php - PDO/MySQL memory consumption with large result set

Question

I'm having a strange time dealing with selecting from a table with about 30,000 rows.

It seems my script is using an outrageous amount of memory for what is a simple, forward only walk over a query result.

Please note that this example is a somewhat contrived, absolute bare minimum example which bears very little resemblance to the real code and it cannot be replaced with a simple database aggregation. It is intended to illustrate the point that each row does not need to be retained on each iteration.

<?php
$pdo = new PDO('mysql:host=127.0.0.1', 'foo', 'bar', array(
    PDO::ATTR_ERRMODE=>PDO::ERRMODE_EXCEPTION,
));
$stmt = $pdo->prepare('SELECT * FROM round');
$stmt->execute();

function do_stuff($row) {}

$c = 0;
while ($row = $stmt->fetch()) {
    // do something with the object that doesn't involve keeping 
    // it around and can't be done in SQL
    do_stuff($row);
    $row = null;
    ++$c;
}

var_dump($c);
var_dump(memory_get_usage());
var_dump(memory_get_peak_usage());

This outputs:

int(39508)
int(43005064)
int(43018120)

I don't understand why 40 meg of memory is used when hardly any data needs to be held at any one time. I have already worked out I can reduce the memory by a factor of about 6 by replacing "SELECT *" with "SELECT home, away", however I consider even this usage to be insanely high and the table is only going to get bigger.

Is there a setting I'm missing, or is there some limitation in PDO that I should be aware of? I'm happy to get rid of PDO in favour of mysqli if it can not support this, so if that's my only option, how would I perform this using mysqli instead?

Answer 1

After creating the connection, you need to set PDO::MYSQL_ATTR_USE_BUFFERED_QUERY to false:

<?php
$pdo = new PDO('mysql:host=127.0.0.1', 'foo', 'bar', array(
    PDO::ATTR_ERRMODE=>PDO::ERRMODE_EXCEPTION,
));
$pdo->setAttribute(PDO::MYSQL_ATTR_USE_BUFFERED_QUERY, false);

// snip

var_dump(memory_get_usage());
var_dump(memory_get_peak_usage());

This outputs:

int(39508)
int(653920)
int(668136)

Regardless of the result size, the memory usage remains pretty much static.