SQL Performance issue when using CASE

Question

I have a SP that returns quite a lot of data every second and is shown in a grid. I'm now trying to reduce bandwidth and thinking of returning only columns currently shown in my grid.

This is of course simplified and minimized but basically what I had was the following SP:

SELECT
[Animals].[AnimalID] AS [AnimalID],
[Animals].[name] AS [AnimalName],
[Foods].[DisplayName] AS [Food],
[Animals].[Age] AS [AnimalAge],
[Animals].[AmountOfFood] AS [AmountOfFood]

What I’m currently trying is to pass a TVP of the names of the fields (@fields) currently shown on the grid and returning only required fields as such:

SELECT
[Animals].[AnimalID] AS [AnimalID],
[Animals].[name] AS [AnimalName],
CASE 
    WHEN ('Food' in (select * from @fields)) 
    THEN [Foods].[DisplayName] 
END AS [Food],
CASE 
    WHEN ('AnimalAge' in (select * from @fields)) 
    THEN [Animals].[Age] 
END AS [AnimalAge],
CASE 
    WHEN ('AmountOfFood' in (select * from @fields)) 
    THEN [Animals].[AmountOfFood] 
END AS [AmountOfFood]

The problem I'm facing is that (as could be expected) my SP went from taking ~200 ms to taking ~1 sec

Is there any way to maybe rewrite this so that it doesn’t kill us?

My kingdom for a foreach!!!

Answer 1

In SQL Server, you can also do this with dynamic SQL. Something like:

declare @sql nvarchar(max);

select @sql = (select ', '+
                      (case when FieldName = 'Food' then 'Foods.DisplayName'
                            when FieldName = 'AnimalAge' then 'Animals.Age'
                         . . .
                       end)
               from @fields
               for xml path ('')
              );

select @sql = 'select [Animals].[AnimalID] AS [AnimalID], [Animals].[name] AS [AnimalName]'+@sql+RESTOFQUERY;

exec(@sql);

Answer 2

I'd try to convert the stored procedure into Table-Valued function, and make your grid select only required columns from it.

So your function would still select

SELECT
[Animals].[AnimalID] AS [AnimalID],
[Animals].[name] AS [AnimalName],
[Foods].[DisplayName] AS [Food],
[Animals].[Age] AS [AnimalAge],
[Animals].[AmountOfFood] AS [AmountOfFood]

If the client only selected for example select * AnimalID, Age from myfunction(..), only these columns would be transferred to the client.