[SQL] Optimizing query inside a function

First of all, sorry for my English. I hope you can understand me. ;)

I made a function using pl/sql. It seems that optimizer doesn't work in the query inside function. Of course, if I
excutethe same query outside function, optimizer works well and uses proper index.
 

Is it true that queries inside a function cannot be optimized? or is there a way I can optimize my query inside a
function?

My pgsql version is 6.4.2.

Thanks in advance!

Hyun-Su Oh