tragicjun opened a new pull request #13921:
URL: https://github.com/apache/flink/pull/13921
## What is the purpose of the change
Fetching first N rows is a special case of TopN, which currently is
supported by the Rank operator. However, the full-fledged TopN implementation
is heavyweight that involves sort key comparison and raw row buffering. This
leads to large state and memory buffer that is not needed for the first-n-row
case. This PR introduces a lightweight variant of TopN function to handle it
more efficiently.
## Brief change log
- Add `AppendOnlyFirstNFunction` as a variant of AppendOnlyTopNFunction
- Modify `StreamExecRank` to instantiate `AppendOnlyFirstNFunciton` when
the first-n-row case is determined
- Add `AppendOnlyFirstNFunctionTest` as unit test of
`AppendOnlyFirstNFunction`
## Verifying this change
This change added tests and can be verified as follows:
- Added unit tests in`AppendOnlyFirstNFunctionTest`
## 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)`: ( no)
- The serializers: (no)
- The runtime per-record code paths (performance sensitive): (no)
- Anything that affects deployment or recovery: JobManager (and its
components), Checkpointing, Kubernetes/Yarn/Mesos, ZooKeeper: (no)
- The S3 file system connector: (no)
## Documentation
- Does this pull request introduce a new feature? (no)
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