Performance tuning
Read EXPLAIN, find slow queries and fix the cause instead of guessing.
What you will be able to do
- Use EXPLAIN ANALYZE to see real timings.
- Look for full table scans on large tables.
- Avoid SELECT * and functions on indexed columns in WHERE.
- Measure before and after every change.
In plain English
Performance work starts with measurement. Enable the slow query log, run EXPLAIN ANALYZE on the worst query, then change the query or index. Adding indexes blindly often makes writes slower without helping.
EXPLAIN ANALYZE
SELECT l.title
FROM lessons l
JOIN courses c ON c.slug = l.course_slug
WHERE c.slug = 'css'
ORDER BY l.minutes DESC
LIMIT 10;Your turn
MediumAvoid a function on an indexed column
Rewrite WHERE YEAR(created_at) = 2026 as a range comparison.
SQL · starter
WHERE YEAR(created_at) = 2026Check your understanding before moving on.
Knowledge checkQuestion 1
What does EXPLAIN ANALYZE add over EXPLAIN?
Select an answer to continue
Study tools
Make it yours.
Keep a private note for this lesson, then turn the key ideas into flashcards you can revisit.
Sign in to save notes and flashcards.Both are private to your account and stay with your progress.
Sign inFinished reading?
Lock in what you just learned.
0%
Mark this lesson complete to save your place and move one step closer to the certificate.