Message-ID: <ab4184b7ab84623be10c4676e090cc27ae78b355.camel@cybertec.at>
Subject: Re: Mailing list search engine: surprising missing results?
From: Laurenz Albe <laurenz.albe@cybertec.at>
To: James Addison <jay@jp-hosting.net>, pgsql-www@postgresql.org
Date: Mon, 24 Jan 2022 08:27:41 +0100
In-Reply-To: 
 <CALDQ5NxzgeXHRCD4dS_6qz+nn01ivi3i1ZEtD2DmC779i0=iSQ@mail.gmail.com>
References: 
	<CALDQ5NxzgeXHRCD4dS_6qz+nn01ivi3i1ZEtD2DmC779i0=iSQ@mail.gmail.com>
Content-Type: text/plain; charset="UTF-8"
User-Agent: Evolution 3.40.4 (3.40.4-2.fc34) 
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Archived-At: 
 <https://www.postgresql.org/message-id/ab4184b7ab84623be10c4676e090cc27ae78b355.camel%40cybertec.at>
Precedence: bulk

On Sun, 2022-01-23 at 12:49 +0000, James Addison wrote:
> Hello,
> 
> I noticed that the mailing list search engine[1] seems to unexpectedly
> miss results for some queries.
> 
> For example:
> 
> A search for "boyer"[2] returns five results, including result
> snippets that contain the text "Boyer-More-Horspool" [sic] and
> "Boyer-Moore-Horspool".
> 
> However, a more specific search for "boyer-moore"[3] does not return
> any results -- that seems surprising.
> 
> Specializing the query further and searching for
> "boyer-moore-horspool"[4] *does* again return results -- two documents
> -- with the terms "boyer" and "horspool" highlighted.

This is caused by the peculiarities of PostgreSQL full text search:

SELECT to_tsvector('english', 'Boyer-Moore-Horspool')
       @@ websearch_to_tsquery('english', 'boyer-moore');

 ?column?
══════════
 f
(1 row)

The reason is that the 'moore' in 'boyer-moore' is stemmed, since it
is at the end of the word, while the 'moore' in 'Boyer-Moore-Horspool'
isn't:

SELECT to_tsvector('english', 'Boyer-Moore-Horspool');

                       to_tsvector
══════════════════════════════════════════════════════════
 'boyer':2 'boyer-moore-horspool':1 'horspool':4 'moor':3
(1 row)

SELECT websearch_to_tsquery('english', 'boyer-moore');

         websearch_to_tsquery
═════════════════════════════════════
 'boyer-moor' <-> 'boyer' <-> 'moor'
(1 row)

'boyer-moor' is not present in the first result.

As a workaround, I suggest that you search for 'boyer moore'
or (even better) '"boyer moore"' (with the double quotes):

SELECT websearch_to_tsquery('english', 'boyer moore');

 websearch_to_tsquery
══════════════════════
 'boyer' & 'moor'
(1 row)

SELECT websearch_to_tsquery('english', '"boyer moore"');

 websearch_to_tsquery
══════════════════════
 'boyer' <-> 'moor'
(1 row)

Yours,
Laurenz Albe