Index on VARCHAR with text_pattern_ops inside PL/PGSQL procedure.

Hello,

I've created table:

quaker=> \d users
Table "public.users"
Column | Type | Modifiers

-----------+-------------------+----------------------------------------------------
id | integer | not null default
nextval('users_id_seq'::regclass)
user_name | character varying | not null
extra | integer |
Indexes:
"users_pkey" PRIMARY KEY, btree (id)
"users_user_name_unique_text_ops" UNIQUE, btree (user_name text_ops)
"users_user_name_unique_text_pattern_ops" btree (user_name
text_pattern_ops)

Filled with random data (100k records).

I do simple queries using above indexes (asking for existing record).

explain analyze select id from users where user_name = 'quaker';
QUERY
PLAN
---------------------------------------------------------------------------------------------------------------------------------------
Index Scan using users_user_name_unique_text_ops on users
(cost=0.00..8.28 rows=1 width=4) (actual time=0.040..0.043 rows=1 loops=1)
Index Cond: ((user_name)::text = 'quaker'::text)
Total runtime: 0.084 ms
(3 rows)

explain analyze select id from users where user_name like 'quaker';
QUERY
PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------
Index Scan using users_user_name_unique_text_pattern_ops on users
(cost=0.00..8.28 rows=1 width=4) (actual time=0.022..0.024 rows=1 loops=1)
Index Cond: ((user_name)::text ~=~ 'quaker'::text)
Filter: ((user_name)::text ~~ 'quaker'::text)
Total runtime: 0.050 ms
(4 rows)

Everything looks fine.

Now, I've created PL/PGSQL function:

create or replace function user_login(
_v_user_name varchar
) returns integer as $$
declare
_i_user_id integer;
begin
select id into _i_user_id from users where user_name = _v_user_name
limit 1;
if found then
return _i_user_id;
end if;
return -1;
end;
$$ language plpgsql security definer;

As shown above, I use "=" operator, which should use
users_user_name_unique_text_ops index:

explain analyze select user_login('quaker');
QUERY PLAN

------------------------------------------------------------------------------------
Result (cost=0.00..0.01 rows=1 width=0) (actual time=0.320..0.322
rows=1 loops=1)
Total runtime: 0.340 ms
(2 rows)

Some performance loss, but OK. Now I've changed "=" into "LIKE" to use
users_user_name_unique_text_pattern_ops index and rerun query:

explain analyze select user_login('quaker');

QUERY PLAN

--------------------------------------------------------------------------------------
Result (cost=0.00..0.01 rows=1 width=0) (actual time=41.606..41.608
rows=1 loops=1)
Total runtime: 41.629 ms
(2 rows)

Second run give 61.061 ms. So no improvements.

Why PL/PGSQL is unable to proper utilize
users_user_name_unique_text_pattern_ops?

quaker=> select version();
version

----------------------------------------------------------------------------------------------------------------------
PostgreSQL 8.2.5 on i486-pc-linux-gnu, compiled by GCC cc (GCC) 4.1.3
20070831 (prerelease) (Ubuntu 4.1.2-16ubuntu1)
(1 row)