sdf-jkl commented on code in PR #10313:
URL: https://github.com/apache/arrow-rs/pull/10313#discussion_r3731631478
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parquet-variant-compute/src/variant_to_arrow.rs:
##########
@@ -661,6 +674,216 @@ impl<'a> StructVariantToArrowRowBuilder<'a> {
}
}
+/// Builder for converting variant values into a [`UnionArray`].
+///
+/// Each value is dispatched to the union field that most exactly represents
its runtime type
+/// (see [`union_child_rank`]), with ties broken by declaration order. Unions
have no top-level
+/// null buffer, so null rows -- and, in safe mode, values no field can
represent -- become a
+/// null in the [`DataType::Null`] child if the union declares one, otherwise
in the first child.
+pub(crate) struct UnionVariantToArrowRowBuilder<'a> {
+ fields: &'a UnionFields,
+ mode: UnionMode,
+ children: Vec<UnionChildBuilder<'a>>,
+ type_ids: Vec<i8>,
+ /// Dense mode only
+ offsets: Vec<i32>,
+ null_child: usize,
+ cast_options: &'a CastOptions<'a>,
+}
+
+struct UnionChildBuilder<'a> {
+ type_id: i8,
+ builder: VariantToArrowRowBuilder<'a>,
+ len: i32,
+}
+
+impl<'a> UnionVariantToArrowRowBuilder<'a> {
+ fn try_new(
+ fields: &'a UnionFields,
+ mode: UnionMode,
+ cast_options: &'a CastOptions<'a>,
+ capacity: usize,
+ ) -> Result<Self> {
+ // null rows need a child to land in
+ if fields.is_empty() {
+ return Err(ArrowError::InvalidArgumentError(
+ "Casting Variant to a union requires at least one union
field".to_string(),
+ ));
+ }
+ let mut children = Vec::with_capacity(fields.len());
+ for (type_id, field) in fields.iter() {
+ children.push(UnionChildBuilder {
+ type_id,
+ builder: make_typed_variant_to_arrow_row_builder(
+ field.data_type(),
+ cast_options,
+ capacity,
+ )?,
+ len: 0,
+ });
+ }
+ let null_child = fields
+ .iter()
+ .position(|(_, field)| field.data_type() == &DataType::Null)
+ .unwrap_or(0);
Review Comment:
I checked the existing Arrow union policy. `UnionBuilder::append_null`
intentionally writes nulls into child arrays, while `UnionBuilder::build` still
creates those child `Field`s with `nullable = false`. `UnionArray::try_new`
does not validate field nullability, and union nullness is derived from the
selected child array. Therefore the fallback does not need to select a
nullable-declared field.
https://github.com/apache/arrow-rs/blob/e28fd0d0f1f45e5608928eb52af3b12ffe9fee2e/arrow-array/src/builder/union_builder.rs#L285-L317
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