Dynamic cursor in stored procedure - mysql

Question

I would like to use LIMIT in a cursor. The cursor should be used and updated several times within a loop, each time with different parameters of LIMIT. Here some code:
DELIMITER $$
CREATE PROCEDURE `updateIt`() READS SQL DATA
BEGIN
declare done int(1) default 0;
declare counter int(10) default 0;
declare xabc int(10) default 0;
declare tab1Cursor cursor for select abc from tab1 limit 100000*counter, 100000;
declare continue handler for not found set done=1;
loopCounter: LOOP
set done = 0;
open tab1Cursor;
igmLoop: loop
fetch tab1Cursor into xabc;
if done = 1 then leave igmLoop; end if;
-- do something
end loop igmLoop;
close tab1Cursor;
if (counter = 1039)
leave loopCounter;
end if;
set counter = counter + 1;
END LOOP loopCounter;
END $$
DELIMITER ;
This, however, does not work (I also tried it with the cursor in the LOOP counterLoop). Can Mysql deal with dynamic cursors?

Answer 1

From the MySQL Manual
a cursor cannot be used for a dynamic statement that is prepared and
executed with PREPARE and EXECUTE. The statement for a cursor is
checked at cursor creation time, so the statement cannot be dynamic.
However there are 2 ways, according to this post in mysql forums:
The first is for cases where absolutely only one user at a time will be running the procedure. A prepare statement can be used to create a view with the dynamic SQL and the cursor can select from this statically-named view. There's almost no performance impact. Unfortunately, these views are also visible to other users (there's no such thing as a temporary view), so this won't work for multiple users.
Analogously, a temporary table can be created in the prepare statement and the cursor can select from the temporary table. Only the current session can see a temporary table, so the multiple user issue is resolved. But this solution can have significant performance impact since a temp table has to be created each time the proc runs.
Bottom line: We still need cursors to be able to be created dynamically!
Here's an example of using a view to pass the table name and column name into a cursor.
DELIMITER //
DROP PROCEDURE IF EXISTS test_prepare//
CREATE PROCEDURE test_prepare(IN tablename varchar(255), columnname varchar(50))
BEGIN
DECLARE cursor_end CONDITION FOR SQLSTATE '02000';
DECLARE v_column_val VARCHAR(50);
DECLARE done INT DEFAULT 0;
DECLARE cur_table CURSOR FOR SELECT * FROM test_prepare_vw;
DECLARE CONTINUE HANDLER FOR cursor_end SET done = 1;
SET #query = CONCAT('CREATE VIEW test_prepare_vw as select ', columnname, ' from ', tablename);
select #query;
PREPARE stmt from #query;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
OPEN cur_table;
FETCH cur_table INTO v_column_val;
WHILE done = 0 DO
SELECT v_column_val;
FETCH cur_table INTO v_column_val;
END WHILE;
CLOSE cur_table;
DROP VIEW test_prepare_vw;
END;
//
DELIMITER ;