查询时临时锁定MYSQL表？

Question

I'm very new to working with database. I'm currently using Google Cloud SQL and accessing the database via PHP PDO.

I managed to find a website ( http://wiki.hashphp.org/PDO_Tutorial_for_MySQL_Developers ) that had some example code:

<?php
try {
    $db->beginTransaction(); //<---- 1

    $db->exec("SOME QUERY"); //<---- 2

    $stmt = $db->prepare("SOME OTHER QUERY?");
    $stmt->execute(array($value));

    $stmt = $db->prepare("YET ANOTHER QUERY??");
    $stmt->execute(array($value2, $value3));

    $db->commit(); //<---- 3
} catch(PDOException $ex) {
    //Something went wrong rollback!
    $db->rollBack();
    echo $ex->getMessage();
}

According to the above code, if I did '$db->beginTransaction();', would that commence locking the table? Then '$db->exec("SOME QUERY");' would be as normal. And finally, '$db->commit();' would save everything above and unlock the table?

If everything were to run correctly, then should I assume that as line 2 is running, no other call to the table can be finished (they'll be queued) and other calls can only begin once line 3 here is called? I just want to know if my understanding of this process (as well as how the code is written) is accurate.

Answer 1

First, since you are using try/catch, you want to make sure PDO's error mode is exceptions: http://php.net/manual/en/pdo.error-handling.php , by default it is silent.

Then, yes, when a query fails, PDO should issue an exception which will stop the execution of the try section. During the catch , you should issue a rollBack() and if everything succeeds, you should issue a commit() . So how you have that looks correct to me.

After you start a transaction, nothing in that connection will be saved until you run commit() . rollBack() ends a transaction and returns to autocommit mode without saving any changes.

From http://php.net/manual/en/pdo.begintransaction.php :

Turns off autocommit mode. While autocommit mode is turned off, changes made to the database via the PDO object instance are not committed until you end the transaction by calling PDO::commit(). Calling PDO::rollBack() will roll back all changes to the database and return the connection to autocommit mode.