浅谈DataFrame和SparkSql取值误区

数据库  /  管理员 发布于 5年前   192

1、DataFrame返回的不是对象。

2、DataFrame查出来的数据返回的是一个dataframe数据集。

3、DataFrame只有遇见Action的算子才能执行

4、SparkSql查出来的数据返回的是一个dataframe数据集。

原始数据

scala> val parquetDF = sqlContext.read.parquet("hdfs://hadoop14:9000/yuhui/parquet/part-r-00004.gz.parquet")df: org.apache.spark.sql.DataFrame = [timestamp: string, appkey: string, app_version: string, channel: string, lang: string, os_type: string, os_version: string, display: string, device_type: string, mac: string, network: string, nettype: string, suuid: string, register_days: int, country: string, area: string, province: string, city: string, event: string, use_interval_cat: string, use_duration_cat: string, use_interval: bigint, use_duration: bigint, os_upgrade_from: string, app_upgrade_from: string, page_name: string, event_name: string, error_type: string]

代码

package DataFrameimport org.apache.spark.sql.SQLContextimport org.apache.spark.{SparkConf, SparkContext}/** * Created by yuhui on 2016/6/14. */object DataFrameTest { def main(args: Array[String]) { DataFrameInto() } def DataFrameInto() { val conf = new SparkConf() val sc = new SparkContext(conf) val sqlContext = new SQLContext(sc) val df = sqlContext.read.parquet("hdfs://hadoop14:9000/yuhui/parquet") //df.map(line => printinfo(line.getString(0))) //df.foreach(line => printinfo(line.getString(0)+" , "+line.getString(14)+" , "+line.getString(15))) //df.select("timestamp","country","area").foreach(line=>printinfo(line.toString)) df.registerTempTable("infotable") sqlContext.sql("SELECT timestamp , country , area from infotable").foreach(line=>printinfo(line.toString)) } def printinfo(msg: String) {println("printinfo函数-->" + msg) }}

代码解析

1、df.map(line => printinfo(line.getString(0)))

这段代码不行执行printinfo()函数，因为只有map算子，没有Action算子。

2、df.foreach(line => printinfo(line.getString(0)+" , "+line.getString(14)+" , "+line.getString(15)))

通过Spark的Action算子接收数据进行操作，执行结果如下：

3、df.select("timestamp","country","area").foreach(line=>printinfo(line.toString))

通过DataFrame的API进行操作，再通过Spark的Action算子打印出来，执行结果如下：

4、sqlContext.sql("SELECT timestamp , country , area from infotable").foreach(line=>printinfo(line.toString))

执行结果如下：

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