Re: Mailing list search engine: surprising missing results?

On 25.01.2022 19:22, Tom Lane wrote:
> Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at> writes:
>> On Tue, 2022-01-25 at 14:04 +0300, Oleg Bartunov wrote:
>>> On Mon, Jan 24, 2022 at 11:47 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>>> Bruce Momjian <bruce(at)momjian(dot)us> writes:
>>>>> On Mon, Jan 24, 2022 at 08:27:41AM +0100, Laurenz Albe wrote:
>>>>>> 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:
>> Not quite. The problem is question is the "'boyer-moore':1".
>> If that were "'boyer-moor':1" instead, the problem would disappear.
> Actually, when I try this here, it seems like the stemming *is*
> consistent:
>
> regression=# SELECT to_tsvector('english', 'Boyer-Moore-Horspool');
> to_tsvector
> ----------------------------------------------------------
> 'boyer':2 'boyer-moore-horspool':1 'horspool':4 'moor':3
> (1 row)
>
> regression=# SELECT to_tsvector('english', 'Boyer-Moore');
> to_tsvector
> -----------------------------------
> 'boyer':2 'boyer-moor':1 'moor':3
> (1 row)
>
> If you try variants of that where the first or third term is stemmable,
> say
>
> regression=# SELECT to_tsvector('english', 'Boyers-Moore-Horspool');
> to_tsvector
> -----------------------------------------------------------
> 'boyer':2 'boyers-moore-horspool':1 'horspool':4 'moor':3
> (1 row)
>
> it sure appears that each component word is stemmed independently
> already. So I think the original explanation here is wrong and
> we need to probe more closely.

The actual explanation can be seen from comparing a tsvector with a tsquery.
To avoid stemming effects, we use the simple configuration below.

# select plainto_tsquery('simple','boyers-moore');

           plainto_tsquery
-------------------------------------
 'boyers-moore' & 'boyers' & 'moore'

# select to_tsvector('simple','boyers-moore-horspool');

                         to_tsvector
-------------------------------------------------------------
 'boyers':2 'boyers-moore-horspool':1 'horspool':4 'moore':3

Obviously, such tsvector does not match the above tsquery. I think,a better tsquery for this query would be

 'boyers-moore' | ('boyers' & 'moore')

May be, it is worth changing to_tsquery() behavior for such cases.

>
> regards, tom lane
>
>
Regards,
Ivan