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

On Friday, June 3, 2016, Greg Navis <contact(at)gregnavis(dot)com> 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?
>
>
No. Indexing is tied to operators.

I don't know which search terms would work best but I gave this same answer
less than a week ago. List searching before asking is appreciated.

David J.