Airblader opened a new pull request #17549:
URL: https://github.com/apache/flink/pull/17549
## What is the purpose of the change
This adds support for `JSON_OBJECTAGG` in SQL, Table API, and PyFlink.
supersedes #11369
## Brief change log
* Introduce a test base for built-in aggregate functions. This is akin to
the one we have for scalar functions, but a bit more general to allow providing
arbitrarily many rows of input data.
* Implement `JSON_OBJECTAGG`.
This has been tricky at times, because for SQL the call is hard-coded into
the parser, and aggregate functions are just not easy. After discussion we
decided to apply a similar workaround to what Calcite does, which is actually
implement two separate functions (for NULL / ABSENT ON NULL). This avoids
shipping symbols across the network for each record, and also a bunch of issues
in the current stack related to that, such as symbols being represented as raw
types of `Enum` etc.
We also add a new rule here which wraps arguments of `JSON_OBJECTAGG` (and
soon `JSON_ARRAYAGG`) into `JSON_STRING`. This tremendously simplifies the
runtime implementation for the aggregation function as it only needs to deal
with strings and can simply insert them as raw values.
## Verifying this change
TODO
## Does this pull request potentially affect one of the following parts:
- Dependencies (does it add or upgrade a dependency): (yes / no)
- The public API, i.e., is any changed class annotated with
`@Public(Evolving)`: (yes / no)
- The serializers: (yes / no / don't know)
- The runtime per-record code paths (performance sensitive): (yes / no /
don't know)
- Anything that affects deployment or recovery: JobManager (and its
components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (yes / no / don't know)
- The S3 file system connector: (yes / no / don't know)
## Documentation
- Does this pull request introduce a new feature? (yes / no)
- If yes, how is the feature documented? (not applicable / docs / JavaDocs
/ not documented)
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