实测可以从1s降低到76ms
scala> val df1 = spark.read.json("/testdata/emp.json") //需要上传到hdfs
df1: org.apache.spark.sql.DataFrame = [comm: string, deptno: bigint ... 6 more fields]
scala> df1.registerTempTable("emp")
warning: there was one deprecation warning; re-run with -deprecation for details
scala> spark.sql("select * from emp").show
//标记缓存某张表
scala> spark.sqlContext.cacheTable("emp")
//清理所有的缓存
scala> spark.sqlContext.clearCache
//清理某张表
scala> spark.sqlContext.uncacheTable("emp")
scala> spark.sql("select * from emp").show
