Airblader opened a new pull request #16984:
URL: https://github.com/apache/flink/pull/16984
## What is the purpose of the change
Currently, if a source implements `SupportsReadingMetadata`, but not
`SupportsProjectionPushDown`, all metadata declared in the table's schema will
be pushed into the source. With this change, sources can let the planner push
only required metadata keys (i.e. after projection) into the source even if the
source does not support projection push-down of physical columns.
To preserve compatibility this change is an opt-in for a source through a
new method in the `SupportsReadingMetadata` interface. This might be a problem
otherwise for sources which shouldn't be instantiated more than once as
different projections would lead to different instances.
This depends on #16956 since it reworks the change made there. Opening as a
draft in the meantime.
## Verifying this change
This change added tests and can be verified as follows:
* `PushProjectIntoTableSourceScanRuleTest`
## 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, Kubernetes/Yarn, 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? JavaDocs
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