I'm trying to use the ranking function ts_rank_cd to rank the result. I understand that the function ts_rank '
Ranks vectors based on the frequency of their matching lexemes.'.However the definition of ts_rank_cd mention that ' This function computes the cover density ranking for the given document vector and query, as described in Clarke, Cormack, and Tudhope's "Relevance Ranking for One to Three Term Queries" in the journal "Information Processing and Management", 1999. Cover density is similar to ts_rank ranking except that the proximity of matching lexemes to each other is taken into consideration.'
In the example of the official documentation:
SELECT title, ts_rank_cd(textsearch, query) AS rank
FROM apod, to_tsquery('neutrino|(dark & matter)') query
WHERE query @@ textsearch
ORDER BY rank DESC
LIMIT 10; title | rank
-----------------------------------------------+----------Neutrinos in the Sun | 3.1The Sudbury Neutrino Detector | 2.4Why the first result has a rank higher than the second example?