{
    "type": "record",
    "name": "main",
    "fields": [
        {"name": "a", "type": "long"},
        {"name": "b", "type": {
            "type": "record",
            "name": "sub1",
            "fields": [
                {"name": "b1_str", "type": "string"},
                {"name": "b2_null_str", "type": ["null", "string"]},
                {"name": "b3_null_str_double", "type": ["null", "string", "double"]},
                {"name": "b4_null_sub1", "type": ["null", "sub1"]}
            ]
        }}
    ]
  }