Query performance issue after upgrade help needed

[awahidt3]

Hi All,
I am experiencing a performance issue for a batch process after migration on ASE 15.5 from 12.5. The batch process used to take just 4 mins on ASE 12.5, now it hangs up for 3 days. The batch process does execute few query very fast but as soon as it come across one of the table, it get hang. But after creating an index on the table the batch moves ahead. I don't understand why should I create an index on the table if the batch works fine without index on 12.5. I also tried using different optimization goals, force plan on, merge_join off,update statistics but it didn't help me. The query looks like something as below :

Select Distinct
trade_id,
deal_id,
maturity_date,
product,
columna,
columb
from
problemtable a,
term b,
product_classification c
where
a.term_id=b.term_id and
a.product_id=c.product_id and
/* the below sub query is problematic, on creating index on trade_id, it run fast */
convert(datetime,maturity_date) in ( select max(convert(datetime,maturity_date)) from problemtable d
where
a.trade_id=d.trade_id)

Please suggest