value型：
输入输出一对一：map flatMap mapPartitions
输入输出多对一：union cartesian
输入输出多对多：groupBy
输出为输入子集：filter distinct subtract sample takeSample
cache型：cache persist
key-value型：
一对一：mapValues
单个RDD聚集：combineByKey reduceByKey partitionBy
两个RDD:cogroup
连接：join leftOutJoin rightOutJoin
无输出（不到hdfs、本地）：foreach
HDFS：saveAsTextFile saveAsObjectFile
scala集合数据类型:collect collectAsMap reduceByKeyLocally lookup count
top reduce fold aggregate
