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Clustered Index Scan Vs Clustered Index Seek

Posted on 2004-04-20
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Last Modified: 2007-12-19
Please view the query/UDF posted in the following related question.

I'd like to know how I can design my indices so that a clustered index scan can be avoided.

http://www.experts-exchange.com/Databases/Microsoft_SQL_Server/Q_20960243.html

Thanks,

Billy.
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Question by:billy21
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LVL 6

Author Comment

by:billy21
ID: 10867640
I've also posted the related table structure.
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LVL 26

Expert Comment

by:Hilaire
ID: 10867992
Create Function dbo.SOSCurrentVary(@StartDate DateTime)
Returns Table
AS
Return
     Select Va.SOSID,
     Max(Case When Va.VDate = @StartDate Then Units End) CurUnits1,
     Max(Case When Va.VDate = @StartDate+1 Then Units End) CurUnits2,
     Max(Case When Va.VDate = @StartDate+2 Then Units End)  CurUnits3,
     Max(Case When Va.VDate = @StartDate+3 Then Units End)  CurUnits4,
     Max(Case When Va.VDate = @StartDate+4 Then Units End)  CurUnits5,
     Max(Case When Va.VDate = @StartDate+5 Then Units End)  CurUnits6,
     Max(Case When Va.VDate = @StartDate+6 Then Units End)  CurUnits7
     From SOSVary Va
     Group By Va.SOSID
)


In the query above, there's no restriction on fields that could make the query more selective and force index seek rather than index scan.
The optimiser will rely on statisitcs to decide what's better.

An index seek will be performed if the query is selective.
The query above obviously needs to make a scan because there's no condition on the SOSID

Index seek will result in a performance boost if, say, 95% of the rows can be excluded by your where clause

You could maybe make it more selective using an additional where clause

Check execution plan for
    Select Va.SOSID,
     Max(Case When Va.VDate = @StartDate Then Units End) CurUnits1,
     Max(Case When Va.VDate = @StartDate+1 Then Units End) CurUnits2,
     Max(Case When Va.VDate = @StartDate+2 Then Units End)  CurUnits3,
     Max(Case When Va.VDate = @StartDate+3 Then Units End)  CurUnits4,
     Max(Case When Va.VDate = @StartDate+4 Then Units End)  CurUnits5,
     Max(Case When Va.VDate = @StartDate+5 Then Units End)  CurUnits6,
     Max(Case When Va.VDate = @StartDate+6 Then Units End)  CurUnits7
     From SOSVary Va
     Where Va.VDate between  @StartDate and @StartDate+6
     Group By Va.SOSID

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Author Comment

by:billy21
ID: 10868191
It still uses a clustered index scan but based on what you said, it sounds like it's necessary to use the scan.
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Accepted Solution

by:
Hilaire earned 1400 total points
ID: 10868253
I think so, since there's a "group by" and no filter.
I guess the same query with a filter on a particular SOSID should use an index seek.
(again, if and only if, based on the statistics, the optimizer decides that the query is selective enough to justify seek vs scan)
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