(PECL solr >= 2.2.0)
SolrQuery::setGroupMain — If true, the result of the first field grouping command is used as the main result list in the response, using group.format=simple
    If true, the result of the first field grouping command is used as the main result list in the response, using group.format=simple.
  
value
      If true, the result of the first field grouping command is used as the main result list in the response.
     
Returns an instance of SolrQuery.