Airblader opened a new pull request #16956:
URL: https://github.com/apache/flink/pull/16956
## What is the purpose of the change
Suppose we have a source which supports both SupportsProjectionPushDown and
SupportsReadingMetadata. Now we create a table with a declared schema
containing some metadata columns.
If we select from this table including at least one metadata column,
`#applyReadableMetadata` will be called with exactly the metadata keys
remaining after the projection. If we select from the table without selecting
any metadata column, however, `#applyReadableMetadata` is called with _all_
declared metadata columns instead of an empty list. This causes the source to
perform unnecessary work.
This PR fixes this behavior by ensuring that even if no metadata keys remain
after projection, we apply the metadata spec to inform the source properly.
As a smaller side fix we also emphasize in the documentation that
`applyReadableMetadata` must be idempotent since the planner always calls it
once initially with all declared metadata columns.
## Verifying this change
This change is largely covered by existing tests.
## 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, ZooKeeper: no
- The S3 file system connector: no
## Documentation
- Does this pull request introduce a new feature? no
- If yes, how is the feature documented? not applicable
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