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 = 'de' 
  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 
  (
    status IN('A')
  ) 
  AND (
    FIND_IN_SET(1, storefronts) 
    OR storefronts IS NULL 
    OR storefronts = ''
  ) 
ORDER BY 
  position asc

Query time 0.00038

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.022835768,
    "nested_loop": [
      {
        "read_sorted_file": {
          "filesort": {
            "sort_key": "cscart_vendor_plans.position",
            "table": {
              "table_name": "cscart_vendor_plans",
              "access_type": "ALL",
              "loops": 1,
              "rows": 4,
              "cost": 0.0115068,
              "filtered": 100,
              "attached_condition": "cscart_vendor_plans.`status` = 'A' and (find_in_set(1,cscart_vendor_plans.storefronts) or cscart_vendor_plans.storefronts is null or cscart_vendor_plans.storefronts = '')"
            }
          }
        }
      },
      {
        "table": {
          "table_name": "cscart_vendor_plan_descriptions",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "10",
          "used_key_parts": ["plan_id", "lang_code"],
          "ref": ["u428615623_ecartifygonje.cscart_vendor_plans.plan_id", "const"],
          "loops": 4,
          "rows": 1,
          "cost": 0.00439696,
          "filtered": 100,
          "attached_condition": "trigcond(cscart_vendor_plan_descriptions.lang_code = 'de')"
        }
      },
      {
        "table": {
          "table_name": "manual_rating",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "102",
          "used_key_parts": ["object_id", "object_type"],
          "ref": ["u428615623_ecartifygonje.cscart_vendor_plans.plan_id", "const"],
          "loops": 4,
          "rows": 1,
          "cost": 0.00439696,
          "filtered": 100,
          "attached_condition": "trigcond(manual_rating.object_type = 'vendor_plan')"
        }
      }
    ]
  }
}