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SQLAdvanced#indexes
SQLIntermediate#indexes#performance
What is an index? What are the trade-offs?
An index is usually a B-tree that lets the engine locate rows without scanning the whole table. It speeds up filtering, joining and sorting but slows down INSERT/UPDATE/DELETE and consumes storage.
Example
CREATE INDEX idx_orders_customer ON orders(customer_id); -- SELECT * FROM orders WHERE customer_id = 42; now uses an index seek