WP Search by custom fields

function cf_search_join( $join ) {

	if ( is_admin() || ! is_search() ) {
		return $join;
	}

	global $wpdb;

	$join .= " LEFT JOIN {$wpdb->postmeta} ON {$wpdb->posts}.ID = {$wpdb->postmeta}.post_id ";

	return $join;
}

add_filter( 'posts_join', 'cf_search_join' );

function cf_search_where( $where ) {

	if ( is_admin() || ! is_search() ) {
		return $where;
	}

	global $wpdb;

	$cf_where = '';
	$cf_keys  = array( '_sku' );

	$or_op = '';

	foreach ( $cf_keys as $cf_key ) {
		$cf_where .= "{$or_op}({$wpdb->postmeta}.meta_key = '{$cf_key}' AND {$wpdb->postmeta}.meta_value LIKE $1)";
		$or_op = ' OR ';
	}

	$where = preg_replace(
		"/\(\s*{$wpdb->posts}.post_content\s+LIKE\s*(\'[^\']+\')\s*\)/",
		"({$wpdb->posts}.post_content LIKE $1) OR {$cf_where}", $where );

	return $where;
}

add_filter( 'posts_where', 'cf_search_where' );

function cf_search_distinct( $distinct ) {

	if ( ! is_admin() && is_search() ) {
		return 'DISTINCT';
	}

	return $distinct;
}
add_filter( 'posts_distinct', 'cf_search_distinct' );