| cost based optimizer [message #60630] |
Fri, 20 February 2004 22:01  |
ora
Messages: 47
Registered: June 2002
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Member |
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Hi,
I want to know that if the optimizer mode is set to COST then is the order of tables and order of conditions in a join query matters or not?
Means, consider the following query:
select t1.* from t1,t2,t3
where
t1.c1 = t2.c1
and t2.c1 = t3.c1
In above query, if the optimizer mode is COST so the order of t1,t2,t3 and the order of conditions in "where" clause matters or not?
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| Re: cost based optimizer [message #60634 is a reply to message #60630] |
Mon, 23 February 2004 00:45  |
Frank Naude
Messages: 4587
Registered: April 1998
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Senior Member |
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Hi,
Yes, if you use the cost based optimizer, the join order is determined by optimizer statistics.
Nevertheless, you can override the join order with hints. The "ORDERED" hint causes Oracle to join tables in the order in which they appear in the FROM clause. This is documented in the "Oracle9i Database Performance Tuning Guide and Reference" manual.
Best regards.
Frank
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