在Mysql中运行第二次提交的结果是什么？

While using a MySQL transaction, is there a problem if you commit twice? e.g.

START TRANSACTION
/* Various DB UPDATEs */
COMMIT
COMMIT

Having tested it out, it seems the 2nd COMMIT is a NO OP and could equally be a ROLLBACK, is that correct? Is it harmless or could it ever be a problem?

As I understand it, after the 1st commit, auto-commit would be enabled and so the 2nd commit is meaningless... But would it cause any harm?

Background info:

I'm using PHP and I have a database transaction, e.g.

$dbconnection->getPdoInstance()->beginTransaction();
/* ... Various DB UPDATEs */
$dbconnection->getPdoInstance()->commit();

However, I want to put a MYSQL COMMENT on my commit so that I can identify it when I see it in the SHOW PROCESSLIST, therefore I can't use PDO, I have to use "raw" SQL:

$dbconnection->getPdoInstance()->beginTransaction();
/* ... various DB UPDATEs */
$dbconnection->createCommand("COMMIT /* My Comment */")->execute();

Except this doesn't work because as far as PDO is concerned the transaction would still be active so it would throw an exception on the next beginTransaction(), e.g.

exception 'PDOException' with message 'There is already an active transaction'

One workaround would be this:

$dbconnection->getPdoInstance()->beginTransaction();
/* ... various DB UPDATEs */
$dbconnection->createCommand("COMMIT /* My Comment */")->execute();
$dbconnection->getPdoInstance()->commit();

which would mean two COMMITs are done, the first with my comment, the 2nd as a no-op but would close the transaction for pdo. (hence the question).

Another workaround would be to leave pdo out altogether:

$dbconnection->createCommand("START TRANSACTION")->execute();
/* ... various DB UPDATEs */
$dbconnection->createCommand("COMMIT /* My Comment */")->execute();

But then you lose the pdo features such as intransaction() and you won't get an exception if you try to start a 2nd transaction while already inside a prior transaction.

On Mysql, two commits are not a problem.

On PDO, im not sure. What if you change the first commit to another command like:

$dbconnection->getPdoInstance()->beginTransaction();
/* ... various DB UPDATEs */
$dbconnection->createCommand("SET @dummy_var=1; /* My Comment */")->execute();
$dbconnection->getPdoInstance()->commit();