database encoding problem

i have problem in database encoding with indexing

case :1
when i initdb -E win874 mydb and create database with createdb -E win874
dbname
and create table emp with index in empname field

i can select sorting name correct in thai alphabet
such select empName from emp order by empName;
but in
select empName from emp where empname like 'xxx%'
it not using index scan , it use seq scan so it may slow in find name

case :2
when i initdb  mydb (use default) and create database with createdb -E
win874 dbname
and create table emp with index in empname field

i can not select sorting name  correct in thai alphabet
but in
select empName from emp where empname like 'xxx%'
it using index scan , very fast in find name

problem:
how can i configure database that can correct in sorting name and using
index scan in like 'xxxx%' search

using FreeBSD 5.4
postgreql 8.0.

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