What is an inverted index and why is it faster than scanning documents?
An inverted index maps terms to the list of document IDs containing them. Instead of scanning every document for a query term (linear in corpus size), you look it up once in the index and get all matching documents instantly—O(log terms) to find the term, plus one disk seek to read the posting list.
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The Inverted IndexA sorted dictionary mapping terms to document IDs. Transform search from O(corpus size) to O(1) lookup, enabling full-text search at scale.
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More system design questions
- Why does an index's internal data structure matter if it all ends up 'faster than a scan'?
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- Why does a database need an index at all — why can't it just scan the table?
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