浅谈DataFrame和SparkSql取值误区

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 dataframe
import org.apache.spark.sql.sqlcontext
import 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：

以上这篇浅谈dataframe和sparksql取值误区就是小编分享给大家的全部内容了，希望能给大家一个参考，也希望大家多多支持服务器之家。

原文链接：https://blog.csdn.net/silentwolfyh/article/details/51669839