Why is there a scan on my clustered index?

Question

SQL 2000
The NED table has a foreign key to the SIGN table NED.RowID to SIGN.RowID
The SIGN table has a foreign key to the NED table SIGN.SignID to NED.SignID
Th

Answer 1

You have several restrictions on the SIGN A table, if I read this correctly:

WHERE  
        (A.DeptID = @DeptID OR   
        S.DeptID = @DeptID  
        AND   
        A.[EndTime] > @StartDateTime AND A.[StartTime] < @EndDateTime  
        AND   
        A.NEDStatusID = 2

Are any of those restrictions (like DeptID, StartTime, EndTime, NEDStatusID) indexed? How well are those field selecting from your set of data?

If you have 10 mio. rows and NEDStatusID has only 10 possible values, then any restriction on that field would always yield approx. 1 mio. rows - in that case, it might just be easier (and less costly) for SQL Server to do a full table scan (clustered index scan), especially if it also needs to check additional WHERE clauses on the same table that aren't indexed, either (StartTime, EndTIme etc.).

Marc