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SQL tuning [message #361095]

Tue, 25 November 2008 00:47

mhdmehraj
Messages: 3
Registered: November 2008

Junior Member

DELETE FROM sal_head
WHERE cm_number NOT IN (SELECT DISTINCT cm_number
FROM sal_detl)

iam using oracle 8.0.6

no of rows in sal_head=2.5lac
no of rows in sal_detl=8.5 lac
still i have index on sal_head.cm_number and sal_detl.cm_number..
..
iam getting the wait event db_file_scattered _read,,,

i have attached the expalin plan with this ,,,

please suggest me to encounter this issue..
regards,
mehraj hussain

Attachment: explainplan.JPG
(Size: 126.45KB, Downloaded 684 times)

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Re: SQL tuning [message #361536 is a reply to message #361095]

Wed, 26 November 2008 20:13

rleishman
Messages: 3728
Registered: October 2005
Location: Melbourne, Australia

Senior Member

DELETE 
FROM sal_head
WHERE nvl(cm_number,-1) NOT IN (
    SELECT nvl(cm_number,-1)
    FROM sal_detl
)

Alternatively, make cm_number non-nullable in both tables.

Ross Leishman

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Re: SQL tuning [message #376175 is a reply to message #361536]

Tue, 16 December 2008 07:24

sibgat
Messages: 45
Registered: November 2006
Location: Chennai

Member

How Using a nvl(cm_number,-1) when used for replacing a null column with -1, will enhance the performance, (if so)

Thanks
Sibgat

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Re: SQL tuning [message #376308 is a reply to message #376175]

Tue, 16 December 2008 21:38

rleishman
Messages: 3728
Registered: October 2005
Location: Melbourne, Australia

Senior Member

NOT IN can be resolved with an ANTI-JOIN, but not if either side of the join can be NULL. If either side is nullable, then Oracle MUST use a nested filter to process the subquery.

The NVL() enables an anti-join, which can then be un-nested.

Ross Leishman

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