RETURN statement before DEALLOCATE CURSOR

Question

Question about use of Cursors in combination with RETURN in a SQL Server 2008 Stored Procedure

Consider this example:

CREATE PROCEDURE [dbo].[test]
    @ReturnEarly BIT = 0
AS
BEGIN
    SET NOCOUNT ON

    SELECT 1 AS Result INTO #Test

    DECLARE @Result INT, @HasResult INT = 1

    DECLARE TestCursor CURSOR FOR 
        SELECT Result
        FROM #Test
        WHERE Result = 0

    OPEN TestCursor
    FETCH NEXT FROM TestCursor INTO @Result

    IF (@@FETCH_STATUS <> 0)
    BEGIN
        IF (@ReturnEarly = 1)
            RETURN 0
        ELSE 
            SET @HasResult = 0
    END

    CLOSE TestCursor
    DEALLOCATE TestCursor

    DROP TABLE #test --technically superflous

    IF (@HasResult = 0)
        RETURN 0
    ELSE
        RETURN 1 -- in reality many more checks on the actual results
END
GO

-- Allow DEALLOCATE CURSOR to be called
DECLARE   @RC int, @ReturnEarly bit
SET @ReturnEarly = 1
EXECUTE @RC = [test] @ReturnEarly
PRINT @RC
GO

-- Exit before DEALLOCATE CURSOR is called
DECLARE   @RC int, @ReturnEarly bit
SET @ReturnEarly = 0
EXECUTE @RC = [test] @ReturnEarly
PRINT @RC
GO

DROP PROCEDURE [dbo].[test]

Both EXEC calls of the Stored Procedure return the same result. My question is this: is there a potential leak associated with returning from inside the Cursor? There is not for the temporary table (#test), for example.

Bonus: Can you point me to docs suggesting either way? My google skills weren't up to it.

Extra Bonus: If there is a problem, is there any way I could find evidence of the leak? perhaps a dmv, event log, etc which would show the problem?

Thanks in advance!

Answer 1

Copying part of my answer from here...

How long the resources hang around depends on whether you declared the cursor locally or globally (and what the default is in your environment - default is global but you can change it).

If the cursor is global, then it can stay "alive" in SQL Server until the last piece of code is touched in the scope in which it was created - and can often persist even beyond that. For example if you call a stored procedure that creates a global cursor, then call 20 other stored procedures, the cursor will live on while those other 20 stored procedures are running, until the caller goes out of scope. I believe it will stay alive at the session level, not the connection level, but haven't tested this thoroughly. If the cursor is declared as local, then it should only stay in scope for the current object (but again, this is theoretical, and I haven't done extensive, low-level memory tests to confirm). I don't know of any DMVs that show this usage individually, or even if on a global level you could detect memory changes in DMVs that probably don't get updated often enough to reflect the truth at any microsecond.

The general concept, though, should be: when you're done with something, say so. I believe there are several ways to write your code such that the RETURN doesn't have to happen within the cursor, and probably ways to re-write the code to eliminate the cursor.

In order to make my cursors as efficient as possible, I always use the following declarations:

DECLARE c CURSOR
  LOCAL STATIC FORWARD_ONLY READ_ONLY
  FOR SELECT ...

I've also heard that there can be memory issues if you only CLOSE or only DEALLOCATE so - even though I haven't proven or disproven it - I always do both when I'm done:

CLOSE c;
DEALLOCATE c;

Answer 2

Try sys.dm_exec_cursors:

-- Create local fast_forward ( forward-only, read-only ) cursor 
DECLARE test_cursor1 CURSOR FAST_FORWARD LOCAL FOR 
SELECT object_id
FROM sys.objects

select 'after declare' s, * from sys.dm_exec_cursors(null)

-- Cursor variables
DECLARE @object_id  INT

OPEN test_cursor1

select 'after open' s, * from sys.dm_exec_cursors(null)

FETCH NEXT FROM test_cursor1 INTO @object_id
WHILE @@fetch_status = 0
BEGIN

    -- cursor code here 
    -- ...

    FETCH NEXT FROM test_cursor1 INTO @object_id
END

CLOSE test_cursor1

select 'after close' s, * from sys.dm_exec_cursors(null)

DEALLOCATE test_cursor1

-- This won't show anything ...
select 'after deallocate' s, * from sys.dm_exec_cursors(null)

Answer 3

Well well. One obvious proof of a problem would seem to be that the example cannot be executed twice against the same database in short succession.

Attempting to do so yields:

Msg 16915, Level 16, State 1, Procedure test, Line 14
A cursor with the name 'TestCursor' already exists.
Msg 16905, Level 16, State 1, Procedure test, Line 18
The cursor is already open.

Still, interested in what insight the community can provide!

edit: removing the STATIC declaration on the cursor eliminates this problem. Not sure why the cursor was declared static > 5 years ago. Edited the example to remove this declaration.