Extending the site search to include a single custom field posts_join, posts_search (ok)

add_action('posts_where', '__adapted_search_function',1,2);
Or
add_action('posts_search', '__adapted_search_function',1,2);
function __adapted_search_function($search, $query) {
  if(is_admin() || !$query->is_main_query() || !$query->is_search)
    return; //determine if we are modifying the right query
  global $wpdb;
  $search_term = $query->get('s');
  $search = ' AND (';
  //point 1
  $search .= "($wpdb->posts.post_title LIKE '%$search_term%')";
  //need to add an OR between search conditions
  $search .= " OR ";
  //point 2
  $search .= "($wpdb->posts.post_excerpt LIKE '%$search_term%')";
  //need to add an OR between search conditions
  $search .= " OR ";
  //point 3
  $search .= "($wpdb->postmeta.meta_key = 'mcb-product' AND $wpdb->postmeta.meta_value LIKE '%$search_term%')";
  //add the filter to join, sql will error out without joining the tables to the query
  add_filter('posts_join', '__custom_join_tables');   
  return $search . ') ';
}
function __custom_join_tables($joins) {
  global $wpdb;
  $joins = "JOIN $wpdb->postmeta ON ($wpdb->postmeta.post_ID = $wpdb->posts.ID)";
  return $joins;
}