Source

File: wp-includes/revision.php

function wp_get_post_autosave( $post_id, $user_id = 0 ) {
	global $wpdb;

	$autosave_name = $post_id . '-autosave-v1';
	$user_id_query = ( 0 !== $user_id ) ? "AND post_author = $user_id" : null;

	// Construct the autosave query.
	$autosave_query = "
		SELECT *
		FROM $wpdb->posts
		WHERE post_parent = %d
		AND post_type = 'revision'
		AND post_status = 'inherit'
		AND post_name   = %s " . $user_id_query . '
		ORDER BY post_date DESC
		LIMIT 1';

	$autosave = $wpdb->get_results(
		$wpdb->prepare(
			$autosave_query,
			$post_id,
			$autosave_name
		)
	);

	if ( ! $autosave ) {
		return false;
	}

	return get_post( $autosave[0] );
}