Composite Indexes: Multi-Column Power

The Scenario

My query filters on `customer_id` AND `order_date`. A single-column index only helps with one.

CREATE INDEX idx_orders_cust_date ON orders (customer_id, order_date);

The index on `(customer_id, order_date)` helps queries that filter:

By `customer_id` alone: ✓

By `customer_id` AND `order_date`: ✓

By `order_date` alone: ✗ (The index MUST be used left-to-right).

Think of it like a phone book sorted by Last Name, then First Name. You can look up "Smith," but you can't easily look up everyone named "John."

*Day 75: Avoiding Index Killers.*