How to translate PostgreSQL "merge_db" (aka upsert) function into MySQL - mysql

Question

Straight from the manual, here's the canonical example of merge_db in PostgreSQL:
CREATE TABLE db (a INT PRIMARY KEY, b TEXT);
CREATE FUNCTION merge_db(key INT, data TEXT) RETURNS VOID AS
$$
BEGIN
LOOP
-- first try to update the key
UPDATE db SET b = data WHERE a = key;
IF found THEN
RETURN;
END IF;
-- not there, so try to insert the key
-- if someone else inserts the same key concurrently,
-- we could get a unique-key failure
BEGIN
INSERT INTO db(a,b) VALUES (key, data);
RETURN;
EXCEPTION WHEN unique_violation THEN
-- Do nothing, and loop to try the UPDATE again.
END;
END LOOP;
END;
$$
LANGUAGE plpgsql;
SELECT merge_db(1, 'david');
SELECT merge_db(1, 'dennis');
Can this be expressed as a user-defined function in MySQL, and if so, how? Would there be any advantage over MySQL's standard INSERT...ON DUPLICATE KEY UPDATE?
Note: I'm specifically looking for a user-defined function, not INSERT...ON DUPLICATE KEY UPDATE.

Answer 1

Tested on MySQL 5.5.14.
CREATE TABLE db (a INT PRIMARY KEY, b TEXT);
DELIMITER //
CREATE PROCEDURE merge_db(k INT, data TEXT)
BEGIN
DECLARE done BOOLEAN;
REPEAT
BEGIN
-- If there is a unique key constraint error then
-- someone made a concurrent insert. Reset the sentinel
-- and try again.
DECLARE ER_DUP_UNIQUE CONDITION FOR 23000;
DECLARE CONTINUE HANDLER FOR ER_DUP_UNIQUE BEGIN
SET done = FALSE;
END;
SET done = TRUE;
SELECT COUNT(*) INTO #count FROM db WHERE a = k;
-- Race condition here. If a concurrent INSERT is made after
-- the SELECT but before the INSERT below we'll get a duplicate
-- key error. But the handler above will take care of that.
IF #count > 0 THEN
UPDATE db SET b = data WHERE a = k;
ELSE
INSERT INTO db (a, b) VALUES (k, data);
END IF;
END;
UNTIL done END REPEAT;
END//
DELIMITER ;
CALL merge_db(1, 'david');
CALL merge_db(1, 'dennis');
Some thoughts:
You can't do an update first and then check #ROW_COUNT() because it returns the number of rows actually changed. This could be 0 if the row already has the value you are trying to update.
Also, #ROW_COUNT() is not replication safe.
You could use REPLACE...INTO.
If using InnoDB or a table with transaction support you might be able to use SELECT...FOR UPDATE (untested).
I see no advantage to this solution over just using INSERT...ON DUPLICATE KEY UPDATE.