If you’re building a web app or working with a database, you’ve probably used joins — they help you combine data from multiple tables.
But you might wonder:
"Are joins slow or expensive?"

Let’s break it down in a simple way.

What is a Join?

A join is a way to get related data from two or more tables.
Example:

SELECT users.name, orders.total
FROM users
JOIN orders ON users.id = orders.user_id;

This gives you the user’s name along with their order total.

When Joins Are Fast

Joins work well when:
✅ You use indexes on foreign keys (like user_id)
✅ You limit the number of rows with WHERE or LIMIT
✅ You select only the fields you need
✅ You don’t join too many tables at once

Modern databases like MySQL or PostgreSQL are built to handle joins efficiently.

When Joins Can Be Slow

Joins get slow when:
❌ You don’t use indexes
❌ You join huge tables without filters
❌ You use too many nested joins
❌ You join unnecessary data (e.g., SELECT * everywhere)

Also, repeating joins in loops (N+1 problem) can make things worse, especially with ORMs.

💡 How to Keep Joins Fast

  1. Always index your join fields
  2. Use SELECT column_name, not SELECT *
  3. Filter your queries with WHERE
  4. Don’t fetch unnecessary related data

If you’re using an ORM like Prisma, it does joins smartly under the hood. Just don’t over-fetch.

Are joins expensive?

🤞 Not really — if used the right way.

They’re safe, powerful, and essential for working with relational data.

Just follow a few best practices, and you’ll be fine!

Regards,
N I Rimon