Retrieving last inserted ID for InnoDB composite key table - php

Question

I have the following table:
CREATE TABLE `my_table` (
composite_pk1 INT NOT NULL ,
composite_pk2 INT NOT NULL ,
data VARCHAR(255) NOT NULL ,
primary key (composite_pk1, composite_pk2)
) ENGINE=InnoDB;
For a given composite_pk1, I wish composite_pk2 to act as an autoincrement primary key. I don't wish to lock the table, and as such plan on using a trigger such as the following:
DELIMITER $$
CREATE TRIGGER my_trigger BEFORE INSERT ON my_table
FOR EACH ROW BEGIN
SET NEW.composite_pk2 = (
SELECT IFNULL(MAX(composite_pk2), 0) + 1
FROM issue_log
WHERE composite_pk1 = NEW.composite_pk1
);
END $$
I can now insert a record:
$stmt=$myDB->prepare('INSERT INTO my_table(composite_pk1, data) VALUES (?,?)');
$stmt->execute([123,'hello']);
How do I get the last inserted composite_pk2? PDO::lastInsertId only works with native autoincrement tables (i.e. not the trigger approach). I "could" later do a SELECT query to get the max value, however, there is no guarantee that another record has snuck in.

Answer 1

You can make composite_pk2 an unique key with auto_increment:
CREATE TABLE `my_table` (
composite_pk1 INT NOT NULL ,
composite_pk2 INT NOT NULL unique auto_increment,
data VARCHAR(255) NOT NULL ,
primary key (composite_pk1, composite_pk2)
) ENGINE=InnoDB;
Now last_insert_id() will return the recently created id for composite_pk2.