From: "Kevin Grittner" <Kevin.Grittner@wicourts.gov>
> "MauMau" <maumau307@gmail.com> wrote:
>> For information, what kind of breakage would occur?
>
>> I imagined removing KEEPONLYALNUM would just accept
>> non-alphanumeric characters and cause no harm to those who use
>> only alphanumeric characters.
>
> This would break our current usages because of the handling of
> trigrams at the "edges" of groups of qualifying characters. It
> would make similarity (and distance) values less useful for our
> current name searches using it. To simulate the effect, I used an
> '8' in place of a comma instead of recompiling with the suggested
> change.
>
> test=# select show_trgm('smith,john');
> show_trgm
> -----------------------------------------------------------
> {" j"," s"," jo"," sm","hn ",ith,joh,mit,ohn,smi,"th "}
> (1 row)
>
> test=# select show_trgm('smith8john');
> show_trgm
> -----------------------------------------------------
> {" s"," sm",8jo,h8j,"hn ",ith,joh,mit,ohn,smi,th8}
> (1 row)
>
> test=# select similarity('smith,john', 'jon smith');
> similarity
> ------------
> 0.615385
> (1 row)
>
> test=# select similarity('smith8john', 'jon smith');
> similarity
> ------------
> 0.3125
> (1 row)
>
> So making the proposed change unconditionally could indeed hurt
> current users of the technique. On the other hand, if there was
> fine-grained control of this, it might make trigrams useful for
> searching statute cites (using all characters) as well as names
> (using the current character set); so I wouldn't want it to just be
> controlled by a global GUC.
Thank you for your concise examples. I probably got it.
From your examples, I thought KEEPONLYALNUM controls whether
non-alphanumeric characters are included in trigrams, though I haven't read
the code of pg_trgm. So, removing KEEPONLYALNUM definition produces trigrams
unnecessary for users who handle only alphanumeric text. That would lead to
undesirable query results.
Then, I wonder what would be the ideal specification...to add
alphanumeric/non-alphanumeric boolean switch to similarity() function, add
non-alphanumeric version of operators (ex. %* and <->*) and non-alphanumeric
version of operator classes (ex. gin_allchars_trgm_ops)? At least, I
understood the fix is not appropriate for minor releases.
Regards
MauMau
