Question

我在1.1-SNAPSHOT版本上使用Apache Flink的Table API来评估流上的SQL查询。

以下是我的代码：

private static final int MAX_RACK_ID = 10;
private static final long PAUSE = 100;
private static final double TEMP_STD = 20;
private static final double TEMP_MEAN = 80;

public static void main(String[] args)
{
     StreamExecutionEnvironment env=StreamExecutionEnvironment.getExecutionEnvironment();
     env.setStreamTimeCharacteristic(TimeCharacteristic.EventTime);
     StreamTableEnvironment tableEnv=TableEnvironment.getTableEnvironment(env);

     DataStream<MonitoringEvent> dstream = env.addSource(new MonitoringEventSource(MAX_RACK_ID, PAUSE, TEMP_STD, TEMP_MEAN));
     tableEnv.registerDataStream("TemperatureData", dstream,"rackid,temperature,timestamp");

     Table tab1 = tableEnv.sql("select STREAM rackid,temperature,timestamp from TemperatureData where temperature>=100");
     DataStream<TemperatureEvent>tempstream=tableEnv.toDataStream(tab1, TemperatureEvent.class);
     tempstream.print();
}

执行此程序时，会抛出以下异常：

Exception in thread "main" org.apache.flink.api.table.TableException: Alias on field reference expression expected.
    at org.apache.flink.api.table.TableEnvironment$$anonfun$4.apply(TableEnvironment.scala:299)
    at org.apache.flink.api.table.TableEnvironment$$anonfun$4.apply(TableEnvironment.scala:292)
    at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
    at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
    at scala.collection.IndexedSeqOptimized$class.foreach(IndexedSeqOptimized.scala:33)
    at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:108)
    at scala.collection.TraversableLike$class.map(TraversableLike.scala:244)
    at scala.collection.mutable.ArrayOps$ofRef.map(ArrayOps.scala:108)
    at org.apache.flink.api.table.TableEnvironment.getFieldInfo(TableEnvironment.scala:292)
    at org.apache.flink.api.table.StreamTableEnvironment.registerDataStreamInternal(StreamTableEnvironment.scala:212)
    at org.apache.flink.api.java.table.StreamTableEnvironment.registerDataStream(StreamTableEnvironment.scala:130)
    at com.yash.flink.Program.main(Program.java:31)

我有几个问题：

使用Apache Flink的Table API在流上编写SQL查询的方法是什么？
如何在Flink中实现此查询？
这是Flink的Table API中的错误吗？

Answer 1

您在Table API中发现了一个限制/错误。问题是由DataStream<MonitoringEvent>作为表注册引起的。你应该做

tableEnv.registerDataStream(
  "TemperatureData", 
  dstream,
  "rackid AS rackid, temperature AS temperature, timestamp AS timestamp"
);

让它发挥作用。在Flink 1.1.0中发布Stream SQL之前，我将确保问题得到解决。

org.apache.flink.api.table.TableException：“期望字段引用表达式的别名”

1 个答案: