Why does a database need an index at all — why can't it just scan the table?
A table scan reads every row to find matches, so cost grows linearly with table size — fine for a thousand rows, unusable for a billion. An index is a precomputed structure that lets the database jump close to the matching rows instead of inspecting every one, the same way a book's index lets you skip straight to a page instead of reading cover to cover.
Answered in
Database Indexing Explained: Which Index Type Actually Fits Your QueryB-tree, hash, composite, and covering indexes each solve a different query shape. Pick the wrong one and you pay index overhead without the speedup.
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