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Posted to issues@flink.apache.org by "raminqaf (via GitHub)" <gi...@apache.org> on 2026/09/03 16:37:52 UTC

[PR] [FLINK-37926][table] Support casting from VARIANT to ROW and STRUCTURED [flink]

raminqaf opened a new pull request, #29092:
URL: https://github.com/apache/flink/pull/29092

## What is the purpose of the change

This pull request adds `CAST` and `TRY_CAST` from `VARIANT` to the constructed types `ROW` and `STRUCTURED`, so a variant object can be given a schema. It is stacked on #29073 (VARIANT to `ARRAY`), reuses the same design, and should be reviewed and merged after it.

A constructed cast is the scalar leaf cast applied to every field plus a shape check at each level. The recursion bottoms out at the same scalar casts the primitive and string rules perform, so no new leaf semantics are introduced.

Key semantics:
- Fields match by name, not by position, since a JSON object is unordered. Name matching is case sensitive. A `ROW` declared without field names uses the default names `f0`, `f1`, and so on, which must then be present in the object.
- A field absent from the object fails the cast, whether the target field is nullable or not.
- A field present but set to a variant null maps to SQL `NULL` when the field is nullable and fails the cast when it is `NOT NULL`.
- Object fields the target does not name are dropped, so the row is a projection.
- A `VARIANT` target field takes the identity cast and keeps a variant null as a variant null rather than turning it into SQL `NULL`. This matches `ARRAY<VARIANT>` and the top level VARIANT cast.
- Leaves are never parsed. A stored string does not reach an integer target.
- `STRUCTURED` shares the `RowData` representation and is served by the same rule.
- If any field cast fails, the whole cast fails, and `TRY_CAST` returns `NULL` for the entire value.

## Brief change log

- Add `VariantToRowCastRule` that casts a variant object to `ROW` or `STRUCTURED`, matching fields by name and casting each field with the existing VARIANT to leaf rules.
- Allow the explicit cast in `LogicalTypeCasts` when `VARIANT` casts to every target field type.
- Document the VARIANT to `ROW` and `STRUCTURED` cast in the data types reference.

## Verifying this change

This change added tests and can be verified as follows:
- `CastRulesTest`: by-name matching, free field order, absent field, present variant null in nullable and `NOT NULL` fields, projection of extra fields, nested rows and arrays, an unnamed `ROW`, `ROW<VARIANT>` with and without a variant null, and a `STRUCTURED` target.
- `CastFunctionITCase`: end to end SQL and Table API cases, including `ROW<VARIANT>` round trips back to a concrete row.
- `LogicalTypeCastsTest` and `CastRuleProviderTest`: castability and rule resolution.

## 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? yes
- If yes, how is the feature documented? docs

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