What happens to unused common table expressions ,Does this affect in performance or ?

Question

If I write a query with one or more common table expressions to which I
don't actually refer in the query, do they just get pruned off or do
they get executed regardless? how does it affect in performance
Prem Shah

Answer 1

Try below
seems when the CTE is not refer in the query then statement inside CTE is not executing at all even if it nested CTE, see for your self
Create table UserInfo
UserId int primary key,
UserName varchar(30)
GO
Create table UserInfo1
UserId int primary key,
UserName varchar(30)
GO
insert into UserInfo
select 1001,'X1' union all
select 1002,'X2' union all
select 1009 ,'X9'
GO
insert into UserInfo1
select 1001,'X1' union all
select 1002,'X2' union all
select 1009 ,'X9'
GO
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
GO
Begin tran
select * from UserInfo1 where UserId between 1001 and 1009
and UserName = 'XXXX'
--Commit
PRINT 'WITH out CTE access in select'
SET STATISTICS IO ON
;WITH CTE1 AS
(Select * From UserInfo1)
select * From UserInfo
PRINT 'WITH CTE access in select'
;WITH CTE1 AS
(Select * From UserInfo1)
select * From UserInfo a inner join CTE1 b on a.UserId=b.UserId
Stats IO
    WITH out CTE access in select
    (3 row(s) affected)
    Table 'UserInfo'. Scan count 1, logical reads 2, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
    (1 row(s) affected)
    WITH CTE access in select
    (3 row(s) affected)
    Table 'UserInfo1'. Scan count 0, logical reads 6, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
    Table 'UserInfo'. Scan count 1, logical reads 2, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
    (1 row(s) affected)
Thanks
Saravana Kumar C