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What is an index and how does it speed up queries?

Q: What is an index and how does it speed up queries?

An **index** is a sorted auxiliary structure (usually a B-tree) on one or more columns that lets the database find rows without scanning the whole table — turning O(n) lookups into O(log n). The trade-off: indexes consume space and slow down writes, so index selectively.

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An index is a sorted auxiliary structure (usually a B-tree) on one or more columns that lets the database find rows without scanning the whole table — turning O(n) lookups into O(log n). The trade-off: indexes consume space and slow down writes, so index selectively.

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