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 0.00010

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "storefronts",
      "access_type": "ALL",
      "rows": 3,
      "filtered": 100,
      "attached_condition": "storefronts.is_default = 'Y'"
    }
  }
}