SELECT 
  cscart_vendor_plan_descriptions.*, 
  cscart_vendor_plans.*, 
  manual_rating.rating AS manual_rating 
FROM 
  cscart_vendor_plans 
  LEFT JOIN cscart_vendor_plan_descriptions ON cscart_vendor_plan_descriptions.plan_id = cscart_vendor_plans.plan_id 
  AND cscart_vendor_plan_descriptions.lang_code = 'ko' 
  LEFT JOIN cscart_manual_rating AS manual_rating ON manual_rating.object_id = cscart_vendor_plans.plan_id 
  AND manual_rating.object_type = 'vendor_plan' 
WHERE 
  cscart_vendor_plans.plan_id = 5 
ORDER BY 
  position asc 
LIMIT 
  0, 1

Query time 0.00020

JSON explain

{
  "query_block": {
    "select_id": 1,
    "const_condition": "1",
    "table": {
      "table_name": "cscart_vendor_plans",
      "access_type": "const",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "4",
      "used_key_parts": ["plan_id"],
      "ref": ["const"],
      "rows": 1,
      "filtered": 100
    },
    "table": {
      "table_name": "cscart_vendor_plan_descriptions",
      "access_type": "const",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "10",
      "used_key_parts": ["plan_id", "lang_code"],
      "ref": ["const", "const"],
      "rows": 1,
      "filtered": 100
    },
    "table": {
      "table_name": "manual_rating",
      "access_type": "const",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "102",
      "used_key_parts": ["object_id", "object_type"],
      "ref": ["const", "const"],
      "rows": 1,
      "filtered": 100
    }
  }
}