Re: text_pattern_ops index not being used for prefix query

On Fri, Oct 21, 2016 at 8:42 AM, Alexander Staubo <alex(at)purefiction(dot)net>
wrote:

> Indexing on a text column:
>
> create index index_documents_on_id_with_pattern_ops (id
> text_pattern_ops);
>
> This works fine:
>
> > explain select id from documents where id like 'dingbat%';
> Index Only Scan using index_documents_on_id_with_pattern_ops on
> documents (cost=0.56..8.58 rows=736 width=19)
> Index Cond: ((id >= 'dingbat'::text) AND (id < 'dingbau'::text))
> Filter: (id ~~ 'dingbat%'::text)
>
> But for some reason, if an underscore character appears in my search
> string, it falls back to a disasterously slow seqscan:
>
> > explain select id from documents where id like '_dingbat%';
> Seq Scan on documents (cost=0.00..779238.28 rows=736 width=19)
> Filter: (id ~~ '_dingbat%'::text)
>
> Is this because of PostgreSQL’s collation system? Using “C” doesn’t work
> either.
>
>
>
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​Underscore in like pattern have a special meaning of "any symbol".
From documentation on the
https://www.postgresql.org/docs/9.4/static/functions-matching.html :
"An underscore (_) in pattern stands for (matches) any single character;"
This could be useful as well: "To match a literal underscore or percent
sign without matching other characters, the respective character in pattern
must be preceded by the escape character."

--
Maxim Boguk
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