Understanding EXPLAIN

What is EXPLAIN?

EXPLAIN shows the query execution plan — how the database will retrieve your data.

EXPLAIN QUERY PLAN SELECT * FROM events WHERE user_id = 3;

This tells you:

• SCAN TABLE: Database reads every row (slow for large tables)
• SEARCH TABLE USING INDEX: Database uses an index (fast!)

Why This Matters

If you see SCAN TABLE, your query might be slow on large datasets. An index could speed it up dramatically.

Key Terms

Term	Meaning	Speed
SCAN	Reads every row	🐌 Slow
SEARCH USING INDEX	Uses index lookup	⚡ Fast
COVERING INDEX	Index has all needed columns	⚡⚡ Fastest

Your Task

Find all sessions where page_views exceed 10:

• Show: session_id, user_id, page_views, device
• Order by page_views descending
• This query would benefit from an index on page_views