Re: BUG #17556: ts_headline does not correctly find matches when separated by 4,999 words

At Fri, 22 Jul 2022 14:06:43 +0000, PG Bug reporting form <noreply@postgresql.org> wrote in 
> The following bug has been logged on the website:
> 
> Bug reference:      17556
> Logged by:          Alex Malek
> Email address:      magicagent@gmail.com
> PostgreSQL version: 14.4
> Operating system:   Red Hat
> Description:        
> 
> Correct results when 4,998 words separate search terms:
> 
> # select ts_headline('baz baz baz ipsum ' || repeat(' foo ',4998) || '
> labor',
>            $$'ipsum' & 'labor'$$::tsquery, 'StartSel=>, StopSel=<, 
> MaxFragments=100, MaxWords=7, MinWords=3') ;
>      ts_headline
> ---------------------
>  >ipsum< ... >labor<
> (1 row)
> 
> Add one more word between terms being searched for, to total 4,999, and
> terms are not found:
> 
> # select ts_headline('baz baz baz ipsum ' || repeat(' foo ',4999) || '
> labor',
>            $$'ipsum' & 'labor'$$::tsquery, 'StartSel=>, StopSel=<, 
> MaxFragments=100, MaxWords=7, MinWords=3') ;
>  ts_headline
> -------------
>  baz baz baz
> (1 row)

When ts_headline searches the document, it splits the document into
segments in the length called internally as max_cover, which is not
configurable for now [1].  In the latter case above, it is
MaxFragments * (max(MaxWords * 10, 100)) = 10000 "words" where
whitespaces are counted as words. The docuement has 10007 "words",
where 'ipsum' is the 7th word and 'labor' is the 10007th word. The two
words aren't within a 10000-word segment so it is missed. ts_headeline
returns instead the first MinWords words as you see.

This is not a bug, but a designed behavior.  However, we might want to
document that beahvior.

This could be "improved" as [1], but in this specific case, I doubt
the usefulness of ts_headline picking up it up when the two words are
that far distant each other, in exchange of possible degradation.


[1] For developers, wparser_def.c:2582
>     * We might eventually make max_cover a user-settable parameter, but for
>     * now, just compute a reasonable value based on max_words and
>     * max_fragments.

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center