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NULL toggles off an index

  • July 28, 2009 at 5:36 am

    #95573

    Hi folks!

    I'm experiencing a trouble with nulls. I have a table which has index over a column which could contain nulls.

    When I use a statement like this one:

    select * from table where TheColumn =

    index is used. But when I use another kind of statement:

    select * from table where TheColumn is not null

    the index is NOT used.

    It might be a workaround for the problem is to use some constant value to determine nulls, but is not my case 'cause I can't do this for now. But I'd like index to be used in the latest case.

    Is there a way to get it worked?

  • July 28, 2009 at 6:33 am

    #1030980

    Hi,

    Try any one of this. First one should work.

    1. select * from table with (index(myidxname)) where thecolumn is not null

    or

    2.

    declare @thecolvalue integer

    set @thecolvalue=null

    select * from table where thecolumn=@thecolvalue

    [font="Verdana"]Thanks
    Chandra Mohan[/font]

  • July 28, 2009 at 8:21 am

    #1031042

    isn't the table scan and not using the index due to the SARGable condition?(Search ARGument)

    column = @value can be used to search as boolean match to the index

    column is not null does not resolve as able to boolean match an index, so a scan must be used.

    AFAIK, any time you have a NOT in the where statemnt, a scan will be used to check all values, right?

    Lowell

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  • July 28, 2009 at 10:21 am

    #1031155

    NULL values are not your friend when it comes to indexes. You will get a scan. Even forcing the use of the index through the hint will result in a scan, and it will probably include a key lookup with the scan.

    "The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood"
    - Theodore Roosevelt

    Author of:
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  • July 29, 2009 at 5:21 am

    #1031543

    hi Grant

    please see the below loink's problem

    http://www.sqlservercentral.com/Forums/FindPost761384.aspx

    thanks

    -------Bhuvnesh----------
    I work only to learn Sql Server...though my company pays me for getting their stuff done;-)

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