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constant scan in execution plan

Posted on 2009-04-13
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Last Modified: 2012-05-06
I have found a constant scan in my execution plan. Can someone tell me under what condition a constant scan would happen.
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Question by:david_32
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3 Comments
 
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by:udaya kumar laligondla
ID: 24135890
it looks like you are using GETDATE() function in a insert statement or do you have a partitioned table.
read more at
http://scarydba.wordpress.com/2008/07/01/constant-scan-in-execution-plans/
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Raja Jegan R earned 2000 total points
ID: 24136248
Its not that you will obtain a Constant Scan when you use a partitioned table.

From MSDN,

Constant Scan

The Constant Scan logical and physical operator introduces a constant row into a query. It will return either zero or one row, which usually contains no columns. A Compute Scalar operator is often used to add columns to the row produced by a Constant Scan.

When you add a new column to a row / Result set using a Scalar function, then you will have a Constant Scan.

Say you have a function named dbo.format and if you use it in your query like

SELECT col1, col2, dbo.format(col3)
FROM urtable

It will have a constant scan.

Whenever you have a scalar function in your query to return some value, For eg, GETDATE() as mentioned earlier, SUBSTRING, LEN, CONVERT, CAST
You will end up in a constant scan.
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Author Closing Comment

by:david_32
ID: 31569796
thanks
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