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],
  );

From Nadav011/apex-skills, MIT.

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