Improve error message on invalid casts for structs

[mkleen]

Which issue does this PR close?

• Closes Be able to cast struct to have non-nullable inner field #22709

Rationale for this change

#22709 is a non-issue, the behavior is on purpose. This pr improves the error message for invalid casts for structs, making sure the exact error is not swallowed and avoid that it sound like it's a missing feature:

> select arrow_cast(struct(10), 'Struct("c0": non-null Int64)');
Optimizer rule 'simplify_expressions' failed
caused by
This feature is not implemented: Unsupported CAST from Struct("c0": Int64) to Struct("c0": non-null Int64)

becomes:

> select arrow_cast(struct(10), 'Struct("c0": non-null Int64)');
Optimizer rule 'simplify_expressions' failed
caused by
Error during planning: Cannot cast nullable struct field 'c0' to non-nullable field

The current cast behavior is on purpose see:

datafusion/datafusion/common/src/nested_struct.rs

Lines 400 to 408 in 533ef35

	// Ensure nullability is compatible. It is invalid to cast a nullable
	// source field to a non-nullable target field as this may discard
	// null values.
	if source_field.is_nullable() && !target_field.is_nullable() {
	return _plan_err!(
	"Cannot cast nullable struct field '{}' to non-nullable field",
	target_field.name()
	);
	}

.

Most SQL systems treat NULL as a valid value of any type, so a cast that removes nullability would be logically inconsistent unless it raises an error on NULL input. On structs DuckDB/PSQL avoid this whole topic completely:

• DuckDB structs do not allow to enforce non-nulliable on fields.
• PostgreSQL composite type fields can not have NOT NULL constraints.

Since arrow allows non-null struct fields I would stick with the current behavior.

See also #19674 (comment)

What changes are included in this PR?

see above.

Are these changes tested?

Yes.

Are there any user-facing changes?

No.

[mkleen]

This make sure that the error message is not swallowed here.

[mkleen]

@Jefffrey Could you maybe have a look at this?