SQL Indexes: When and How to Use Them
Indexes are the most powerful performance tool in SQL, but they come with tradeoffs. Here is how I decide what to index. Indexes are the single most effective way to improve SQL query performance. They are also the easiest way to slow down writes and waste storage if used carelessly. After years of optimizing database performance, I have developed a set of principles for when to add indexes and when to leave them out. How Indexes Work An index is a data structure, typically a B-tree, that lets the database find rows without scanning the entire table. Without an index, finding a user by email requires checking every row in the users table. With an index on the email column, the database navigates the B-tree to find matching rows in logarithmic time. The difference between scanning one million rows and traversing a tree with twenty levels is enormous. When to Index The simple rule I follow: index columns that appear in WHERE clauses, JOIN conditions, and ORDER BY clauses. These are the columns the database searches on, and indexing them directly improves query performance. CREATE INDEX idx_users_email ON users(email); CREATE INDEX idx_orders_user_id…