Rowset string concatenation: Which method is best?

Adam Machanic — Thu, 13 Jul 2006 01:15:00 GMT

Yeah, yeah, yeah, let's get this out of the way right from the start: Don't concatenate rows into delimited strings in SQL Server. Do it client side.

Except if you really have to create delimited strings in SQL Server. In which case you should read on.

There was a little discussion on SQLTeam about the best way to concatenate. I recommended a scalar UDF solution, whereas Rob Volk recommended a solution involving a temp table.

I mentioned my dislike for the temp table solution for a couple of reasons. First of all, it relies on a clustered index for ordering. That will probably work in this example, but is not guaranteed to always work and relying on indexes rather than ORDER BY for ordering is definitely not a habit I want anyone to get into. The clustered index as it was described in Rob's example also has another problem that I didn't even notice until I was writing this entry. But I'll get to that in a moment. The second reason I dislike the temp table is that I felt it would be less efficient than the scalar UDF.

Rob didn't agree about the efficiency. And so I set out to prove him wrong...

We'll use the Authors table in Pubs. I want a comma-delimited list, per state, of the last name of each author who lives there.

First, the scalar UDF:

USE pubs
GO

CREATE FUNCTION dbo.ConcatAuthors(@State CHAR(2))
RETURNS VARCHAR(8000)
AS
BEGIN
	DECLARE @Output VARCHAR(8000)
	SET @Output = ''

	SELECT @Output =	CASE @Output 
				WHEN '' THEN au_lname 
				ELSE @Output + ', ' + au_lname 
				END
	FROM Authors
	WHERE State = @State
	ORDER BY au_lname

	RETURN @Output
END
GO

To find the list I want:

SELECT DISTINCT State, dbo.ConcatAuthors(State)
FROM Authors
ORDER BY State

... And the adaptation of Rob's temp table method... I did change two things due to problems I discovered during testing. One, I've altered the au_lname column to VARCHAR(8000); the column in the Authors table is VARCHAR(40), not large enough for all of the California authors. What if we were dealing with a much larger dataset? Second, I added an IDENTITY column, and I'm clustering on that instead of the actual data to get the ordering. I'm doing so because of the VARCHAR(8000). Index rows can be a maximum of 900 bytes, so if we had enough data to exceed that length, this method would fail.

CREATE TABLE #AuthorConcat
(
	State CHAR(2) NOT NULL,
	au_lname VARCHAR(8000) NOT NULL,
	Ident INT IDENTITY(1,1) NOT NULL PRIMARY KEY
)

INSERT #AuthorConcat 
(
	State,
	au_lname
)
SELECT
	State, 
	au_lname
FROM Authors
ORDER BY 
	State, 
	au_lname

DECLARE @Authors VARCHAR(8000)
SET @Authors = ''
DECLARE @State CHAR(2)
SET @State = ''

UPDATE #AuthorConcat
SET @Authors = au_lname =	CASE 
				WHEN @State = State THEN @Authors + ', ' + au_lname 
				ELSE au_lname END,
	@State = State

SELECT State, MAX(au_lname) 
FROM #AuthorConcat
GROUP BY State

Clever, but more complex and harder to read than the scalar UDF version. Output is identical, but that's not why we're here. Which one is more efficient?

Drumroll, please...

Results were tabulated using STATISTICS IO, STATISTICS TIME, and Query Analyzer's Show Execution Plan. DBCC DROPCLEANBUFFERS and DBCC FREEPROCCACHE were run before each test.

Scalar UDF Method
Total cost: 0.0492
Total Scan count: 1
Total Logical reads: 2
Total Physical reads: 2
Total time: 25 ms

Temp Table Method
Total cost: 0.2131
Total Scan count: 4
Total Logical reads: 9
Total Physical reads: 2
Total time: 88 ms

So in conclusion, neither method is incredibly taxing with the tiny Pubs dataset, but I think I have proven that the UDF is far more efficient.

Update, February 28, 2005: Modified the adapation of Rob Volk's method to use a CREATE TABLE instead of SELECT INTO, as the latter is not necessarily guaranteed to insert rows in the right order for the sake of this example. Thanks to "PW" on SQLServerCentral for pointing this problem out. Note that this changed the total costs very slightly -- for the better -- but the UDF still performs better by quite a large margin.

Adam Machanic : Performance, Architecture

Rowset string concatenation: Which method is best?