[FLINK-40410][table-planner] Exclude VARIANT from constant folding

[manner]

What is the purpose of the change

This pull request excludes the VARIANT data type from constant folding VARIANT expressions. This previously resulted in a ClassCastException when running a query such as the following, where a constant expression followed a VARIANT expression in the projection.

SELECT PARSE_JSON('{}'), JSON_STRING(PARSE_JSON('{}'));

The ExpressionReducer.scala in flink-table-planner evaluates constant sub-expressions, so they get computed during compile-time and not during query run-time. It runs in two passes that must agree on which types get a slot in the intermediate (compile-time) result:

• Phase 1 excludes unsupported "object literal" types (ROW/ARRAY/MAP/MULTISET/...) from compile-time evaluation. VARIANT was previously missing from this list and was therefore evaluated at compile-time and added to the intermediate result list.
• Phase 2 has an specific case for VARIANT and treats VARIANT as excluded from compile-time evaluation. So a VARIANT constant gets evaluated and occupies a real slot in phase 1, but phase 2 doesn't know that and never advances its slot index. Then the next reduced expression reads the stale slot containing the VARIANT data and crashes on the bad cast.

Brief change log

• Adds VARIANT to the exclude list of data types that use constant folding

Verifying this change

This change added tests and can be verified as follows:

• Added test in JsonFunctionsITCase.java for VARIANT, where constant folding is enabled

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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Generated-by: Claude Sonnet 5

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CI report:

• 4cee6ba Azure: SUCCESS

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[gustavodemorais]

nit: this comment narrates the bug/ticket story rather than stating the timeless invariant. The test name already establishes it's a regression test, so the JIRA reference and blow-by-blow of what the old bug looked like can move to the commit message. Something tighter reads better long-term, e.g.:

Suggested change

	// Regression test for FLINK-40410: PARSE_JSON(f0) (VARIANT) precedes
	// JSON_STRING(PARSE_JSON(f0)) (STRING) in a constant-folded projection. VARIANT
	// is excluded from folding in ExpressionReducer, like ROW/ARRAY/MAP, and
	// evaluated normally at runtime.
	// VARIANT must be excluded from constant folding, like ROW/ARRAY/MAP,
	// otherwise a constant-folded projection reads a stale slot for it.

[gustavodemorais]

Actually, I'd just drop the comment

[gustavodemorais]

Added one nit. Thanks for working and fixing this @manner

[gustavodemorais]

Actually, I'd just drop the comment