Query runs differently in 9i and 10g

Question

I have a query that implictly does an order by in 9i and does not do the implicit order by in 10g. Nope, I dont have a "group by" clause in the query. Looking at the explain plan, the 9i version does a "SORT UNIQUE" and the 10g version does a "HASH UNIQUE" instead.
Query
select CTMR.CUSTOMER_ID,
     CTMR.PRIMARY_SITE_NO,
     CTMR.NAME,
     CTAC.TAX_ID STATE_TAX_ID,
     CTAC.FEDTAX_ID FED_TAX_ID,
     CTMR.COMMENT_TXT,
     CTAC.ACCT_TYPE ACCT_TYPE,
DECODE(TMXGBL.GET_FLAG('STRICT_CREDIT_LIMIT'), 'Y', DECODE(CTAC.CREDIT_LIMIT, 0, 'NOT ALLOWED', TO_CHAR(CTAC.CREDIT_LIMIT, 'FM999G999G990D00')),
                              'N', DECODE(CTAC.CREDIT_LIMIT, 0, 'UNLIMITED', TO_CHAR(CTAC.CREDIT_LIMIT, 'FM999G999G990D00')),
                              TO_CHAR(CTAC.CREDIT_LIMIT, 'FM999G999G990D00')) CREDIT_LMT,
     CTAC.PO_REQ_FL PO_REQ,
     CTMR.JOB_REQUIRED_FL JOB_CONT_REQ,
     (SELECT EMPLOYEE_ID
     FROM CUSTOMER_EMPLOYEE
     WHERE CUSTOMER_ID = CTMR.CUSTOMER_ID
     AND ROWNUM < 2) EMPLOYEE_ID
FROM TABLE_1_DUMMY CTMR, TABLE_2_DUMMY CTAC
WHERE CTMR.CUSTOMER_ID = CTAC.CUSTOMER_ID(+)
--AND CTMR.STATUS_CD = 'A'
AND CTMR.CUSTOMER_ID IN (SELECT CAV.CUSTOMER_ID
          FROM TABLE_DUMMY3 AN, TABLE_DUMMY4 AU, CUSTOMER_ATTRIBUTE_VALUE CA                                                       WHERE AN.NAME_TEXT = 'TERRITORY'
                                                            AND AU.FUNCTIONAL_AREA_CD = 'CUS'
                                                            AND AN.ATTRIBUTE_ID = AU.ATTRIBUTE_ID
                                                            AND CAV.ATTRIBUTE_ID = AN.ATTRIBUTE_ID
                                                            AND CAV.FUNCTNL_AREA_CD = 'CUS'
                                                            AND CAV.ATTRIBUTE_VALUE = '042401')
Question:
What clause in the query is making a HASH UNIQUE / SORT UNIQUE?
Besides using a ORDER BY in 10g, is there a different solution?

Answer 1

9i explain plan
SELECT STATEMENT, GOAL = CHOOSE               5     1     179
COUNT STOPKEY                         
INDEX RANGE SCAN     TMX17     CUSTOMER_EMPLOYEE_PK     2     1     18
NESTED LOOPS SEMI               5     1     179
NESTED LOOPS OUTER               4     1     173
NESTED LOOPS               4     1     128
VIEW     SYS     VW_NSO_1     3     1     9
SORT UNIQUE                    1     190
NESTED LOOPS               3     1     190
NESTED LOOPS               3     1     37
TABLE ACCESS BY INDEX ROWID     TMX17     ATTRIBUTE_NAME     3     1     27
INDEX UNIQUE SCAN     TMX17     ATTRIBUTE_NAME_UK     2     1     
INDEX UNIQUE SCAN     TMX17     ATTR_FUNCTIONAL_USAGE_PK     1     1     10
TABLE ACCESS BY INDEX ROWID     TMX17     CUSTOMER_ATTRIBUTE_VALUE     1     1     153
INDEX RANGE SCAN     TMX17     CUSTOMER_ATTRIBUTE_ATTR_IDX     1     1     
TABLE ACCESS BY INDEX ROWID     TMX17     CUSTOMER     1     1     119
INDEX UNIQUE SCAN     TMX17     CUSTOMER_PK     1     1     
TABLE ACCESS BY INDEX ROWID     TMX17     CUSTACCT     1     1     45
INDEX UNIQUE SCAN     TMX17     CUSTACCT_PK     1     1     
INDEX RANGE SCAN     TMX17     CUSPRICE_IDX1     2     2     12
10g explain plan
SELECT STATEMENT, GOAL = ALL_ROWS               537     1     401
COUNT STOPKEY                         
INDEX RANGE SCAN     TMX249     CUSTOMER_EMPLOYEE_PK     2     596     10132
NESTED LOOPS OUTER               537     1     401
NESTED LOOPS               536     1     356
VIEW     SYS     VW_NSO_1     534     19     171
HASH UNIQUE                    1     3895
NESTED LOOPS               534     19     3895
NESTED LOOPS               2     1     52
TABLE ACCESS BY INDEX ROWID     TMX249     ATTRIBUTE_NAME     2     1     35
INDEX UNIQUE SCAN     TMX249     ATTRIBUTE_NAME_UK     1     1     
INDEX UNIQUE SCAN     TMX249     ATTR_FUNCTIONAL_USAGE_PK     0     23     391
TABLE ACCESS FULL     TMX249     CUSTOMER_ATTRIBUTE_VALUE     532     19     2907
TABLE ACCESS BY INDEX ROWID     TMX249     CUSTOMER     1     1     347
INDEX UNIQUE SCAN     TMX249     CUSTOMER_PK     0     1     
TABLE ACCESS BY INDEX ROWID     TMX249     CUSTACCT     1     1     45
INDEX UNIQUE SCAN     TMX249     CUSTACCT_PK     0     1     
Noticed that the GOAL is "ALL_ROWS", and there is no hint to force this in the query, and is different from the Goal "Choose" in 9i