billy21
asked on
Clustered Index Scan Vs Clustered Index Seek
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.
https://www.experts-exchange.com/questions/20960243/Converting-Vertical-Data-to-Horizontal-Data-through-Aggregation.html
Thanks,
Billy.
I'd like to know how I can design my indices so that a clustered index scan can be avoided.
https://www.experts-exchange.com/questions/20960243/Converting-Vertical-Data-to-Horizontal-Data-through-Aggregation.html
Thanks,
Billy.
Create Function dbo.SOSCurrentVary(@StartD ate 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
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
ASKER
It still uses a clustered index scan but based on what you said, it sounds like it's necessary to use the scan.
ASKER CERTIFIED SOLUTION
membership
This solution is only available to members.
To access this solution, you must be a member of Experts Exchange.
ASKER