Performance problem caused by joining with the result of a scalar function?

Question

In the following example the performance of the first version using only one SQL query is very bad. The problem is the last join.

When I store the result of the left side into a temporary table and join this, it runs about 50 times quicker.

It is intended to convert this to Oracle, therefore I try to avoid the use of temporary tables.

Any explanations, why the first query behaves so bad or hint to improve it?

Declare @HO int = 2866;
Declare @Dtz int = 35;

---- version 1 takes about 60 seconds
With ratings as
(
    select
        ROW_NUMBER() Over ( ORDER BY SEDate) lfd, 
        SEDate, 
        BBCode 
    from  PPV
        join BB on BBRefnr = SEBBRefnr
    where SEHORefnr = @HORefnr 
    and SEBBRefnr > 0
)
Select
    SEDATE,
    BBCODE,
    Code,
    DTRefnr
FROM (  
Select 
    l.SEDate,
    l.BBCode,
    dbo.GetCode(@Dtz, l.BBCode) Code
from ratings l left join ratings r on l.lfd = r.lfd + 1
where l.BBCode <> r.BBCode
or r.BBCode is null
) as t
    join DT d on code = d.DTCode and d.DTDTKRefnr = @Dtz 
order by 1;

------- version 2 using a temp table takes less than 1 second

With ratings as
(
    select
        ROW_NUMBER() Over ( ORDER BY SEDate) lfd, 
        SEDate, 
        BBCode 
    from  PPV
        join BB on BBRefnr = SEBBRefnr
    where SEHORefnr = @HORefnr 
    and SEBBRefnr > 0
)
Select 
    l.SEDate,
    l.BBCode,
    dbo.GetCode(@Dtz, l.BBCode) Code
into #tmp
from ratings l left join ratings r on l.lfd = r.lfd + 1
where l.BBCode <> r.BBCode
or r.BBCode is null


Select
    SEDATE,
    BBCODE,
    Code,
    DTRefnr
FROM #tmp
    join DT d on code = d.DTCode and d.DTDTKRefnr = @Dtz 
order by 1;

EDIT:

Here the function:

Create function dbo.GetCode (
    @fDtz int, 
    @Value varchar(10)
) returns varchar(10)
as
begin
    declare @CODE varchar(10)
    SET @Code =
        (SELECT Code FROM Ableitungen 
        WHERE DTZ = @fDtz
        AND Value = @Value )

    return @CODE
end

Answer 1

Exactly as expected.

A scalar UDF is a black box to the optimiser: no indexes can be used, the cost can't be worked out correctly.

And if the scalar UDF has table access, then you have running a CURSOR (bad in SQL Server) to do a lookup per row: it isn't a set based operation

To fix it, don't use a UDF this way. It can be written as a JOIN. There is no hint or magic

Edit: remove the UDF. For each row in ratings your query Ableitungen. It's exponential

And I've move the filters into the JOINs

With ratings as
(
    select
        ROW_NUMBER() Over (ORDER BY SEDate) lfd, 
        SEDate, 
        BBCode 
    from  PPV
        join BB on BBRefnr = SEBBRefnr
    where SEHORefnr = @HORefnr 
    and SEBBRefnr > 0
)
Select 
    l.SEDate,
    l.BBCode,
    A.Code,
    D.DTRefnr
from 
    ratings l
    left join 
    ratings r on l.lfd = r.lfd + 1
    JOIN 
    Ableitungen A ON A.Value = l.BBCode
    join
    DT d on A.code = d.DTCode and d.DTDTKRefnr = A.DTZ
where
    A.DTZ = @fDtz
    AND 
   (l.BBCode <> r.BBCode or r.BBCode is null)
order by
    SEDATE;