id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
28194	Add search rank cd function and normalization for Postgres full text search	Andrii Soldatenko	Hannes Ljungberg	"It would be great to have cover density ranking for the given document vector and query function `ts_rank_cd`. It helps to order search results with different normalization strategies.

Also add `normalization` parameter option that specifies whether and how a document's length should impact its rank. 

>0 (the default) ignores the document length
>1 divides the rank by 1 + the logarithm of the document length
>2 divides the rank by the document length
>4 divides the rank by the mean harmonic distance between extents (this is implemented only by ts_rank_cd)
>8 divides the rank by the number of unique words in document
>16 divides the rank by 1 + the logarithm of the number of unique words in document
>32 divides the rank by itself + 1 


Now you can do something like this:

{{{
class SearchRankCD(SearchRank):
    function = 'ts_rank_cd'

    def __init__(self, vector, query, normalization=0, **extra):
        super(SearchRank, self).__init__(
            vector, query, normalization, **extra)

query = SearchQuery('messenger')

Application.objects.annotate(
    rank=SearchRankCD(
        F('search_vector_title'), query,
        normalization=2)  # 2 divides the rank by the document length
).filter(search_vector_title=query).order_by('-rank')
}}}
"	New feature	closed	contrib.postgres	dev	Normal	fixed			Ready for checkin	1	0	0	0	0	0