SQL & PL/SQL

Hi,

I am trying to create the below sql query into CTAS but it is taking huge time for creation of the table. Could you please let me know where i can tune this.

create table zz_po_ds_tmp

nologging

AS

SELECT  GREATEST (      

          NVL (

             (SELECT MAX (pos.lastopendate)

                FROM po_dto_master pos

               WHERE pos.userid = po_master.id

                     AND pos.catdivision IN (1, 5)),

             TO_DATE ('01/01/1900', 'mm/dd/yyyy')),

          NVL (

             po_master.mrktg_sub_dt,

             NVL (

                (SELECT MAX (psm.subdate)

                   FROM po_dos_master psm

                  WHERE psm.userid = po_master.id

                        AND psm.catdivision IN (1, 5)),

                TO_DATE ('01/01/1900', 'mm/dd/yyyy')))) dt from po_master ;

Thanks in advance