Re: [pg_trgm] Making similarity(?, ?) < ? use an index

Hello.

As I know 'lhs % rhs' is equivalent to 'similarity(lhs, rhs) >=
show_limit()'.

And so your query should looks like this:

SELECT * FROM restaurants WHERE city % 'warsw';

And it should use index.

On 03.06.2016 13:35, Greg Navis wrote:
> Hey!
>
> I'm playing with pg_trgm. It seems that `lhs % rhs` is _almost_
> equivalent to `similarity(lhs, rhs) < show_limit()`. The difference that
> I noticed is that `%` uses a GIN index while `similarity` does not.
>
> ```
> grn=# \d restaurants
> Table "public.restaurants"
> Column | Type | Modifiers
> --------+------------------------+-----------
> city | character varying(255) | not null
> Indexes:
> "restaurants_city_trgm_idx" gin (city gin_trgm_ops)
>
> grn=# SELECT COUNT(*) FROM restaurants;
> count
> --------
> 515475
> (1 row)
>
> Time: 45.964 ms
> grn=# EXPLAIN ANALYZE SELECT * FROM restaurants WHERE similarity(city,
> 'warsw') > show_limit();
> QUERY PLAN
>
> --------------------------------------------------------------------------------------------------------------------
> Seq Scan on restaurants (cost=0.00..11692.81 rows=171825 width=10)
> (actual time=16.436..665.062 rows=360 loops=1)
> Filter: (similarity((city)::text, 'warsw'::text) > show_limit())
> Rows Removed by Filter: 515115
> Planning time: 0.139 ms
> Execution time: 665.105 ms
> (5 rows)
>
> Time: 665.758 ms
> ```
>
> My question is: is it possible to make `similarity` use the index? If
> not, is there a way to speed up the query above?
>
> Best regards
> --
> Greg Navis
> I help tech companies to scale Heroku-hosted Rails apps.
> Free, biweekly scalability newsletter for SaaS CEOs
> <http://www.gregnavis.com/newsletter/>
>

--
Artur Zakirov
Postgres Professional: http://www.postgrespro.com
Russian Postgres Company