/**
 * Alters a search query before executing it.
 *
 * @param SearchApiMultiQueryInterface $query
 *   The executed search query.
 */
function hook_search_api_multi_query_alter(SearchApiMultiQueryInterface $query)
{
    $indexes = $query->getIndexes();
    if (isset($indexes['default_node_index'])) {
        $query->condition('default_node_index:author', 0, '!=');
    }
}