Will the query inside the ORDER BY be repeatedly evaluated? - sql

Question

I have several database objects which need to extract a single record (like TOP 1) from a table, but the priority for which one is chosen depends on a BIT value in a settings table, and that settings table will contain only one row.
I have written a view which will perform the required functionality:
CREATE VIEW TopOrganisationAddresses AS
WITH cte AS
(
SELECT OrganisationID,
AddressID,
CASE WHEN EXISTS (SELECT * FROM GlobalSettings WHERE DeliveryAddressInReports=1) THEN IsDeliveryAddress ELSE IsInvoiceAddress END AS OrderFirst,
CASE WHEN EXISTS (SELECT * FROM GlobalSettings WHERE DeliveryAddressInReports=1) THEN IsInvoiceAddress ELSE IsDeliveryAddress END AS OrderSecond
FROM OrganisationAddresses
)
SELECT OrganisationID, AddressID,
ROW_NUMBER() OVER(PARTITION BY OrganisationID
ORDER BY OrderFirst DESC, OrderSecond DESC) AS [Row]
FROM cte
Will the SELECT * FROM GlobalSettings queries be evaluated for every fow in the OrganisationAddresses table? If so this would be incredibly wasteful as it is only a static value that isn't going to change.

Answer 1

In theory it could be optimised away but you can perhaps just use another cte which does that SELECT to guarantee it e.g.
CREATE VIEW TopOrganisationAddresses AS
WITH cteg AS
(
SELECT CASE WHEN EXISTS (SELECT * FROM GlobalSettings WHERE DeliveryAddressInReports=1) THEN 1 ELSE 0 END AS dair
),
cte AS
(
SELECT OrganisationID,
AddressID,
CASE WHEN cteg.dair = 1 THEN IsDeliveryAddress ELSE IsInvoiceAddress END AS OrderFirst,
CASE WHEN cteg.dair = 1 THEN IsInvoiceAddress ELSE IsDeliveryAddress END AS OrderSecond
FROM OrganisationAddresses
CROSS JOIN cteg
)
SELECT OrganisationID, AddressID,
ROW_NUMBER() OVER(PARTITION BY OrganisationID
ORDER BY OrderFirst DESC, OrderSecond DESC) AS [Row]
FROM cte

Answer 2

SQL Server should be smart enough to run the queries only once. The optimizer should recognize exactly what you do -- that these are constant values. For instance, the following query returns the same value for the id:
with cte as (
select v.*,
(select top 1 newid() from (values (1), (2)) v2(n) order by newid()) as val
from (values (1), (2)) v(n)
)
select *
from cte;
(Here is a db<>fiddle.)
If you wanted to be sure that these are evaluated only once, you can move the logic to the FROM clause:
CREATE VIEW TopOrganisationAddresses AS
SELECT oa.OrganisationID, oa.AddressID,
ROW_NUMBER() OVER (PARTITION BY oa.OrganisationID
ORDER BY v.OrderFirst DESC, v.OrderSecond DESC
) AS [Row]
FROM OrganisationAddresses oa CROSS JOIN
(VALUES (CASE WHEN EXISTS (SELECT * FROM GlobalSettings WHERE DeliveryAddressInReports = 1) THEN IsDeliveryAddress ELSE IsInvoiceAddress END,,
CASE WHEN EXISTS (SELECT * FROM GlobalSettings WHERE DeliveryAddressInReports = 1) THEN IsInvoiceAddress ELSE IsDeliveryAddress END)
) v(OrderFirst, OrderSecond);