Source

File: wp-includes/query.php

function _find_post_by_old_slug( $post_type ) {
	global $wpdb;

	$query = $wpdb->prepare( "SELECT post_id FROM $wpdb->postmeta, $wpdb->posts WHERE ID = post_id AND post_type = %s AND meta_key = '_wp_old_slug' AND meta_value = %s", $post_type, get_query_var( 'name' ) );

	// If year, monthnum, or day have been specified, make our query more precise
	// just in case there are multiple identical _wp_old_slug values.
	if ( get_query_var( 'year' ) ) {
		$query .= $wpdb->prepare( ' AND YEAR(post_date) = %d', get_query_var( 'year' ) );
	}
	if ( get_query_var( 'monthnum' ) ) {
		$query .= $wpdb->prepare( ' AND MONTH(post_date) = %d', get_query_var( 'monthnum' ) );
	}
	if ( get_query_var( 'day' ) ) {
		$query .= $wpdb->prepare( ' AND DAYOFMONTH(post_date) = %d', get_query_var( 'day' ) );
	}

	$id = (int) $wpdb->get_var( $query );

	return $id;
}