使用group by查询实例化视图，latest_by_offset两次返回相同的键

Question

我正在按照这个link查询一个实体化视图，并期望一个group by为每个键只返回一行，但事实并非如此(sensor-1在下面的查询中出现了两次)：

ksql> SELECT sensor,                                                                                                                                
>           LATEST_BY_OFFSET(area) AS area,                                                                                                         
>           LATEST_BY_OFFSET(reading) AS last                                                                                                       
>    FROM readings                                                                                                                                  
>    GROUP BY sensor                                                                                                                                
>    EMIT CHANGES;                                                                                                                                  
+------------------------------------------------+------------------------------------------------+------------------------------------------------+
|SENSOR                                          |AREA                                            |LAST                                            |
+------------------------------------------------+------------------------------------------------+------------------------------------------------+
|sensor-1                                        |wheel                                           |45                                              |
|sensor-2                                        |motor                                           |41                                              |
|sensor-1                                        |wheel                                           |92                                              |

与实例化视图的结果相同：

CREATE TABLE latest_readings AS
    SELECT sensor,
           LATEST_BY_OFFSET(area) AS area,
           LATEST_BY_OFFSET(reading) AS last
    FROM readings
    GROUP BY sensor
    EMIT CHANGES;

这似乎与Is it possible to get the latest value for a message key from kafka messages中的robin-moffatt答案不同

我错过了什么吗？

Answer 1

据我所知，emit changes将在更新发生时推出更新，因此，当给定键的集合发生更新时，将发出对该键的更改。