PDO Unbuffered queries - php

Question

I'm trying to get into PDO details. So I coded this:
$cn = getConnection();
// get table sequence
$comando = "call p_generate_seq('bitacora')";
$id = getValue($cn, $comando);
//$comando = 'INSERT INTO dsa_bitacora (id, estado, fch_creacion) VALUES (?, ?, ?)';
$comando = 'INSERT INTO dsa_bitacora (id, estado, fch_creacion) VALUES (:id, :estado, :fch_creacion)';
$parametros = array (
':id'=> (int)$id,
':estado'=>1,
':fch_creacion'=>date('Y-m-d H:i:s')
);
execWithParameters($cn, $comando, $parametros);
my getValue function works fine, and I get the next sequence for the table. But when I get into execWithParameters, i get this exception:
PDOException: SQLSTATE[HY000]: General error: 2014 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. in D:\Servidor\xampp_1_7_1\htdocs\bitacora\func_db.php on line 77
I tried to modify the connection attributes but it doesn't work.
These are my core db functions:
function getConnection() {
try {
$cn = new PDO("mysql:host=$host;dbname=$bd", $usuario, $clave, array(
PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
));
$cn->setAttribute(PDO::MYSQL_ATTR_USE_BUFFERED_QUERY, true);
return $cn;
} catch (PDOException $e) {
print "Error!: " . $e->getMessage() . "<br/>";
die();
}
}
function getValue($cn, $comando) {
$resul = $cn->query($comando);
if (!$resul) return null;
while($res = $resul->fetch()) {
$retorno = $res[0][0];
break;
}
return $retorno;
}
function execWithParameters($cn, $comando, $parametros) {
$q = $cn->prepare($comando);
$q->execute($parametros);
if ($q->errorInfo() != null) {
$e = $q->errorInfo();
echo $e[0].':'.$e[1].':'.$e[2];
}
}
Somebody who can shed a light for this? PD. Please do not suggest doing autonumeric id, cause i am porting from another system.

Answer 1

The issue is that mysql only allows for one outstanding cursor at a given time. By using the fetch() method and not consuming all the pending data, you are leaving a cursor open.
The recommended approach is to consume all the data using the fetchAll() method.
An alternative is to use the closeCursor() method.
If you change this function, I think you will be happier:
<?php
function getValue($cn, $comando) {
$resul = $cn->query($comando);
if (!$resul) return null;
foreach ($resul->fetchAll() as $res) {
$retorno = $res[0];
break;
}
return $retorno;
}
?>

Answer 2

I don't think PDOStatement::closeCursor() would work if you're not doing a query that returns data (i.e. an UPDATE, INSERT, etc).
A better solution is to simply unset() your PDOStatement object after calling PDOStatement::execute():
$stmt = $pdo->prepare('UPDATE users SET active = 1');
$stmt->execute();
unset($stmt);

Answer 3

The problem seems to be---I'm not too familiar with PDO--- that after your getValue call returns, the query is still bound to the connection (You only ever ask for the first value, yet the connection returns several, or expects to do so).
Perhaps getValue can be fixed by adding
$resul->closeCursor();
before the return.
Otherwise, if queries to getValue will always return a single (or few enough) value, it seems that using fetchAll will be preferred.

Answer 4

I just spend 15 minutes googling all around the internet, and viewed at least 5 different Stackoverflow questions, some who claimed my bug apparently arose from the wrong version of PHP, wrong version of MySQL library or any other magical black-box stuff...
I changed all my code into using "fetchAll" and I even called closeCursor() and unset() on the query object after each and every query. I was honestly getting desperate! I also tried the MYSQL_ATTR_USE_BUFFERED_QUERY flag, but it did not work.
FINALLY I threw everything out the window and looked at the PHP error, and tracked the line of code where it happened.
SELECT AVG((original_bytes-new_bytes)/original_bytes) as saving
FROM (SELECT original_bytes, new_bytes FROM jobs ORDER BY id DESC LIMIT 100) AS t1
Anyway, the problem happened because my original_bytes and new_bytes both where unsigned bigints, and that meant that if I ever had a job where the new_bytes where actually LARGER than the original_bytes, then I would have a nasty MySQL "out of range" error. And that just happened randomly after running my minification service for a little while.
Why the hell I got this weird MySQL error instead of just giving me the plain error, is beyond me! It actually showed up in SQLBuddy (lightweight PHPMyAdmin) when I ran the raw query.
I had PDO exceptions on, so it should have just given me the MySQL error.
Never mind, the bottom line is:
If you ever get this error, be sure to check that your raw MySQL is actually correct and STILL working!!!

Answer 5

A friend of mine had very much the same problem with the xampp 1.7.1 build. After replacing xampp/php/* by the 5.2.9-2 php.net build and copying all necessary files to xampp/apache/bin it worked fine.

Answer 6

If you're using XAMPP 1.7.1, you just need to upgrade to 1.7.2.