How can I convert split function to inline table valued udf in SQL server?

Question

Assuming I have this query ( pseudo) :

Select T.a,
       T.b, 
       (select top 1 element from fn_split(c,',') where element=T.element)
From largeTable T

Where fn_split runs for each row , I would like to use inline table valued udf so , that performance will be better.

NB : fn_split just create a table via splitting via , :

But looking at inline table valued udf structure :

create FUNCTION [dbo].[fn_...]
(
    ...
)
RETURNS table
AS
RETURN  SELECT ...(!!!)

It should return the select right away as the first statement !

But what if my UDF looks like :

CREATE FUNCTION [dbo].[FN_Split] 
(
    @InDelimitedString varchar(max),
    @InDelimiter       varchar(10) 
)
RETURNS 
    @tblArray TABLE 
    (
        ElementID  smallint IDENTITY(1,1), 
        Element    varchar(1000)
    )

AS
BEGIN

    DECLARE @StrPos smallint,
    @StrStart smallint,
    @DelimiterLength smallint

    SET @DelimiterLength = LEN(@InDelimiter)    

    WHILE LEN(@InDelimitedString) > 0
        BEGIN
         --Removed for clarity . do some CHARINDEX  manipulation ETc.
        END

    RETURN 
END

Question :

I can't return select right away , but still , I want to change the fn_split to inline table valued udf.

How can I do it ?

Answer 1

The problem is with your split function. It is doing the split in an RBAR fashion. You should use a set-based splitter. Here is the DelimitedSplit8k by Jeff Moden, which is one of the fastest splitter there is:

CREATE FUNCTION [dbo].[DelimitedSplit8K](
    @pString VARCHAR(8000), @pDelimiter CHAR(1)
)
RETURNS TABLE WITH SCHEMABINDING AS
RETURN
WITH E1(N) AS (
    SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL 
    SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1
)
,E2(N) AS (SELECT 1 FROM E1 a, E1 b)
,E4(N) AS (SELECT 1 FROM E2 a, E2 b)
,cteTally(N) AS(
    SELECT TOP (ISNULL(DATALENGTH(@pString), 0)) ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) FROM E4
)
,cteStart(N1) AS(
    SELECT 1 UNION ALL 
    SELECT t.N+1 FROM cteTally t WHERE SUBSTRING(@pString, t.N, 1) = @pDelimiter
),
cteLen(N1, L1) AS(
SELECT 
    s.N1,
    ISNULL(NULLIF(CHARINDEX(@pDelimiter, @pString, s.N1),0) - s.N1, 8000)
FROM cteStart s
)
SELECT 
    ItemNumber = ROW_NUMBER() OVER(ORDER BY l.N1),
    Item       = SUBSTRING(@pString, l.N1, l.L1)
FROM cteLen l

Note: Be sure to look into the article for the updated function

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For more split functions, read these articles by Sir Aaron Bertrand:

• Split strings the right way – or the next best way
• Splitting Strings : A Follow-Up
• Splitting Strings : Now with less T-SQL
• Comparing string splitting / concatenation methods
• Processing a list of integers : my approach
• Splitting a list of integers : another roundup
• More on splitting lists : custom delimiters, preventing duplicates, and maintaining order
• Removing Duplicates from Strings in SQL Server