Question

I have created the following function in my database

Create FUNCTION [dbo].[CountUseer](@couponid INT)
RETURNS INT
AS
 BEGIN
 RETURN
(
  SELECT Count(cu.id) NofUses  --- this can only return one column
  FROM   Coupon as  c
  JOIN CouponUse as cu
    ON c.id = cu.couponid
  WHERE cu.couponid = @couponid
 )
 END 

Then I run this query

   ALTER TABLE dbo.Coupon
   ADD NofUses AS dbo.CountUseer(Id)

Now when I try to create an index on the column NofUses :

CREATE INDEX Noofusesindex ON Coupon (NofUses)  

I get this error:

Column ‘NofUses’ in table ‘Coupon’ cannot be used in an index or
statistics or as a partition key because it is non-deterministic.

Answer 1

Well, the error message really says it all: since this function is non-deterministic (e.g. when you call it with the same input parameters, there’s no guarantee you’ll get the same results every time), it cannot be indexed.

The only way around this would be to make it a regular column (not based on a function), and just update the value stored in that column on a regular basis (by a SQL Agent job, for instance, or by a trigger on the CouponUse table, or some other mechanism)