SQL Server (T-SQL): Avoid call scalar function multiple times - sql-server-2008

Question

I have a stored procedure which does a SELECT to return some rows. Within the SELECT, I need to check a condition in order to return the correct value for some columns. This condition consists on a scalar function. All times scalar function is called with the same parameter for the row being processed, see below:
SELECT
Id,
Name,
Surname,
CASE WHEN (dbo.GetNumTravels(Id) >= 50)
THEN 10
ELSE 99
END as Factor1,
CASE WHEN (dbo.GetNumTravels(Id) >= 50)
THEN 15
ELSE -1
END as Factor2,
CASE WHEN (dbo.GetNumTravels(Id) >= 50)
THEN 30
ELSE 70
END as Factor3
FROM
Employees
WHERE
DepartmentId = 100
I am worried about performance, I mean, I do not like to call scalar function dbo.GetNumTravels multiples times, so how to avoid this and only call it once and then used it all the times I need it?

Answer 1

Scalar user defined functions are infamous for poor performance. If you can convert it to an inline table-valued function you can expect to see performance gains.
If you convert your scalar function to an inline table-valued function you can call it once for each row using cross apply() like so:
select
Id,
Name,
Surname,
case when x.NumTravels >= 50
then 10
else 99
end as Factor1,
case when x.NumTravels >= 50)
then 15
else -1
end as Factor2,
case when x.NumTravels >= 50
then 30
else 70
end as Factor3
from Employees
cross apply dbo.GetNumTravels_itvf(e.Id) x
where DepartmentId = 100
Reference:
When is a sql function not a function? "If it’s not inline, it’s rubbish." - Rob Farley
Inline Scalar Functions - Itzik Ben-Gan
Scalar functions, inlining, and performance: An entertaining title for a boring post - Adam Machanic
tsql User-Defined Functions: Ten Questions You Were Too Shy To Ask - Robert Sheldon

Answer 2

You can achieve this by using derived table concept, In derived table we once called to function dbo.GetNumTravels(Id) only once and used its output in outer query, this may help to gaining performance at some level by avoiding multiple calls to same function.
SELECT
Id,
Name,
Surname,
CASE WHEN (NumTravelsID >= 50) THEN 10 ELSE 99 END as Factor1,
CASE WHEN (NumTravelsID >= 50) THEN 15 ELSE -1 END as Factor2,
CASE WHEN (NumTravelsID >= 50) THEN 30 ELSE 70 END as Factor3
FROM (
SELECT
Id,
Name,
Surname,
dbo.GetNumTravels(Id) as NumTravelsID
FROM Employees
WHERE DepartmentId = 100
)M

Answer 3

I am not sure about performances (anyway, do your tests considering other answers too), but I would like to test this. I tried to reduce use of function and use of CASE too. Pls let me know
SELECT A.*
, 10*F0+99*~F0 AS FACTOR1
, 15*F0-1*~F0 AS FACTOR2
, 30*F0+70*~F0 AS FACTOR3
FROM (
SELECT
Id,
Name,
Surname,
CAST(CASE WHEN (dbo.GetNumTravels(Id) >= 50) THEN 1 ELSE 0 END AS BIT) AS F0
FROM Employees
WHERE DepartmentId = 100
) A