读取特定 Parquet 列时，将读取所有列而不是 Parquet-Sql 中给出的单个列

我在 Parquet Documentation 中读到，只有我查询的列，才会读取并处理该列的数据。但是当我看到 Spark-UI 时，我发现完整的文件已被读取。

以下是编写 parquet 文件并在 Spark-Sql 中读取的代码。

object ParquetFileCreator_simple {
  def datagenerate(schema: Schema, ind: Long): GenericRecord ={
    var data: GenericRecord = new GenericData.Record(schema)
    data.put("first", "Pg20 " + ind )
    data.put("appType", "WAP" + ind)
    data
  }
  def main (args: Array[String]): Unit = {

    val conf = new SparkConf().setMaster("local[2]").set("spark.app.name", "merger").set("spark.eventLog.enabled", "true")
    val sc = new SparkContext(conf)
    val sqlc = new org.apache.spark.sql.SQLContext(sc)

    val schemaPath = "/home/a.avsc"
    val schema = new Schema.Parser().parse(new FileInputStream(schemaPath))
    val outFile = "/home/parquet_simple.parquet"
    val outPath: org.apache.hadoop.fs.Path = new org.apache.hadoop.fs.Path(outFile);
    var writer: AvroParquetWriter[GenericRecord] = new AvroParquetWriter[GenericRecord](outPath, schema);
    for(ind <- 1 to 50000000) {
      var r = datagenerate(schema, ind)
      writer.write(r);
    }

    writer.close();

    val df = sqlc.read.parquet(outFile)
    df.registerTempTable("nestedread")
    //var results = df.select("address.pincode")
    val results = sqlc.sql("SELECT first FROM nestedread ")
    results.count()
    //results.foreach(x=>println(x))

    Thread.sleep(60000)
  }

}

我的 Avro 架构是：a.avsc

 {
    "type": "record",
    "name": "FullName",
    "namespace": "com.snapdeal.event.avro",
    "fields": [{
        "name": "first",
        "type": ["string", "null"]
    },  {
                   "name" : "appType",
                   "type" :   {
                     "name" : "app_types",
                     "type" : "enum",
                     "symbols" : [ "WAP", "WEB", "APP" ]
                   }
                 }
    ]
 }

我已经在本地运行了这个。我首先创建了一个 1.7GB 的文件，Parquet 读取该文件。