Oracle-L: RE: IN vs. EXISTS

From: DENNIS WILLIAMS <DWILLIAMS_at_LIFETOUCH.COM>
Date: Fri, 18 Oct 2002 11:53:57 -0800
Message-ID: <F001.004EDBF6.20021018115357@fatcity.com>

Rick - I would check the EXPLAIN PLAN for both queries, and see whether Oracle uses a different query plan. Usually the prescription for better SQL performance is to avoid NOT IN, as the following snippet that I snagged off the 'net explains.

Use NOT EXISTS in Place of NOT IN
In sub-query statements such as the following, the NOT IN clause causes an internal sort/merge. The NOT IN clause is the all-time slowest test possible as it forces a full read of the table in the sub-query SELECT. Avoid using NOT IN clause either by replacing it with Outer Joins or with a NOT EXISTS clause

Dennis Williams
DBA, 40%OCP
Lifetouch, Inc.
dwilliams_at_lifetouch.com

-----Original Message-----
Sent: Friday, October 18, 2002 1:32 PM
To: Multiple recipients of list ORACLE-L

Hi All,

I have 2 tables

BIG - 100 million records
SMALL - 1 million records.

I want to delete all the records in BIG that are in small. There is a PK on field1.
Which of the below methods would you choose and why?

DELETE FROM big
WHERE field1 IN (SELECT field1 FROM small);

DELETE FROM big a
WHERE EXISTS (SELECT 1 FROM small B

                                    WHERE b.field1 = a.field1);

Thanks
Rick

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Author:
INET: Rick_Cale_at_teamhealth.com

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Author: DENNIS WILLIAMS
INET: DWILLIAMS_at_LIFETOUCH.COM

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