Algol Plus Ltd

add_filter( "posts_where", function($where, $query){
  global $wpdb;
  if ( isset( $query->query_vars['OR_sku_filter'] ) ) {
    $like  = "%" . $wpdb->esc_like( $query->query_vars['OR_sku_filter'] ) . "%";
    $where .= " OR ({$wpdb->posts}.ID IN ( SELECT post_id FROM {$wpdb->postmeta} WHERE {$wpdb->postmeta}.meta_key = '_barcode' AND {$wpdb->postmeta}.meta_value LIKE '" . $like . "' ))";
  }
  return $where;
}, 10, 2 );