index not being used when variable is sent

Hi, 

I encountered a problem while trying to improve the performance of a certain select query I have made. 

here is a simplified code for the function I am using 

CREATE OR REPLACE FUNCTION test_func(STR text) 

RETURNS integer AS 

$BODY$ 

begin 

insert into plcbug(val) values('begin time before perform'); 

perform t1.val FROM t1 WHERE 

(COALESCE(rpad(t1.val, 100),'') ) like COALESCE(STR || '%','')  

order by COALESCE(rpad(t1.val, 100), '') using ~<~ LIMIT 5; 

insert into plcbug(val) values('time after perform'); 

return 1; 

END; 

$BODY$ 

LANGUAGE plpgsql VOLATILE 

COST 100; 

ALTER FUNCTION test_func(text) OWNER TO postgres; 

plcbug is a table I am using in order to see how much time has past between the perform query.

t1 (about 800,000 records) is:

create table t1 (val varchar(200))

this is the code of the index for the query 

CREATE INDEX ixt1 ON t1 USING btree 

((COALESCE(rpad(val::text, 100), ''::text)) varchar_pattern_ops)

the problem is that for some reason the index is not being used when I try to run the function with the STR variable(the running time is about 70 milliseconds), but if I am writing the same text instead of using the variable STR then the index is being used(the runing time is about 6 milliseconds) 

to make it more clear 

COALESCE(STR || '%','') this is when I use the variable and the function is being called by 

select test_func('si') 

COALESCE('si' || '%','') this is when I write the text at hand and the index is being used. 

I tried to cast the expression with every type I could think of with no success of making the index work 

postgresql version is 9.0.4 64-bit on windows server 2008 R2.

more info:

i did not know how to do "explain analyze" for the code inside the function. so i did something which i believe still represent the same problem. instead of using the variable (STR) i did a select from a very simple, one record table t2, which holds the value.

create table t2 (val varchar(200));

insert into t2 (val) values ('si');

analyze t2;

select t1.val FROM t1 WHERE 

(COALESCE(rpad(t1.val, 100),'') ) like COALESCE((select val from t2 limit 1) || '%','')  

order by COALESCE(rpad(t1.val, 100), '') using ~<~ LIMIT 5; 

select t1.val FROM t1 WHERE 

(COALESCE(rpad(t1.val, 100),'') ) like COALESCE('si' || '%','')  

order by COALESCE(rpad(t1.val, 100), '') using ~<~ LIMIT 5; 

Thanks in advance for the help! 

Eran