SQL Server 2008 - For XML Path is very slow - sql

Question

I need help to increase the performance of my query, the example is below. I have a SELECT query list multiple fields of CUSTOMER table, this is really fast, about 15 ms. However, when I include the statement below using FOR XML PATH to grab the Customer POs (multiple) and combine them into one column, it is very slow, but it works.
Any suggestion how to increase the performance, but still getting the same results (combine the Customer POs into one column)? A sample code would be appreciated.
Select
Col1, Col2,
(SELECT
STUFF((SELECT ', ' + CustomerPO
FROM dbo.Tbl_CustomerPO
WHERE CustomerID = cus.CustomerID
FOR XML PATH('')), 1, 1, '')
) AS CustomerPOs
FROM Tbl_Customer cus
Thank you,

Answer 1

Try something like this ....
With a TINY recordset the "string concatenating" function method appears better from an execution plan cost, IO, and Time bases.
IF OBJECT_ID(N'fnConcatenateCustPOs', N'fN') IS NOT NULL
BEGIN
DROP FUNCTION dbo.fnConcatenateCustPOs
END
GO
CREATE FUNCTION dbo.fnConcatenateCustPOs
(
#CustomerID INT
)
RETURNS nvarchar(max)
--WITH ENCRYPTION
AS
BEGIN
DECLARE #StrFP nvarchar(3750)
--DECLARE #Custpo TABLE(CustomerPOId INT, CustomerID INT)
SET #StrFP = ''
SET #StrFP = ''
SELECT #StrFP = + #StrFP + ',' + CAST(CustomerPOId AS nvarchar(50))
FROM Custpo co
WHERE co.CustomerID = #CustomerID
RETURN SUBSTRING(#StrFP, 2, LEN(#StrFP))
END
GO
IF OBJECT_ID(N'Cust', N'U') IS NOT NULL
BEGIN
DROP TABLE Cust
END
IF OBJECT_ID(N'Custpo', N'U') IS NOT NULL
BEGIN
DROP TABLE CustPO
END
CREATE TABLE Cust (CustomerID INT)
CREATE TABLE CustPO (CustomerPOId INT, CustomerId INT)
INSERT Cust
SELECT 1
UNION
SELECT 2
INSERT CustPO
SELECT 10, 1
UNION
SELECT 20, 1
UNION
SELECT 30, 2
UNION
SELECT 31, 2
SET STATISTICS IO ON
SET STATISTICS TIME ON
SELECT CustomerId, dbo.fnConcatenateCustPOs(CustomerID)
FROM Cust cus
Select
CustomerID,
(SELECT
STUFF((SELECT ', ' + CAST(CustomerPOId AS nvarchar(50))
FROM dbo.CustPO
WHERE CustomerID = cus.CustomerID
FOR XML PATH('')), 1, 1, '')
) AS CustomerPOs
FROM Cust cus
SET STATISTICS IO OFF
SET STATISTICS TIME OFFTry:
Select
Col1, Col2,
(
STUFF((', ' + CustomerPO
FOR XML PATH('')), 1, 1, '')
) AS CustomerPOs
FROM Tbl_Customer cus
INNER JOIN dbo.Tbl_CustomerPO cpo ON cus.CustomerID = cpo.CustomerID

Answer 2

You're introducing a JOIN to your query, which will inherently affect performance.
If you index the joining field CustomerID you can speed up this query. Not much else to do here.
Note: Since you're prefixing your CustomerPo list with a comma and a space, you should use:
FOR XML PATH('')), 1, 2, '')
If you don't want your resulting strings to all start with a space.