Loading big json dataset raises pyarrow.lib.ArrowNotImplementedError
State: closed | Author: maxzirps (NONE)
Labels: name: bug

## Describe the bug
When trying to create a dataset from a json file with around 25MB, the following error is raised `pyarrow.lib.ArrowNotImplementedError: Unsupported cast from struct<b: int64, c: int64> to struct using function cast_struct`

Splitting the big file into smaller ones and then loading it with the `load_dataset` method did also not work.

Creating a pandas dataframe from it and then loading it with `Dataset.from_pandas` works

## Steps to reproduce the bug
```python
load_dataset("json", data_files="test.json")
```

test.json ~25MB
```json
{"a": {"c": 8, "b": 5}}
{"a": {"b": 7, "c": 6}}
{"a": {"c": 8, "b": 5}}
{"a": {"b": 7, "c": 6}}
{"a": {"c": 8, "b": 5}}
...
```

working.json ~160bytes
```json
{"a": {"c": 8, "b": 5}}
{"a": {"b": 7, "c": 6}}
{"a": {"c": 8, "b": 5}}
{"a": {"b": 7, "c": 6}}
{"a": {"c": 8, "b": 5}}
```

## Expected results
It should load the dataset from the json file without error.

## Actual results
It raises Exception `pyarrow.lib.ArrowNotImplementedError: Unsupported cast from struct<b: int64, c: int64> to struct using function cast_struct`

```
Traceback (most recent call last):
  File "/Users/m/workspace/xxx/project/main.py", line 60, in <module>
    dataset = load_dataset("json", data_files="result.json")
  File "/opt/homebrew/Caskroom/miniforge/base/envs/xxx/lib/python3.9/site-packages/datasets/load.py", line 1627, in load_dataset
    builder_instance.download_and_prepare(
  File "/opt/homebrew/Caskroom/miniforge/base/envs/xxx/lib/python3.9/site-packages/datasets/builder.py", line 607, in download_and_prepare
    self._download_and_prepare(
  File "/opt/homebrew/Caskroom/miniforge/base/envs/xxx/lib/python3.9/site-packages/datasets/builder.py", line 697, in _download_and_prepare
    self._prepare_split(split_generator, **prepare_split_kwargs)
  File "/opt/homebrew/Caskroom/miniforge/base/envs/xxx/lib/python3.9/site-packages/datasets/builder.py", line 1159, in _prepare_split
    writer.write_table(table)
  File "/opt/homebrew/Caskroom/miniforge/base/envs/xxx/lib/python3.9/site-packages/datasets/arrow_writer.py", line 428, in write_table
    pa_table = pa.Table.from_arrays([pa_table[name] for name in self._schema.names], schema=self._schema)
  File "pyarrow/table.pxi", line 1685, in pyarrow.lib.Table.from_arrays
  File "pyarrow/table.pxi", line 630, in pyarrow.lib._sanitize_arrays
  File "pyarrow/array.pxi", line 338, in pyarrow.lib.asarray
  File "pyarrow/table.pxi", line 304, in pyarrow.lib.ChunkedArray.cast
  File "/opt/homebrew/Caskroom/miniforge/base/envs/xxx/lib/python3.9/site-packages/pyarrow/compute.py", line 309, in cast
    return call_function("cast", [arr], options)
  File "pyarrow/_compute.pyx", line 528, in pyarrow._compute.call_function
  File "pyarrow/_compute.pyx", line 327, in pyarrow._compute.Function.call
  File "pyarrow/error.pxi", line 143, in pyarrow.lib.pyarrow_internal_check_status
  File "pyarrow/error.pxi", line 120, in pyarrow.lib.check_status
pyarrow.lib.ArrowNotImplementedError: Unsupported cast from struct<b: int64, c: int64> to struct using function cast_struct
```

## Environment info
- `datasets` version: 1.14.0
- Platform: macOS-12.0.1-arm64-arm-64bit
- Python version: 3.9.7
- PyArrow version: 6.0.0



Comments: Hi,

this issue is similar to https://github.com/huggingface/datasets/issues/3093, so you can either use the solution provided there or try to load the data in one chunk (you can control the chunk size by specifying the `chunksize` parameter (`int`) in `load_dataset`).

@lhoestq Is this worth opening an issue on Jira? Basically, PyArrow doesn't allow casts that change the order of the struct fields because they treat `pa.struct` as an ordered sequence. Reordering fields manually in Python is probably too slow, so I think this needs to be fixed by them to be usable on our side.

I agree I would expect PyArrow to be able to handle this, do you want to open the issue @mariosasko ?
Although maybe it's possible to fix struct casting on our side without hurting performance too much, if it's simply a matter of reordering the arrays in the StructArray

Fixed in #3575, so I'm closing this issue.