[C++][Compute] Support casting a smaller struct to a bigger one if extra fields are nullable #44555
Description
Describe the enhancement requested
Arrow can already cast to a schema with an extra column if that column is nullable but it can't do the same on structs.
More precisely I want the follow unittests to pass
TEST(Cast, StructToBiggerStruct) {
std::vector<std::string> field_names = {"a", "b"};
std::shared_ptr<Array> a, b;
a = ArrayFromJSON(int8(), "[1, 2]");
b = ArrayFromJSON(int8(), "[3, 4]");
ASSERT_OK_AND_ASSIGN(auto src, StructArray::Make({a, b}, field_names));
const auto dest =
arrow::struct_({std::make_shared<Field>("a", int8()),
std::make_shared<Field>("b", int8()),
std::make_shared<Field>("c", int8(), /*nullable=*/false)});
const auto options = CastOptions::Safe(dest);
EXPECT_RAISES_WITH_MESSAGE_THAT(
TypeError,
::testing::HasSubstr("struct fields don't match or are in the wrong order"),
Cast(src, options));
}
TEST(Cast, StructToBiggerNullableStruct) {
std::vector<std::string> field_names = {"a", "b"};
std::shared_ptr<Array> a, b, c;
a = ArrayFromJSON(int8(), "[1, 2]");
b = ArrayFromJSON(int8(), "[3, 4]");
ASSERT_OK_AND_ASSIGN(auto src, StructArray::Make({a, b}, field_names));
const auto type_dest = arrow::struct_(
{std::make_shared<Field>("a", int8()), std::make_shared<Field>("b", int8()),
std::make_shared<Field>("c", int8(), /*nullable=*/true)});
const auto options = CastOptions::Safe(type_dest);
c = ArrayFromJSON(int8(), "[null, null]");
ASSERT_OK_AND_ASSIGN(auto dest, StructArray::Make({a, b, c}, {"a", "b", "c"}));
CheckCast(src, dest);
}
Does this sound like a reasonable thing to support? The motivation comes from delta-io/delta-rs#1610 but I think the fix would require a change to arrow.
Component(s)
C++