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14 April, 2008

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.

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