raminqaf opened a new pull request, #29125:
URL: https://github.com/apache/flink/pull/29125
## What is the purpose of the change
Casting a `VARIANT` to a character string renders a stored scalar the way a
regular SQL cast of that kind would (a boolean becomes `TRUE`, a timestamp uses
the SQL format, a binary value is read as UTF-8). A `VARIANT` holding an object
or an array had no scalar form, so the cast failed and pointed the user at
`JSON_STRING`.
This makes `CAST(v AS STRING)` render an object or array as its JSON
representation instead of failing, reusing `Variant#toJson()` — the same text
`JSON_STRING` and the result-print path already produce. Redirecting users to a
different function for a value that already has a well-defined textual form was
surprising.
Scalar rendering is unchanged, so the only asymmetry is the expected one: a
top-level scalar string still casts unquoted (`foo`), while a string nested
inside a rendered container stays quoted (`"foo"`), matching `JSON_STRING`.
```sql
CAST(PARSE_JSON('["a","b"]') AS STRING) -- ["a","b"] (was: runtime error)
CAST(PARSE_JSON('{"a":1}') AS STRING) -- {"a":1} (was: runtime error)
CAST(PARSE_JSON('"foo"') AS STRING) -- foo (unchanged)
```
Only `OBJECT` and `ARRAY` change here. Scalar kinds whose string rendering
is still unsupported (`TIME` and nanosecond timestamps, tracked in FLINK-40492)
keep failing.
## Brief change log
- `VariantCastUtils.getVariantTypeAsString` renders an `OBJECT`/`ARRAY`
variant via `toJson() `rather than throwing; other unsupported kinds still fail.
- A bounded `CHAR(n)`/`VARCHAR(n)` target trims the rendered JSON like any
other over-length value; `TRY_CAST` returns the JSON rather than `NULL`; a
stored JSON null still casts to SQL NULL.
- Updated the cast-support matrix and prose in data-types.md.
## Verifying this change
This change added tests and can be verified as follows:
- `CastRulesTest`: VARIANT → STRING for an object and an array now assert
the JSON rendering.
- `CastFunctionITCase`: `CAST`/`TRY_CAST` of an object and an array to
STRING return JSON, plus a container cast to a bounded VARCHAR(n) that trims
the rendered JSON.
## 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 (a new castable pair
for VARIANT)
- If yes, how is the feature documented? docs (cast-support matrix)
---
#### Was generative AI tooling used to co-author this PR?
- [x] Yes (please specify the tool below)
Generated-by: Claude Code (Claude Opus 4.8)
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