What does an INDEX do?

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Accepted Answer

An index is a separate data structure (usually a B-tree) that lets the database jump directly to rows matching a condition instead of reading every row in the table. The trade-off is that indexes consume extra disk space and must be updated on every INSERT, UPDATE, and DELETE. A common mistake is adding an index to every column - over-indexing slows down writes without helping reads on low-cardinality columns.