Re: pg_trgm vs. Solr ngram

On 10.02.23 04:48, Laurenz Albe wrote:
> On Fri, 2023-02-10 at 03:20 +0100, Chris wrote:
>> In Solr I was using ngrams and customized the
>> TokenizerFactories until more or less only whitespace was as separator,
>> while [.:-_\d] remains part of the ngrams. This allows to search for
>> ".12.255/32" or "xzy-eth5.example.org" without any false positives.
>>
>> It looks like a straight conversion of this method is not possible 


On 10.02.23 04:48, Laurenz Albe wrote:
> Here is a hack that you can try: pre-process your strings and replace
> symbols with rare characters:
> 
>    SELECT show_trgm(translate('127.0.0.1/32', './', 'qx'));
> 
>                            show_trgm
>    ═════════════════════════════════════════════════════════
>     {"  1"," 12",0q0,0q1,127,1x3,27q,"32 ",7q0,q0q,q1x,x32}
>    (1 row)
> 
> Then you could search like
> 
>    WHERE translate(search_string, './', 'qx') LIKE translate('%127.0.0.1/32%', './', 'qx')
>      AND search_string LIKE '%127.0.0.1/32%'
> 
> The first condition can use a trigram index, and the second filters out
> false positives.


Hehe that is a nifty idea. I went to try this but then it turned out 
that I was probably overthinking the whole issue already. Using a 
gist_trgm_ops index and % as operator works perfectly well:

insert into docs (orig) values ('120.2.10.22');
insert into docs (orig) values ('120 2 10 22');
CREATE INDEX iorig ON docs USING GIST (orig gist_trgm_ops);

set enable_seqscan = off;
explain analyze verbose select * from docs where orig like '%.10.22%';

Index Scan using iorig on public.docs  (cost=0.14..8.16 rows=1 width=32) 
(actual time=0.952..1.018 rows=1 loops=1)
   Output: orig
   Index Cond: (docs.orig ~~ '%.10.22%'::text)
   Rows Removed by Index Recheck: 1

Even though this query has the same trigrams like e.g. '% 10 22%', the 
index recheck takes care of it and only the matching row is returned. 
Excellent, not quite sure why I was expecting false positives in the 
first place, it would be a pretty stark violation of how % is supposed 
to behave.

Not quite sure how big the performance hit of not having the optimal 
trigrams with punctuation in the index and rechecking some hits will be, 
but for now I'll assume it's negligible. Otherwise I'll try the 
translate variant.

Many thanks!