CROSS APPLY on Scalar function
-
19-12-2020 - |
Domanda
I have this:
SELECT
A
,B
,dbo.Func(C)
,dbo.Func(D)
,dbo.Func(E)
,F
FROM abcdef
WHERE
0 = dbo.Func(C) + dbo.Func(D)
I have read that this is not good practice because the function is called million of times and it has a bad impact on performance.
I tried to rewrite it with CROSS APPLY:
SELECT *
FROM abcdef
CROSS APPLY dbo.Func(D) as FD
but it returns this error:
Invalid object name 'dbo.Func'
Can I use CROSS APPLY only on TABLE VALUED functions ?
And is CROSS APPLY suitable for scalar functions (converted to table valued function) anyway ? Because on my small performance test CROSS APPLY is a bit slower.
Soluzione
I read that this is not good practice because function is called 'zilion' times and it have bad impact on performance.
While CROSS APPLY
can be useful in some cases, I don't expect any difference in performance between calling the function in WHERE
or CROSS APPLY
in the specific case. If the table has a million rows (and columns C
and D
possibly a million different values), a million times the function will be called. How can it be otherwise?
I tried to rewrite it with
CROSS APPLY
.
Here's how:
SELECT
t.A,
t.B,
ca.Fc,
ca.Fd,
dbo.Func(t.E) AS Fe
t.F,
FROM abcdef AS t
CROSS APPLY
( SELECT
dbo.Func(t.C) AS Fc,
dbo.Func(t.D) AS Fd
) AS ca
WHERE 0 = ca.Fc + ca.Fd ;
or:
SELECT
...
FROM abcdef AS t
CROSS APPLY
( SELECT
dbo.Func(t.C) AS Fc,
dbo.Func(t.D) AS Fd
FROM (SELECT NULL) AS dummy
WHERE 0 = dbo.Func(t.C) + dbo.Func(t.d)
) AS ca ;
Again, I don't think this will have any effects on efficiency.