What makes a bitmap index different from a B-tree, and when is it better?
A bitmap index stores one bit per row per distinct value — a row's bit is 1 if it has that value, 0 otherwise. This is extremely space-efficient and fast to combine with AND/OR operations across multiple bitmap indexes, but only when the column has few distinct values (like a status flag or a boolean). On a high-cardinality column like a user ID, a bitmap index would need one bitmap per unique value, which stops making sense.
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What Actually Happens Inside a Database Index: The Data StructuresAn index trades disk I/O for lookup speed. Here's how B-trees, hash tables, and bitmap indexes each make that trade differently.
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