IMHO, this is valid. As you have demonstrated in the example, a sliced struct array will result in a length shorter than its child arrays. This kind of flexibility can make it easy to reuse child arrays within the struct array.
> Struct: a nested layout consisting of a collection of named child fields > each having the same length but possibly different types. I think the `the same length` means the length of the struct array, this is similar in the case of RecordBatch where the `num_rows` of a RecordBatch can be different to the length of its fields. Best, Gang On Sat, May 6, 2023 at 1:45 AM Weston Pace <weston.p...@gmail.com> wrote: > We allow arrays to have a shorter length than their buffers. Is it also > legal for a struct array to have a shorter length than its child arrays? > For example, in C++, I can create this today by slicing a struct array: > > ``` > std::shared_ptr<StructArray> my_array = > std::dynamic_pointer_cast<StructArray>(array); > ASSERT_EQ(my_array->length(), 4); > ASSERT_EQ(my_array->field(0)->length(), 4); > auto sliced = std::dynamic_pointer_cast<StructArray>(my_array->Slice(2)); > ASSERT_EQ(sliced->length(), 2); > // Note: StructArray::field pushes its offset and length into the created > array > ASSERT_EQ(sliced->field(0)->length(), 2); > // However, the actually ArrayData objects show the truth > ASSERT_EQ(sliced->data()->child_data[0]->length, 4); > // Our validator thinks this is ok > ASSERT_OK(sliced->ValidateFull()); > ``` > > The only reference I can find in the spec is this: > > > Struct: a nested layout consisting of a collection of named child fields > each > > having the same length but possibly different types. > > This seems to suggest that the C++ implementation is doing something > incorrect. > > I'm asking because I've started to encounter some issues relating to > this[1][2] and I'm not sure if the struct array itself is the issue or the > fact that we aren't expecting these kinds of struct arrays is the problem. > > [1] https://github.com/apache/arrow/issues/35450 > [2] https://github.com/apache/arrow/issues/35452 >
