if you are using a join in your sql and the column in the where clause is not having an indx on it and/ or its position is in a composite index is not allowing the usage of index the sort will be done in the temp tablespace. so depending on the cardinality of the column adding a index to this column will greatly improve the queries performance. if that does not help
- coalescing the temp tablespace.
- increasing the sort_area_size parameter.
- increasing the temp tablespace.