Avoid Cursors, Rewrite Into While Loops
A few months back, we had a stored proc at work that was using CURSORs pretty heavily. It had CURSORs inside of CURSORs. This proc was one of our oldest, biggest, and most involved procs at over 2050 lines. On larger clients, it could be called thousands of times a day. All of the cursors were FAST_FORWARD types, which should have at least been somewhat performance friendly. However they were SELECTing from tables that had triggers which implicitly converted the cursor type to be STATIC. That’s not good, because it was locking up the SELECTed table and causing all kinds of headaches.
So the decision was made to rewrite it to use table variables and WHILE LOOPs. This sped the proc up immensely, as measured by elapsed time, at the cost of using more in-process memory. You see, when a CURSOR is used, it basically creates a temp table on TempDB and iterates through it. With a STATIC cursor, the selected table has locks placed on it in the transaction tables. The CURSOR alternative is to create a table variable containing the rows that would have gone in the CURSOR and use a counter variable to loop through it using the IDENTITY column on the table variable. It’s still row-by-row processing, but it’s faster, more efficient, and doesn’t lock. The only downside is that it will use more RAM. So beware if you’re using SQL 2000, SQL Express, or an MSDE version as you may see performance degrade if the sqlservr process hits it’s memory ceiling.
I don’t think I need to say this, but will anyway. Changing CURSORs into WHILE LOOPs are better, but CURSORs into a set-based query is best. It’s always best to do a query in a single batch as opposed to row-by-row.
DECLARE @SomeID INT DECLARE myCursor CURSOR LOCAL FAST_FORWARD FOR SELECT DISTINCT SomeID FROM dbo.TableA WHERE ID = @ID OPEN myCursor FETCH NEXT FROM myCursor INTO @SomeID WHILE(@@FETCH_STATUS <> -1) BEGIN --do activity END
TABLE VARIABLE, WHILE/LOOP code:
DECLARE @rowCount INT, @currentRow INT, @SomeID INT DECLARE @tableVariable TABLE (RowID INT IDENTITY(1,1), SomeID INT NULL) INSERT INTO @tableVariable(SomeID) SELECT SomeID FROM dbo.TableA WHERE ID = @ID SELECT @rowCount = @@RowCount, @currentRow = 1 WHILE @currentRow⇐@rowCount BEGIN SELECT @SomeID = SomeID FROM @tableVariable WHERE RowID = @currentRow --do activity SET @currentRow = @currentRow + 1 END