What is an alternative to cursors for sql looping?

Question

Using SQL 2005 / 2008

I have to use a forward cursor, but I don't want to suffer poor performance. Is there a faster way I can loop without using cursors?

Answer 1

Recursive Queries using Common Table Expressions.

Answer 2

It is not accurate to say "Cursors affect performance of SQL". They certainly have a tendency to, but a lot of that has to do with how people use them.

First option is to try to find a set-based approach to the problem.

If logically there is no set-based approach, and the query for the Cursor is hitting real (non-Temp) Tables, then use the STATIC keyword which will put the results of the SELECT statement into a Temp Table and hence will not lock the base-tables of the query as you iterate through the results. Using STATIC will not help if you need to be sensitive of records that might disappear while processing the result set, but that is a moot point if you are considering converting to a WHILE loop against a Temp Table (since that will also not know of changes to underlying data).

Answer 3

Depending on what you want it for, you may be able to use a tally table.

Jeff Moden has an excellent article on tally tables Here

Answer 4

I have to use a forward cursor, but I don't want to suffer poor performance. Is there a faster way I can loop without using cursors?

This depends on what you do with the cursor.

Almost everything can be rewritten using set-based operations in which case the loops are performed inside the query plan and since they involve no context switch are much faster.

However, there are some things SQL Server is just not good at, like computing cumulative values or joining on date ranges.

These kinds of queries can be made faster using a CURSOR:

• Flattening timespans: SQL Server

But again, this is a quite a rare exception, and normally a set-based way performs better.

If you posted your query, we could probably optimize it and get rid of a CURSOR.

Answer 5

You can do a WHILE loop, however you should seek to achieve a more set based operation as anything in SQL that is iterative is subject to performance issues.

http://msdn.microsoft.com/en-us/library/ms178642.aspx

Answer 6

Here is the example using cursor:

DECLARE @VisitorID int
DECLARE @FirstName varchar(30), @LastName varchar(30)
-- declare cursor called ActiveVisitorCursor 
DECLARE ActiveVisitorCursor Cursor FOR
SELECT VisitorID, FirstName, LastName 
FROM Visitors
WHERE Active = 1
-- Open the cursor
OPEN ActiveVisitorCursor 
-- Fetch the first row of the cursor and assign its values into variables
FETCH NEXT FROM ActiveVisitorCursor INTO @VisitorID, @FirstName, @LastName 
-- perform action whilst a row was found
WHILE @@FETCH_STATUS = 0
BEGIN
 Exec MyCallingStoredProc @VisitorID, @Forename, @Surname
 -- get next row of cursor
 FETCH NEXT FROM ActiveVisitorCursor INTO @VisitorID, @FirstName, @LastName 
END
 -- Close the cursor to release locks
CLOSE ActiveVisitorCursor 
 -- Free memory used by cursor
DEALLOCATE ActiveVisitorCursor 

Now here is the example how can we get same result without using cursor:

/* Here is alternative approach */

-- Create a temporary table, note the IDENTITY
-- column that will be used to loop through
-- the rows of this table
CREATE TABLE #ActiveVisitors (
       RowID int IDENTITY(1, 1), 
       VisitorID int,
       FirstName varchar(30),
       LastName varchar(30)
 )
DECLARE @NumberRecords int, @RowCounter int
DECLARE @VisitorID int, @FirstName varchar(30), @LastName varchar(30)

-- Insert the resultset we want to loop through
-- into the temporary table
INSERT INTO #ActiveVisitors (VisitorID, FirstName, LastName)
SELECT VisitorID, FirstName, LastName
FROM Visitors
WHERE Active = 1 

-- Get the number of records in the temporary table
SET @NumberRecords = @@RowCount 
--You can use: SET @NumberRecords = SELECT COUNT(*) FROM #ActiveVisitors
SET @RowCounter = 1

-- loop through all records in the temporary table
-- using the WHILE loop construct
WHILE @RowCounter <= @NumberRecords
BEGIN
 SELECT @VisitorID = VisitorID, @FirstName = FirstName, @LastName = LastName 
 FROM #ActiveVisitors
 WHERE RowID = @RowCounter

 EXEC MyCallingStoredProc @VisitorID, @FirstName, @LastName

 SET @RowCounter = @RowCounter + 1
END

-- drop the temporary table
DROP TABLE #ActiveVisitors