bug?

Hi, Tom,
we're using version 7.2.

I've a document table, the cdi column is indexed.
I tried to do a query by using cdi in two ways -

1. the simple query as expected is using index scan
edbs=# explain select emrxid from document where cdi='1031-15402';
NOTICE: QUERY PLAN:

Index Scan using document_cdi_key on document (cost=0.00..4.43 rows=1
width=27)

2. the one using plsql function is not using index scan!
EXPLAIN
edbs=# explain select getEmrxid('1031-15402') from document;
NOTICE: QUERY PLAN:

Seq Scan on document (cost=0.00..447377.72 rows=13018272 width=0)

here's the simple fuction -
create FUNCTION getEmrxid(text)
RETURNS text AS'
DECLARE
cdi_in ALIAS FOR $1;
docrec document%ROWTYPE;
BEGIN
select into docrec * from document where cdi = cdi_in;
RETURN docrec.emrxid;
END;
'LANGUAGE 'plpgsql';

This is not what I expected. If it's normal in pgsql, would you
providing some suggestions how to let function use index.

thanks in advance.

john