Term frequency counts how many times a term occurs in a document or text representation. In PostgreSQL full-text search, ts_rank uses term frequency and term position to rank matching documents. section 6.4
Comparing occurrences in movie descriptions
The lexemes for Ghost contain 'ghost':1,2,15, showing three occurrences. A Girl Walks Home Alone at Night contains 'ghost':13, showing one. Frequency alone does not determine the final order: ts_rank also considers position, while ts_rank_cd considers term density. section 6.4
Listing 6.11 Ranking search results with the ts_rank function
SELECT id, name, vote_average,
ts_rank(lexemes, to_tsquery('ghosts')) AS search_rank
FROM omdb.movies
WHERE lexemes @@ to_tsquery('ghosts')
ORDER BY search_rank DESC, vote_average DESC NULLS LAST LIMIT 10;
Run a PostgreSQL query that calculates search_rank with ts_rank, then compare ranking output against the observed lexeme counts. section 6.4
