 Positional index

A positional index is an inverted index whose postings lists retain the positions of a term's occurrences within each document. These positions distinguish a phrase query from a request for documents merely containing the same words. Positions usually count the terms produced by the indexing pipeline; they need not be character offsets.^[ref]

Number words from one. For document 1, red boat red sail, store positions red: 1,3, boat: 2, sail: 4. For document 2, boat red sail, store red: 2, boat: 1, sail: 3. The phrase query red boat asks whether a position $p$ of red has a position $p+1$ of boat in the same document. Document 1 qualifies at $p=1$; document 2 contains both words but fails. The phrase query red sail matches both documents.

A positional index spends additional storage to preserve ordering information. Removing words during preprocessing without preserving their position gaps can change phrase semantics: the index must specify what counts as adjacent. The same position data can support queries allowing a bounded separation, rather than exact adjacency.