feat(arrow): read unshredded variant columns#2880
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Project variant columns on read by mapping their metadata/value leaves (which carry no field id — only the enclosing group does) to the variant's field id, mirroring Java's PruneColumns: the whole group is projected with no type-promotion check. Shredded variants (typed_value present) are rejected until we can reconstruct them. collect_variant_leaves numbers leaves exactly like Fields::filter_leaves, including its Dictionary/RunEndEncoded unwrapping and Union descent, so a sibling of those kinds can't shift a variant's leaves onto the wrong column. The pruned schema used for the type-promotion check includes variant leaves too, keeping map<_, variant> a well-formed two-field map. Variant identity is recovered from the Iceberg schema: fields the table schema declares as variants are tagged with the arrow.parquet.variant extension so arrow_schema_to_schema folds the storage struct back into Type::Variant. This mirrors iceberg-java's TypeWithSchemaVisitor, which recognizes a variant by the Parquet annotation OR the Iceberg type, and avoids depending on the Parquet variant annotation being present.
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What changes are included in this PR?
Project variant columns on read by mapping their metadata/value leaves (which carry no field id — only the enclosing group does) to the variant's field id, mirroring Java's PruneColumns: the whole group is projected with no type-promotion check. Shredded variants (typed_value present) are rejected until we can reconstruct them.
collect_variant_leaves numbers leaves exactly like Fields::filter_leaves, including its Dictionary/RunEndEncoded unwrapping and Union descent, so a sibling of those kinds can't shift a variant's leaves onto the wrong column. The pruned schema used for the type-promotion check includes variant leaves too, keeping map<_, variant> a well-formed two-field map.
Variant identity is recovered from the Iceberg schema: fields the table schema declares as variants are tagged with the arrow.parquet.variant extension so arrow_schema_to_schema folds the storage struct back into Type::Variant. This mirrors iceberg-java's TypeWithSchemaVisitor, which recognizes a variant by the Parquet annotation OR the Iceberg type, and avoids depending on the Parquet variant annotation being present.
Are these changes tested?
yes
Based on #2188 and #2840 and