Query Optimization

Hi guys. I have four similar queries like this. I just cut off I don’t why but this query is an expensive query. It takes like 20 minutes to be executed. To your info, the two tables participating in this and the other queries were heaps.  What would be the reason? Is there other mechanism to optimize this query? I am wondering any one could lend me a hand. Do you think adding a constraint or creating a clustered index would improve the performance?

UPDATEdbo.tbl_cal_db

SETPAY = (

               SELECT isnull(sum(coc), 0)

               FROM ##tempa AS aod

               WHERE aod.agent IN (

                               95

                               ,206

                               )

                       AND aod.midnight = h.midnight

                       AND aod.callservice = h.service

                       AND aod.callhour = h.CallHour

                       AND aod.calltz = h.TZ

                       AND aod.ac = h.st

               GROUP BY midnight

                       ,callhour

                       ,callservice

                       ,calltz

                       ,ac

               )

FROM##tempa

        ,dbo.tbl_cal_db AS h