MySQL RAND() function in Doctrine2 DQL

d2-bootstrap.php

<?php

$config = new \Doctrine\ORM\Configuration();
/** OTHER STUFF */
$config->addCustomNumericFunction('RAND', 'My\Custom\Doctrine2\Function\Rand');
/** CONTINUES */

Rand.php

<?php
namespace My\Custom\Doctrine2\Function;

/**
 * RandFunction ::= "RAND" "(" ")"
 */
class Rand extends FunctionNode
{

    public function parse(\Doctrine\ORM\Query\Parser $parser)
    {
        $parser->match(Lexer::T_IDENTIFIER);
        $parser->match(Lexer::T_OPEN_PARENTHESIS);
        $parser->match(Lexer::T_CLOSE_PARENTHESIS);
    }

    public function getSql(\Doctrine\ORM\Query\SqlWalker $sqlWalker)
    {
        return 'RAND()';
    }
}

@nocomment17 http://www.doctrine-project.org/2010/03/29/doctrine2-custom-dql-udfs.html

@jeroensen @dvapelnik IIRC, sorting by RAND() forces a scan of all matching records, as RAND() is just a numerical function that returns a random number. If there is a small number of matching records, it should be alright. But if you have millions of matching records and attempt something like ORDER BY RAND() LIMIT 10, you might be in trouble, as the database will have to scan all these rows, remember all of them along with the number returned by RAND() for each of them, then extract the few ones with the lowest number (someone correct me if I'm wrong on the details). This is very resource intensive.

There are more efficient ways to select randomly. See for example:
http://jan.kneschke.de/projects/mysql/order-by-rand/