I would like to know how to specify key column argument in
the following situation:

In PB help its only saying how to specify a key column when
it is a retrieval aragument (col=:as_arg)

My dw will have multiple rows. So based on row's particular
columns' value the pic needs to be retrieved.

It should be like this while getting pic:

emp_id=:as_empid (retrieval aragument)
and child_id=child_id (based on row's column value)
and type='ret' (static condition where in other type pic
will not be there)

Also when trying to specify different key clause & see the
preview, the PB getting killed.