Question

I have the following PHP code:

$dbh->beginTransaction();
$dbh->exec("LOCK TABLES
    `reservations` WRITE, `settings` WRITE");
$dbh->exec("CREATE TEMPORARY TABLE
        temp_reservations
    SELECT * FROM reservations");
$dbh->exec("ALTER TABLE
        `temp_reservations`
    ADD INDEX ( conf_num ) ; ");

// [...Other stuff here with temp_reservations...]

$dbh->exec("DELETE QUICK FROM `reservations`");
$dbh->exec("OPTIMIZE TABLE `reservations`");
$dbh->exec("INSERT INTO `reservations` SELECT * FROM temp_reservations");

var_dump(GlobalContainer::$dbh->inTransaction()); // true
$dbh->exec("UNLOCK TABLES");
$dbh->rollBack();

Transactions are working fine for regular updates/inserts but the above code for some reason is not. When an error happens above, I’m left with a completely empty reservations table. I read on the PDO::beginTransaction page that “some databases, including MySQL, automatically issue an implicit COMMIT when a database definition language (DDL) statement such as DROP TABLE or CREATE TABLE is issued within a transaction“. The MySQL manual has a list of “Data Definition Statements”, which I would assume is the same as DDL mentioned above which lists CREATE TABLE but I am only creating a temporary table. Is there any way around this?

Also, does the fact that I’m left with an empty reservations table show that a commit occurred after the DELETE QUICK FROM reservations query?

Edit: On an additional note, the INSERT INTO reservations line also produces the following error:

Cannot execute queries while other unbuffered queries are active. Consider using PDOStatement::fetchAll(). Alternatively, if your code is only ever going to run against mysql, you may enable query buffering by setting the PDO::MYSQL_ATTR_USE_BUFFERED_QUERY attribute.

I tried doing $dbh->setAttribute( PDO::MYSQL_ATTR_USE_BUFFERED_QUERY , true); but this doesn’t seem to affect it. I’m assuming it would have something to do with the transaction, but I’m not sure. Can anyone pinpoint what exactly is causing this error as well?

Answer 1

Your OPTIMIZE TABLE statement is causing an implicit commit.

I’m not sure exactly what you’re trying to do, but it looks you can shorten your code to:

$dbh->exec("OPTIMIZE TABLE `reservations`");

All the other code is just making the job more complex, for no gain.

I’m also assuming you’re using InnoDB tables, because MyISAM tables wouldn’t support transactions anyway. Every DDL or DML operation on a MyISAM table implicitly commits immediately.

By the way, buffered queries have nothing to do with transactions. They have to do with fetching SELECT result sets one row at a time, versus fetching the whole result set into memory in PHP, then iterating through it. See explanation at: http://php.net/manual/en/mysqlinfo.concepts.buffering.php