Database Indexing Explained: How to Make Your Queries 1000x Faster

Slow queries are the #1 performance killer. Understanding indexes is the fastest way to fix them. Here's a practical guide. What Is an Index? An index is like a book's table of contents. Without it, the database scans every row (full table scan). With it, the database jumps directly to matching rows. -- Without index: scans all 10 million rows SELECT * FROM orders WHERE customer_id = 12345 ; -- Time: 3200ms -- With index: jumps to matching rows CREATE INDEX idx_orders_customer ON orders ( customer_id ); -- Same query now: 2ms Types of Indexes B-Tree (Default) Best for equality and range queries: CREATE INDEX idx_users_email ON users ( email ); CREATE INDEX idx_orders_date ON orders ( created_at ); -- These queries use the index: SELECT * FROM users WHERE email = 'alice@example.com' ; SELECT * FROM orders WHERE created_at > '2024-01-01' ; SELECT * FROM orders WHERE created_at BETWEEN '2024-01-01' AND '2024-06-30' ; Composite Indexes Multiple columns — order matters: CREATE INDEX idx_ord