SELECT 
  code 
FROM 
  ushop_cp_profile_types 
WHERE 
  user_type = 'C' 
  AND is_default > 0

Query time 0.00057

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.72"
    },
    "table": {
      "table_name": "ushop_cp_profile_types",
      "access_type": "ALL",
      "rows_examined_per_scan": 12,
      "rows_produced_per_join": 1,
      "filtered": "8.33",
      "cost_info": {
        "read_cost": "1.63",
        "eval_cost": "0.10",
        "prefix_cost": "1.73",
        "data_read_per_join": "800"
      },
      "used_columns": [
        "code",
        "user_type",
        "is_default"
      ],
      "attached_condition": "((`marketplace`.`ushop_cp_profile_types`.`user_type` = 'C') and (`marketplace`.`ushop_cp_profile_types`.`is_default` > 0))"
    }
  }
}