Performance problem: Converting rows to a comma separated column using STUFF() and self join

Question

Hi,
This might be a really dumb one to ask but I am currently working on a table that has sequential data for steps that an invoice goes through in a particular system. Here is how it looks:
ID InvoiceID
InvoiceSteps
Timestamp
283403 0000210121_0002_2013
Post FI Invoice
2013-07-01 19:07:00.0000000
389871 0000210121_0002_2013
Clear Invoice
2013-08-25 14:02:00.0000000
Here is my extremely slow query that converts multiple rows of an invoice into a single one with 'InvoiceSteps' listed according to their timestamps in a sequential manner separated by commas.
SELECT [InvoiceID],
[InvoiceSteps] = STUFF((
SELECT ',' + ma.InvoiceSteps
FROM invoices ma
WHERE m.InvoiceID = ma.InvoiceID
ORDER BY [Timestamp]
FOR XML PATH(''), TYPE).value('.', 'NVARCHAR(MAX)'), 1, 1, '')
FROM invoices m
GROUP BY InvoiceID
ORDER BY InvoiceID;
Here is the end result:
InvoiceID InvoiceSteps
0000210121_0002_2013
Post FI Invoice,Clear Invoice
My question: How can I improve the query so that it can process thousands of records as fast as possible (>600K in this case).
Thanks you!

Answer 1

There are many methods to concatenate the rows to columns. Assuming you have necessary indexes to support your query as Rishabh suggested, if you still find issues with performance, then you need to look at various other approaches as well. I have seen at
certain places(huge data), CLR outperformed . Having said, we need to assess each and come to a conclusion for your scenario.
Refer the below link for various approach, (please also look at the comment session as well):
https://www.simple-talk.com/sql/t-sql-programming/concatenating-row-values-in-transact-sql/