Understanding Indexes

What is an Index?

An index is like a book's index - it helps find data without scanning every row.

Common indexed columns:

• Primary keys (automatic)
• Foreign keys
• Columns often used in WHERE or JOIN

Index-Friendly Patterns

✅ Good for indexes:

WHERE user_id = 5          -- exact match
WHERE status = 'active'    -- equality
WHERE created_at > '2026-01-01'  -- range on indexed column

❌ Often prevents index use:

WHERE LOWER(email) = '...'  -- function on column
WHERE status != 'deleted'   -- NOT equal (sometimes)
WHERE description LIKE '%word%'  -- wildcard at start

Your Task

Write an efficient query to find all events for a specific user (user_id = 3):

• Show: event_id, event_type, event_time
• This type of query benefits from an index on user_id