使用registerTempTable注册表是一个临时表，生命周期只在所定义的sqlContext或hiveContext实例之中。换而言之，在一个sqlontext（或hiveContext）中registerTempTable的表不能在另一个sqlContext（或hiveContext）中使用。
而saveAsTable则是永久的，只要连接存在，spark再启的时候，这个表还是在的。
官网上的描述：
Unlike theregisterTempTable command, saveAsTable will materialize the contents of the dataframe and create a pointer to the data in the HiveMetastore. Persistent tables will still exist even after your Spark program has restarted, as long as you maintain your connection to the same metastore. A DataFrame for a persistent table can be created by calling the table method on a SQLContext with the name of the table.