PDO::fetchAll vs. PDO::fetch in a loop

Question

Just a quick question.

Is there any performance difference between using PDO::fetchAll() and PDO::fetch() in a loop (for large result sets)?

I'm fetching into objects of a user-defined class, if that makes any difference.

My initial uneducated assumption was that fetchAll might be faster because PDO can perform multiple operations in one statement while mysql_query can only execute one. However I have little knowledge of PDO's inner workings and the documentation doesn't say anything about this, and whether or not fetchAll() is simply a PHP-side loop dumped into an array.

Any help?

Answer 1

One thing about PHP that I've found to be true almost always is that a function you implement yourself will almost always be slower than the PHP equivalent. This is because when something is implemented in PHP it doesn't have all the compile time optimizations that C has (which PHP is written in) and there is high overhead of PHP function calls.

Answer 2

Little benchmark with 200k random records. As expected, the fetchAll method is faster but require more memory.

Result :
fetchAll : 0.35965991020203s, 100249408b
fetch : 0.39197015762329s, 440b

The benchmark code used :

<?php
// First benchmark : speed
$dbh = new PDO('mysql:dbname=testage;dbhost=localhost', 'root', '');
$dbh->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$sql = 'SELECT * FROM test_table WHERE 1';
$stmt = $dbh->query($sql);
$data = array();
$start_all = microtime(true);
$data = $stmt->fetchAll();
$end_all = microtime(true);

$stmt = $dbh->query($sql);
$data = array();
$start_one = microtime(true);
while($data = $stmt->fetch()){}
$end_one = microtime(true);

// Second benchmark : memory usage
$stmt = $dbh->query($sql);
$data = array();
$memory_start_all = memory_get_usage();
$data = $stmt->fetchAll();
$memory_end_all = memory_get_usage();

$stmt = $dbh->query($sql);
$data = array();
$memory_end_one = 0;
$memory_start_one = memory_get_usage();
while($data = $stmt->fetch()){
  $memory_end_one = max($memory_end_one, memory_get_usage());
}

echo 'Result : <br/>
fetchAll : ' . ($end_all - $start_all) . 's, ' . ($memory_end_all - $memory_start_all) . 'b<br/>
fetch : ' . ($end_one - $start_one) . 's, ' . ($memory_end_one - $memory_start_one) . 'b<br/>';

Answer 3

@Arkh

// $data in this case is an array of rows;

$data = $stmt->fetchAll();


// $data in this case is just one row after each loop;

while($data = $stmt->fetch()){}


// Try using

$i = 0;

while($data[$i++] = $stmt->fetch()){}

The memory difference should become neglijable