viirya commented on PR #2040:
URL: https://github.com/apache/arrow-rs/pull/2040#issuecomment-1184793174
I was looking for how the reader knows how to ignore it. But I didn't find
it. Then I realized that previously the writer wrote the entire buffers. For
the reader, it sets offsets from zero and reads the buffers from incorrect
positions.
So I think it is a bug that sliced record batch cannot be read correctly
after IPC. We can reproduce it:
```rust
fn create_batch() -> RecordBatch {
let schema = Schema::new(vec![
Field::new("a", DataType::Int32, true),
Field::new("b", DataType::Utf8, true),
]);
let a = Int32Array::from(vec![1, 2, 3, 4, 5]);
let b = StringArray::from(vec!["a", "b", "c", "d", "e"]);
RecordBatch::try_new(Arc::new(schema), vec![Arc::new(a), Arc::new(b)])
.unwrap()
}
let big_record_batch = create_batch();
let offset = 2;
let record_batch_slice = big_record_batch.slice(offset, 3);
```
Original record batches:
```
RecordBatch { schema: Schema { fields: [Field { name: "a", data_type: Int32,
nullable: true, dict_id: 0, dict_is_ordered: false, metadata: None }, Field {
name: "b", data_type: Utf8, nullable: true, dict_id: 0, dict_is_ordered: fal
se, metadata: None }], metadata: {} }, columns: [PrimitiveArray<Int32>
[
1,
2,
3,
4,
5,
], StringArray
[
"a",
"b",
"c",
"d",
"e",
]], row_count: 5 }
RecordBatch { schema: Schema { fields: [Field { name: "a", data_type: Int32,
nullable: true, dict_id: 0, dict_is_ordered: false, metadata: None }, Field {
name: "b", data_type: Utf8, nullable: true, dict_id: 0, dict_is_ordered: fal
se, metadata: None }], metadata: {} }, columns: [PrimitiveArray<Int32>
[
3,
4,
5,
], StringArray
[
"c",
"d",
"e",
]], row_count: 3 }
```
Record batches after IPC serialization/deserialzation:
```
RecordBatch { schema: Schema { fields: [Field { name: "a", data_type: Int32,
nullable: true, dict_id: 0, dict_is_ordered: false, metadata: None }, Field {
name: "b", data_type: Utf8, nullable: true, dict_id: 0, dict_is_ordered: fal
se, metadata: None }], metadata: {} }, columns: [PrimitiveArray<Int32>
[
1,
2,
3,
4,
5,
], StringArray
[
"a",
"b",
"c",
"d",
"e",
]], row_count: 5 }
RecordBatch { schema: Schema { fields: [Field { name: "a", data_type: Int32,
nullable: true, dict_id: 0, dict_is_ordered: false, metadata: None }, Field {
name: "b", data_type: Utf8, nullable: true, dict_id: 0, dict_is_ordered: fal
se, metadata: None }], metadata: {} }, columns: [PrimitiveArray<Int32>
[
null,
null,
5,
], StringArray
[
"a",
"b",
"c",
]], row_count: 3 }
```
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