Source

File: wp-admin/includes/post.php

function has_meta( $postid ) {
	global $wpdb;

	return $wpdb->get_results(
		$wpdb->prepare(
			"SELECT meta_key, meta_value, meta_id, post_id
			FROM $wpdb->postmeta WHERE post_id = %d
			ORDER BY meta_key,meta_id",
			$postid
		),
		ARRAY_A
	);
}