Index columns used in JOIN, WHERE, and ORDER BY

Index columns used in JOIN, WHERE, and ORDER BY | BeforeMerge Rules | BeforeMerge

Why this matters

Columns used to filter (WHERE), join, or sort (ORDER BY) need supporting indexes; otherwise MySQL scans every row and sorts in memory or on disk. The right index turns an O(n) scan into an O(log n) lookup.

-- Bad: no index on customer_id -> full scan on every lookup
SELECT * FROM orders WHERE customer_id = 42;
 
-- Good
CREATE INDEX idx_orders_customer_id ON orders (customer_id);

Why This Matters

Related Rules

Catch this automatically on every PR

Why this matters