Question

How would I rewrite this query to be performant by executing the SQL function only once?

SELECT Top 1 Id, Name
  FROM Users U 
       INNER JOIN UserDetail D on U.Id = D.Id
       LEFT OUTER JOIN CreditCards C ON C.Id = U.Id AND UserHasCC(U.Id) = 1
       LEFT OUTER JOIN CreditCardDetails CD on C.CCID = C.CCID AND UserHasCC(U.Id) = 1
WHERE
       ((CD.active = 1 and UserHasCC(U.Id) = 1) OR UserHasCC(U.Id) = 0) and
       U.active = 1 and
       ((C.IsInternational = 1 and UserHasCC(U.Id) = 1) OR UserHasCC(U.id = 0)

Basically, the query gets all users that don’t have credit cards and those that have active international credit cards.

Answer 1

This looks like it accomplishes the same logic.

 SELECT Top 1 Id, Name
  FROM Users U 
       INNER JOIN UserDetail D on U.Id = D.Id
       LEFT OUTER JOIN CreditCards C ON C.Id = U.Id 
       LEFT OUTER JOIN CreditCardDetails CD on C.CCID = C.CCID 
   WHERE
         U.active = 1
         AND
         (
             UserHasCC(U.Id) = 0
             OR
             (
                UserHasCC(U.Id) = 1
                and
                (
                  C.IsInternational = 1
                  OR
                  CD.active = 1
                )

         )