Hi Surendra,
I had a similar problem and oddly the following worked for me with the to_date() function:
to_date(date, 'YYYYMMDD') -> note the format string.
Try experimenting with the dummy pseudo table to see what works for you, e.g.:
select to_date(date_string, 'YYYYMMDD'/another_format_string) from dummy;
Hope this helps.
Regards,
Ramesh