According to Oleg in a previous discussion, ts_rank does not use index because index does not store enough information for ranking:
http://archives.postgresql.org/pgsql-general/2011-07/msg00351.phpOn Sat, Feb 18, 2012 at 12:39, Andrey Chursin
<andll@danasoft.ws> wrote:
Hello
I have two unrelated questions about fts function ts_rank:
1) I've created GiST index on column with fts vector, but query
SELECT * FROM table ORDER BY ts_rank(field, :query) LIMIT 20
is perfomed with sequential table scan. Index was created on field
column. Does it mean FTS indexes does not support order by ranking? Or
I need somehow to create separated index for ranking?
2) I have a misunderstanding with proximity ranking work. Given two
vectors 'a:1 b:2' and 'a:1 b:1000', i am measuring ts_rank(vector, 'a'
| 'b'). And it is equal! But when i am replacing query with &
operator, e.g. asking for ts_rank(vector, 'a' & 'b') i am getting
different numbers. Why do I get proximity ranking only for AND fts
queries? This is a problem as far as to_tsquery produces OR queries,
so i need self-written postprocessing of query to replace OR with AND.
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Regards,
Andrey
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