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reduce the elapsed time [message #461882]

Tue, 22 June 2010 01:29

akilabaskaran
Messages: 29
Registered: May 2007
Location: chennai

Junior Member

Hi,

i have a sample code as below.
Kindly let me know what can be done to reduce the elapsed time of the below query

 select col1 from a where a.end_date is null and a.start_date <(sysdate-(xhours/24))
       order by  a.start_date

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Re: reduce the elapsed time [message #461893 is a reply to message #461882]

Tue, 22 June 2010 01:43

rahulvb
Messages: 924
Registered: October 2009
Location: Somewhere Near Equator.

Senior Member

1/ Post the Execution plan of above Query.
2/ Might want to "(sysdate-(xhours/24))" calculate this value in some variable so for each line this will not be calcualted again.( will make difference for large table)

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Re: reduce the elapsed time [message #461894 is a reply to message #461893]

Tue, 22 June 2010 01:44

rahulvb
Messages: 924
Registered: October 2009
Location: Somewhere Near Equator.

Senior Member

if the result set is very large and if you dont care of Sorting Remove it.

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Re: reduce the elapsed time [message #461901 is a reply to message #461894]

Tue, 22 June 2010 01:51

akilabaskaran
Messages: 29
Registered: May 2007
Location: chennai

Junior Member

The explain plan is as below

SELECT STATEMENT  ALL_ROWSCost: 7 K  Bytes: 3 K  Cardinality: 132          
    2 SORT ORDER BY  Cost: 7 K  Bytes: 3 K  Cardinality: 132      
        1 TABLE ACCESS FULL TABLE a Cost: 7 K  Bytes: 3 K  Cardinality: 132

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Re: reduce the elapsed time [message #461953 is a reply to message #461901]

Tue, 22 June 2010 04:08

cookiemonster
Messages: 13958
Registered: September 2008
Location: Rainy Manchester

Senior Member

How many rows in the table?
Do you have an index on start_date?

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Re: reduce the elapsed time [message #461955 is a reply to message #461953]

Tue, 22 June 2010 04:19

akilabaskaran
Messages: 29
Registered: May 2007
Location: chennai

Junior Member

There are 11459274 rows in it as of now but would potentially increase in future.
No we do not have any index on the dates.

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Re: reduce the elapsed time [message #461961 is a reply to message #461955]

Tue, 22 June 2010 04:56

cookiemonster
Messages: 13958
Registered: September 2008
Location: Rainy Manchester

Senior Member

Did you run the explain plan against a dev/test db that has a lot less rows in it?
The explain plan thinks that table is practically empty.

Regardless, if you don't have any indexes a full table scan is the only option, try adding an index on start_date.

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Re: reduce the elapsed time [message #461971 is a reply to message #461961]

Tue, 22 June 2010 05:12

akilabaskaran
Messages: 29
Registered: May 2007
Location: chennai

Junior Member

The explain plan in the live environment is also same.It also uses the Full Table Scan.
So i was also thinking of creating a function based index on the
end date. But since we sort and use start date it would be a better option to create a index on start date.

cookiemonster : Thanks For your suggestions and immediate help.

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Re: reduce the elapsed time [message #462034 is a reply to message #461882]

Tue, 22 June 2010 08:12

michael_bialik
Messages: 621
Registered: July 2006

Senior Member

IMHO

1. If xhours is a table column then the expression
a.start_date < (sysdate-(xhours/24)) is not indexable and Oracle must use full table scan.

2. Let's assume that xhours is parameter. In that case you are selecting all rows older then (sysdate-(xhours/24)).
So (unless your table holds future dates) you are selecting almost all data - full table scan is used to get a better performance.

3. How many rows your query is supposed to select?

HTH.

[Updated on: Tue, 22 June 2010 08:14]

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Re: reduce the elapsed time [message #467179 is a reply to message #461901]

Thu, 22 July 2010 08:25

prashant_ora
Messages: 196
Registered: July 2010

Senior Member

have u checked the v$sql_plan for explain plan , becuase some time it differs from the explain plan which we took from any tool like pl-sql developper.

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