Why Your SQL Queries Are Slow (And How to Fix Them)

It's a classic story: you write a query, it runs instantly on your test database with 50 rows, and you deploy it. Two months later, your app is crashing because that same query is now trying to sift through 5 million records.

SQL performance can feel like a dark art, but 90% of performance issues come down to a few common mistakes. In this post, we'll explore why queries get slow and how you can use SQL Boy's Visual Tools to spot the problems.

1. The SELECT * Trap

We've all done it. It's easy to write:

SELECT * FROM users WHERE id = 123;

But SELECT * is a performance killer for two reasons:

1. Network Overhead: You're fetching data you might not need. If your users table has a massive bio text field, you're dragging that across the network every time.
2. Index Inefficiency: Databases can often retrieve data directly from an index (Covering Index) if you only ask for specific columns. SELECT * forces the database to look up the full row on disk.

The Fix: Be specific.

SELECT id, username, email FROM users WHERE id = 123;

2. The Missing Index (The Library Analogy)

Imagine walking into a massive library to find a book titled "The Art of SQL", but there's no catalog system. You'd have to walk every aisle, look at every shelf, and check every single book. That's a Full Table Scan.

Without an index, your database does exactly this.

SELECT * FROM orders WHERE customer_id = 450;

If customer_id isn't indexed, the database reads every single order to find the ones for customer 450.

The Fix: Add an index on columns you frequently filter by.

CREATE INDEX idx_orders_customer ON orders(customer_id);

3. How to "See" the Problem: The EXPLAIN Command

You don't have to guess why a query is slow. SQL databases have a built-in feature called EXPLAIN (or EXPLAIN QUERY PLAN) that tells you exactly what they are doing.

On SQL Boy, we make this visual. When you run a query, you can switch to the "Explain" tab to see the execution plan.

Look for these keywords:

• SCAN TABLE: Bad. This usually means a full table scan.
• SEARCH TABLE: Good. This means it's using an index (Binary Search).

Try it yourself!

Copy this query into the SQL Boy Playground and check the Explain tab:

-- This will likely trigger a SCAN if 'salary' isn't indexed
SELECT * FROM employees WHERE salary > 100000;

Summary

1. Never use SELECT * in production.
2. Index columns used in WHERE, JOIN, and ORDER BY.
3. Use EXPLAIN to verify your query is actually using those indexes.

Mastering these basics puts you ahead of many developers. Happy optimizing!