Correlated update

I have the following problem
The update is very long (more than 4 hours) table T_SUBJECTVISITFORMITEMS is 3M rows  

How can I do that more quickly ?  

Thanks in advance

CREATE TABLE T_ITEMFIRSTDATA_STATUS_ICLD AS
SELECT
 MIN (ITEMFIRSTDATA) ITEMFIRSTDATA ,
 SUBJECTID,
 VISITID,
 DB_SOURCE
FROM T_SUBJECTVISITFORMITEMS
WHERE FORMMNEMONIC = 'STATU'
GROUP BY SUBJECTID,VISITID,FORMID,DB_SOURCE;   ALTER TABLE T_SUBJECTVISITFORMITEMS ADD ITEMFIRSTDATA_STATU DATE;   UPDATE T_SUBJECTVISITFORMITEMS B SET ITEMFIRSTDATA_STATU = (SELECT DISTINCT ITEMFIRSTDATA
FROM T_ITEMFIRSTDATA_STATUS_ICLD A
WHERE A.SUBJECTID=B.SUBJECTID
AND A.VISITID=B.VISITID
AND A.DB_SOURCE= B.DB_SOURCE);        Received on Sat Dec 04 2010 - 01:11:14 CST