Why can't a hash index handle range queries?
A hash function scrambles input values into effectively random bucket positions by design — that's what makes lookups O(1). But it destroys any ordering relationship between keys: two values that are numerically close can hash to buckets that are nowhere near each other. Range scanning depends on adjacent values being stored near each other, which hashing specifically prevents.
Answered in
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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