how to optimise MySql function to work for concurrent users - mysql

Question

I am new to MySQL. Please can you advice on how can i modify below function to ensure it does not throw locking errors when called by multiple users at the same time.
CREATE FUNCTION `get_val`(`P_TABLE` VARCHAR(50)) RETURNS int(11)
DETERMINISTIC
BEGIN
DECLARE pk_value INT DEFAULT 0;
DECLARE pk_found INT DEFAULT 0;
SELECT 1 INTO pk_found FROM pk_keys WHERE TABLE_NAME = P_TABLE;
IF pk_found = 1
THEN
UPDATE pk_keys SET TABLE_VALUE = (TABLE_VALUE + 1 ) WHERE TABLE_NAME = P_TABLE;
ELSE
INSERT INTO pk_keys VALUES ( P_TABLE, 1 );
END IF;
SELECT TABLE_VALUE INTO pk_value FROM pk_keys WHERE TABLE_NAME = P_TABLE;
RETURN pk_value;
END
thanks

Answer 1

CREATE FUNCTION `get_val`(`P_TABLE` VARCHAR(50)) RETURNS int(11)
DETERMINISTIC
MODIFIES SQL DATA
BEGIN
DECLARE pk_value INT DEFAULT 0;
IF EXISTS (SELECT 1 FROM pk_keys WHERE TABLE_NAME = P_TABLE)
THEN
SELECT TABLE_VALUE + 1 INTO pk_value FROM pk_keys WHERE TABLE_NAME = P_TABLE FOR UPDATE;
UPDATE pk_keys SET TABLE_VALUE = pk_value WHERE TABLE_NAME = P_TABLE;
ELSE
SET pk_value = 1;
INSERT INTO pk_keys VALUES ( P_TABLE, pk_value );
END IF;
RETURN pk_value;
END
Have a look at SELECT ... FOR UPDATE and SELECT ... LOCK IN SHARE MODE Locking Reads
Let us look at another example: We have an integer counter field in a
table child_codes that we use to assign a unique identifier to each
child added to table child. It is not a good idea to use either
consistent read or a shared mode read to read the present value of the
counter because two users of the database may then see the same value
for the counter, and a duplicate-key error occurs if two users attempt
to add children with the same identifier to the table.
Here, LOCK IN SHARE MODE is not a good solution because if two users
read the counter at the same time, at least one of them ends up in
deadlock when it attempts to update the counter.
To implement reading and incrementing the counter, first perform a
locking read of the counter using FOR UPDATE, and then increment the
counter. For example:
SELECT counter_field FROM child_codes FOR UPDATE;
UPDATE child_codes SET counter_field = counter_field + 1;
A SELECT ... FOR UPDATE reads the latest available data, setting
exclusive locks on each row it reads. Thus, it sets the same locks a
searched SQL UPDATE would set on the rows.
Also I replaced your if condition. EXISTS stops as soon as a row is found.