PDO beginTransaction rollback function - database

Question

I'm new to PDO transaction, I want to know is the rollback() is rollback the database or only the tables inside the beginTransaction().
For example :
$db->beginTransaction();
try{
$stmt = $db->prepare('SELECT * FROM user_detail .......');
$stmt -> bindParam(........);
......
$stmt -> execute();
while($s = $stmt->fetch(PDO::FETCH_ASSOC)){
$update = $db->prepare('UPDATE tableB .......');
$update -> execute();
$update = $db->prepare('UPDATE tableC .......');
$update -> execute();
$delete = $db->prepare('Delete tableD .......');
$delete -> execute();
}
$db->commit();
}
catch(PDOException $e){
$db->rollback();
}
At the same time, there is another query which will update tableC and tableG been submitted when the first query (shown above) still executing. Will the new query execute immediately or after the first query has done.
If both of them execute at the same time, what if there is an error
found and trigger the rollback(), will the second query (update tableC and tableG) rollback if it is done before the rollback start?

Answer 1

Rollback undoes all the changes made by INSERT/UPDATE/DELETE since the beginning of the transaction.
Whether you commit or rollback, that only those changes made within the current transaction, not other changes going on in other sessions. They can commit or rollback independently.
Concurrent updates running in different sessions will run at the same time, unless they are trying to update the same rows (even partially-overlapping sets of rows). One session or the other will get to those rows first, and lock the rows before it updates them. The session that acquired the locks will hold those locks until it either commits or rolls back its transaction.
There's a lot more to learn about locking. It's a very complex subject. You might like my presentation InnoDB Locking Explained with Stick Figures to get started (but it only scratches the surface).
My presentation is specifically about MySQL. You didn't say what brand of RDBMS you're using. All different brands will each have their own quirks regarding transactions and locking.