Hint ordered and cartesian product

Question

I have a query which needs to be tuned.
The initial Query does not show cartesian product in the plan , when I change the order of the tables in the from clause ( I have placed the least # of rows first and the most rows tables in the last) and I get MERGE JOIN CARTESIAN product the where clause remaining the same and the cost escalates from 7180 to 454M .
This happens with ordered hint.

Answer 1

ranjus wrote:
I have a query which needs to be tuned.
The initial Query does not show cartesian product in the plan , when I change the order of the tables in the from clause ( I have placed the least # of rows first and the most rows tables in the last) and I get MERGE JOIN CARTESIAN product the where clause remaining the same and the cost escalates from 7180 to 454M .
This happens with ordered hint.Let's take a look at this with a simple test case. Assume that:
* Table T1 contains 10,000 rows
* Table T2 contains 100 rows
* Table T3 contains 100,000 rows
* Table T4 contains 10 rows
You have the following query:
SELECT
  T1.C1,
  T2.C2,
  T3.C3,
  T4.C4
FROM
  T1,
  T2,
  T3,
  T4
WHERE
  T1.C1=T2.C2
  AND T2.C2=T3.C2
  AND T3.C3=T4.C3;In the above, you can see that in the WHERE clause:
* Table T1 is joined to table T2
* Table T2 is joined to tables T1 and T3
* Table T3 is joined to tables T2 and T4
* Table T4 is joined to table T3
What it sounds like you are trying to do is something like this based on the number of rows in the tables:
SELECT /*+ ORDERED */
  T1.C1,
  T2.C2,
  T3.C3,
  T4.C4
FROM
  T4,
  T2,
  T1,
  T3
WHERE
  T1.C1=T2.C2
  AND T2.C2=T3.C2
  AND T3.C3=T4.C3;In the above, we are forcing Oracle's optimizer to join tables T4 and T2 - but there is no direct relationship between the two tables described in the WHERE clause. In such a case, Oracle's optimizer will be forced to perform a Cartesian join between tables T4 and T2. Next, table T1 has a relationship defined in the WHERE clause with table T2, so that join would not result in a Cartesian product. Finally, table T3 has a relationship between tables T2 and T4, so that would not result in a Cartesian product.
Going back to this example, assume that table T4 contained a single row. The execution plan will still show a Cartesian join, but that Cartesian join is no worse than a nested loops join because there is a single row in one of the row sources.
In short, think about what you are attempting to force the optimizer to do. You cannot always join in the order of increasing row cardinality estimates - it simply does not make sense to try in some cases.
Charles Hooper
Co-author of "Expert Oracle Practices: Oracle Database Administration from the Oak Table"
http://hoopercharles.wordpress.com/
IT Manager/Oracle DBA
K&M Machine-Fabricating, Inc.