Query tuning - please help!!

[newbie5]

How do I get rid of the sort merge join? The explain plan and the query are given below:

 ----------------------------------------------------------------------------------------------------
| Id  | Operation                           |  Name              | Rows  | Bytes | Cost  | Pstart| Pstop |
----------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                    |                    |     1 |   575 |    19 |       |       |
|   1 |  SORT UNIQUE                        |                    |     1 |   575 |    19 |       |       |
|*  2 |   TABLE ACCESS BY GLOBAL INDEX ROWID| ISSUE              |     1 |    29 |     2 | ROWID | ROW L |
|   3 |    NESTED LOOPS                     |                    |     1 |   575 |    15 |       |       |
|   4 |     NESTED LOOPS                    |                    |     4 |  2184 |    13 |       |       |
|   5 |      NESTED LOOPS                   |                    |     7 |  2457 |    11 |       |       |
|   6 |       MERGE JOIN CARTESIAN          |                    |  6406 |  1707K|    11 |       |       |

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
|   7 |        PARTITION HASH ALL           |                    |       |       |       |     1 |     8 |
|*  8 |         INDEX FAST FULL SCAN        | SYS_IOT_TOP_30857  |     7 |  1365 |     2 |     1 |     8 |
|   9 |        BUFFER SORT                  |                    |   980 | 76440 |     9 |       |       |
|  10 |         PARTITION HASH ALL          |                    |       |       |       |     1 |    12 |
|* 11 |          INDEX FAST FULL SCAN       | SYS_IOT_TOP_30830  |   980 | 76440 |     2 |     1 |    12 |
|  12 |       PARTITION HASH ITERATOR       |                    |       |       |       |   KEY |   KEY |
|* 13 |        INDEX UNIQUE SCAN            | SYS_IOT_TOP_30830  |     1 |    78 |       |   KEY |   KEY |
|  14 |      PARTITION HASH ITERATOR        |                    |       |       |       |   KEY |   KEY |
|* 15 |       INDEX RANGE SCAN              | SYS_IOT_TOP_30857  |     1 |   195 |     1 |   KEY |   KEY |
|* 16 |     INDEX RANGE SCAN                | XIF9ISSUE1         |   115 |       |     1 |       |       |
----------------------------------------------------------------------------------------------------