What Is Connection Pooling? Database Performance Guide

Creating a new database connection for every query? Getting "too many connections" errors? Connection pooling reuses connections and can improve performance 10x.

The Problem

Creating a database connection is expensive: TCP handshake, SSL, auth — ~10ms overhead when the query itself takes ~1ms.

How Pooling Works

App start: [conn1] [conn2] [conn3] (ready)
Request 1: borrow conn1 → query → return conn1
Request 2: borrow conn2 → query → return conn2

Node.js + PostgreSQL

const pool = new Pool({ min: 5, max: 20 });

const client = await pool.connect();
try {
  const result = await client.query('SELECT * FROM users WHERE id = $1', [id]);
} finally {
  client.release(); // Return to pool, don't close!
}

Pool Sizing

Optimal size = (CPU cores × 2) + disk count
Example (4 cores, 1 SSD): 9 connections

PgBouncer

External pool proxy between app and database. Reduces 60 app connections to 20 DB connections.

Tools

| Tool | Language | Highlights | |------|----------|-----------| | HikariCP | Java | Fastest Java pool | | PgBouncer | PostgreSQL | External proxy | | ProxySQL | MySQL | Load balancer + pool | | Prisma | Node.js | Integrated pool |

Best Practices

1. Don't oversize the pool | 2. Always release connections in finally
2. Health checks | 4. Detect connection leaks | 5. Monitor active/idle/waiting counts

Conclusion

Connection pooling is the lowest-cost database optimization. Proper pool sizing reduces request latency, decreases database load, and prevents connection exhaustion.

Learn database optimization on LabLudus.