[GitHub] [flink] hequn8128 opened a new pull request #8550: [FLINK-12401][table] Support incremental emit under AccRetract mode for non-window streaming FlatAggregate on Table API

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hequn8128 opened a new pull request #8550: [FLINK-12401][table] Support incremental emit under AccRetract mode for non-window streaming FlatAggregate on Table API
URL: https://github.com/apache/flink/pull/8550
 
 
   
   ## What is the purpose of the change
   
   This pull request supports incremental emit under AccRetract mode for non-window streaming FlatAggregate on Table API
   
   As described in [Flip-29](https://cwiki.apache.org/confluence/pages/viewpage.action?pageId=97552739), there are two output modes for non-window streaming flatAggregate. One is emitting with full values, the other is emitting with incremental values. [FLINK-10977](https://issues.apache.org/jira/browse/FLINK-10977) supports the former one, this jira is going to support the latter one under AccRetract mode. The reason that Acc mode has not been converted by this jira is that we can't define keys on TableAggregateFunction. Once we support define keys on the output of flatAggregate, we can support the AccMode. 
   
   
   ## Brief change log
   
   There are four commits for this PR:
     - 3a39d63 adds the RetractableCollector. The collector collects a record and forwards it. The collector can output retract messages with the retract. The collector only use it in `emitRetractValueIncrementally`.
     - 03019de adjusts the codgen part. It takes the `emitRetractValueIncrementally` method into consideration and also generates the retract methods if it is a Retractable collector.
     - 611a02c This is the runtime function part. Minor changes in this part. The big difference is retractions will be generated by TableAggregateFunction manually if incremental emit value methods have been defined.
     - a9921d5 Adjusts documents about `emitRetractValueIncrementally`.
   
   
   ## Verifying this change
   
   This change added tests and can be verified as follows:
     - Add `testEmitRetractValueIncrementally()` ITCase to test the incremental emit.
     - Add `testTableAggregateEmitRetractValueIncrementally()` harness test to test the incremental emit.
   
   ## Does this pull request potentially affect one of the following parts:
   
     - Dependencies (does it add or upgrade a dependency): (no)
     - The public API, i.e., is any changed class annotated with `@Public(Evolving)`: (yes)
     - The serializers: (no)
     - The runtime per-record code paths (performance sensitive): (no)
     - Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (no)
     - The S3 file system connector: (no)
   
   ## Documentation
   
     - Does this pull request introduce a new feature? (yes)
     - If yes, how is the feature documented? (docs)
   

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