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

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/>