SELECT 
  storefronts.* 
FROM 
  cscart_storefronts AS storefronts 
WHERE 
  1 = 1 
  AND storefronts.is_default IN ('Y') 
GROUP BY 
  storefronts.storefront_id 
ORDER BY 
  storefronts.is_default desc

Query time 0.00074

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "0.93"
    },
    "ordering_operation": {
      "using_filesort": false,
      "grouping_operation": {
        "using_filesort": false,
        "table": {
          "table_name": "storefronts",
          "access_type": "ALL",
          "possible_keys": [
            "PRIMARY"
          ],
          "rows_examined_per_scan": 4,
          "rows_produced_per_join": 1,
          "filtered": "25.00",
          "cost_info": {
            "read_cost": "0.83",
            "eval_cost": "0.10",
            "prefix_cost": "0.93",
            "data_read_per_join": "2K"
          },
          "used_columns": [
            "storefront_id",
            "url",
            "redirect_customer",
            "is_default",
            "status",
            "access_key",
            "name",
            "theme_name",
            "is_accessible_for_authorized_customers_only"
          ],
          "attached_condition": "(`s2cart`.`storefronts`.`is_default` = 'Y')"
        }
      }
    }
  }
}