Re: Full text search ranking: ordering using index and proximiti ranking with OR queries

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.php

On Sat, Feb 18, 2012 at 12:39, Andrey Chursin <andll(at)danasoft(dot)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.
>
> --
> Regards,
> Andrey
>
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