Flink SQL：在 GROUP BY 查询的结果中重复分组键

Question

我想在一个包含 group by 语句的表中使用 Flink SQL 进行简单查询。但在结果中，group by 语句中指定的列有重复的行。是不是因为我使用的是流媒体环境，它不记得状态？

final StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();
final StreamTableEnvironment tableEnv = TableEnvironment.getTableEnvironment(env);
// configure Kafka consumer
Properties props = new Properties();
props.setProperty("bootstrap.servers", "localhost:9092"); // Broker default host:port
props.setProperty("group.id", "flink-consumer"); // Consumer group ID

FlinkKafkaConsumer011<BlocksTransactions> flinkBlocksTransactionsConsumer = new FlinkKafkaConsumer011<>(args[0], new BlocksTransactionsSchema(), props);
flinkBlocksTransactionsConsumer.setStartFromEarliest();

DataStream<BlocksTransactions> blocksTransactions = env.addSource(flinkBlocksTransactionsConsumer);


tableEnv.registerDataStream("blocksTransactionsTable", blocksTransactions);

Table sqlResult
        = tableEnv.sqlQuery(
                "SELECT block_hash, count(tx_hash) " +
                "FROM blocksTransactionsTable " +
                "GROUP BY block_hash");

DataStream<Test> resultStream = tableEnv
        .toRetractStream(sqlResult, Row.class)
        .map(t -> {
            Row r = t.f1;
            String field2 = r.getField(0).toString();
            long count = Long.valueOf(r.getField(1).toString());
            return new Test(field2, count);
        })
        .returns(Test.class);

resultStream.print();

resultStream.addSink(new FlinkKafkaProducer011<>("localhost:9092", "TargetTopic", new TestSchema()));

env.execute();

我对 block_hash 列使用 group by 语句，但我有好几次相同的 block_hash。这是 print() 的结果：

测试{field2='0x2c4a021d514e4f8f0beb8f0ce711652304928528487dc7811d06fa77c375b5e1', count=1} 测试{field2='0x2c4a021d514e4f8f0beb8f0ce711652304928528487dc7811d06fa77c375b5e1', count=1} 测试{field2='0x2c4a021d514e4f8f0beb8f0ce711652304928528487dc7811d06fa77c375b5e1', count=2} 测试{field2='0x780aadc08c294da46e174fa287172038bba7afacf2dff41fdf0f6def03906e60', count=1} 测试{field2='0x182d31bd491527e1e93c4e44686057207ee90c6a8428308a2bd7b6a4d2e10e53', count=1} 测试{field2='0x182d31bd491527e1e93c4e44686057207ee90c6a8428308a2bd7b6a4d2e10e53', count=1}

如何在不使用 BatchEnvironment 的情况下解决此问题？

Answer 1

在流上运行的GROUP BY 查询必须产生更新。考虑以下示例：

SELECT user, COUNT(*) FROM clicks GROUP BY user;

每次clicks 表收到一个新行，相应user 的计数都需要递增和更新。

当您将Table 转换为DataStream 时，这些更新必须在流中进行编码。 Flink 使用撤回和添加消息来做到这一点。通过调用tEnv.toRetractStream(table, Row.class)，您可以将Table table 转换为DataStream<Tuple2<Boolean, Row>。 Boolean 标志很重要，它指示 Row 是从结果表中添加还是撤消。

给定上面的示例查询和输入表clicks as

user | ...
------------
Bob  | ...
Liz  | ...
Bob  | ...

您将收到以下撤稿流

(+, (Bob, 1)) // add first result for Bob
(+, (Liz, 1)) // add first result for Liz
(-, (Bob, 1)) // remove outdated result for Bob
(+, (Bob, 2)) // add updated result for Bob

您需要自己主动维护结果，并按照撤回流的Boolean 标志的指示添加和删除行。