The Worst Ever SQL Rewrite

What is the worst ever re-write you've done in attempting to tune an SQL statement ?

This is what I did a few weeks ago. I took a complicated (it had 3 correlated complex sub-queries) of the form :

SELECT A.column_1
     , A.column_2
...
 FROM table_A A
     ,table_P P
...
 WHERE A.column_1=E.column_1
...
  AND D.column_15 = (
      SELECT /*+ NO_MERGE */ MAX(column_25)
      FROM
....
.. 71 line SQL statement

SQL> select * from table(dbms_xplan.display());

PLAN_TABLE_OUTPUT                                                                                  
------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 888888888                                                                        
                                                                                                   
------------------------------------------------------------------------------------------------------------------------
 Id   Operation                                    Name              Rows   Bytes TempSpc Cost (%CPU) Time
------------------------------------------------------------------------------------------------------------------------
   0  SELECT STATEMENT                                                   1    337            47M  (1)159:03:36
*  1   FILTER                                                                             
*  2    HASH JOIN RIGHT OUTER                                           48M    15G          177K  (1) 00:35:26
   3     INDEX FAST FULL SCAN                        A_TABLE           2687  67175             5  (0) 00:00:01

so you can see that it had already been hinted during the course of a tuning exercise ...

and I transformed it with

SELECT /*+ ORDERED */ A.column_1
...
-- FROM table_A A
-- , table_P P
FROM
table_E E
, table_D D
... 74lines

SQL> select * from table(dbms_xplan.display());

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 999999999

------------------------------------------------------------------------------------------------------------------------      
 Id   Operation                                    Name              Rows   Bytes TempSpc Cost (%CPU) Time           
------------------------------------------------------------------------------------------------------------------------      
   0  SELECT STATEMENT                                                   1    337           250T  (2)999:59:59       
*  1   FILTER                                                                                                        
*  2    HASH JOIN RIGHT OUTER                                           48M    15G          250T  (2)999:59:59       
   3     INDEX FAST FULL SCAN                      A_TABLE             2687  67175             5  (0) 00:00:01

I managed to increase the "COST" from 47million to 250trillion.
Oracle's projected query runtime went up from 159hours to to in excess of 1000 hours. !

And my mistake ? Wrongly ordering the tables in the FROM clause after putting in an ORDERED Hint ! Very fortunately, I hadn't run the query else I would have waited too long before realising that I had forced Oracle into an atrocious execution plan.

Moral of the story ? Hints do work as Directives ! Be careful with Hints.