Date: Sat, 28 Oct 2017 10:22:29 +0200
From: Arthur Zakirov <a.zakirov@postgrespro.ru>
To: Cristiano Coelho <cristianocca@hotmail.com>
Cc: "pgsql-bugs@postgresql.org" <pgsql-bugs@postgresql.org>
Subject: Re: pg_trgm word_similarity inconsistencies or bug
Message-ID: <20171028082225.GA2157@arthur.localdomain>
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On Fri, Oct 27, 2017 at 06:48:08PM +0000, Cristiano Coelho wrote:
> Hello all, this is related to postgres 9.6 (9.6.4) and a good description can be found here https://stackoverflow.com/questions/46966360/postgres-word-similarity-not-comparing-words
> 
> But in summary, word_similarity doesn’t seem to do exactly what the docs say, since it will match trigrams from multiple words rather tan doing a word by word comparison.
> 
> Below is a table with output and expected output, thanks to kiln from stackoverflow to provide it.
> 

Interesting. An klin's answer from stackoverflow.com is right.

The initial example can be reduced to the next:

=# select word_similarity('sage', 'age sag');
 word_similarity 
-----------------
               1

It computes maximum similarity using closest trigrams not considering order of
'sage' trigrams. It determines that all
trigrams from 'sage' match trigrams from 'age sag'.

Initial order of 'age sag' trigrams:
'  a', ' ag', 'age', 'ge ', '  s', ' sa', 'sag', 'ag '
                ^                           ^
                |from                       |to
Sorted 'sage' trigrams (all of them occured within 'age sag' trigrams
continuously):
'  s', ' sa', 'age', 'ge ', 'sag'

Maybe the problem should be solved by considering 'sage' trigrams
initial order.

-- 
Arthur Zakirov
Postgres Professional: http://www.postgrespro.com
Russian Postgres Company


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