inside a transaction
- Use `SERIALIZABLE` isolation only when you need true snapshot consistency; `READ COMMITTED` is sufficient for most OLTP workloads

---

## Migration Safety

Unsafe migrations can lock tables and cause downtime on live databases.

```sql
-- ❌ WRONG — ALTER TABLE ... ADD COLUMN with NOT NULL and no DEFAULT
-- On large tables this rewrites every row and locks the table for minutes
ALTER TABLE users ADD COLUMN preferences JSONB NOT NULL;

-- ✅ CORRECT — add nullable first, backfill, then add constraint
ALTER TABLE users ADD COLUMN preferences JSONB;
UPDATE users SET preferences = '{}' WHERE preferences IS NULL;
ALTER TABLE users ALTER COLUMN preferences SET NOT NULL;
ALTER TABLE users ALTER COLUMN preferences SET DEFAULT '{}';
```

```sql
-- ❌ WRONG — dropping a column immediately (app still reads it until deploy)
ALTER TABLE users DROP COLUMN legacy_field;

-- ✅ CORRECT — ignore → deprecate → remove over multiple deploys
-- Deploy 1: stop reading legacy_field in application code
-- Deploy 2: run migration to drop the column
ALTER TABLE users DROP COLUMN legacy_field;
```

**Rules:**
- Every migration must be reversible — write a `down` migration
- Never rename a column directly; add the new column, backfill, switch reads/writes, then drop the old one
- Test migrations against a production-size data snapshot before deploying
- Run migrations before deploying new app code (not after)

---

## Parameterized Queries (SQL Injection Prevention)

String interpolation into SQL is a direct injection vulnerability. Always use parameterized queries.

```typescript
// ❌ WRONG — SQL injection: attacker sends userId = "1 OR 1=1"
const result = await db.query(
  `SELECT * FROM users WHERE id = ${userId}`,
);

// ✅ CORRECT — driver handles escaping
const result = await db.query(
  'SELECT id, email FROM users WHERE id = $1',
  [userId],
);
```

---

## Checklist

- [ ] No `SELECT *` — columns are always explicit
- [ ] Related data fetched with JOIN or `include` — no N+1 loops
- [ ] Every filtered column has an appropriate index
- [ ] `EXPLAIN ANALYZE` reviewed for slow queries before merging
- [ ] Connection pool configured with a max and timeout
- [ ] Multi-table mutations wrapped in a transaction
- [ ] Migrations are backward-compatible and have a `down` script
- [ ] All queries use parameterized placeholders — no string interpolation
- [ ] Serverless functions use a pooler (PgBouncer / Neon serverless)
- [ ] No network I/O inside a transaction