Allow converting empty pyarrow.RecordBatch to arrow::RecordBatch

[Michael-J-Ward]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
datafusion-python currently errors when calling select count(*) from t when t is a pyarrow.Dataset.

The resulting pyarrow.RecordBatch contains no rows and has a schema with no columns, but it does have num_rows set to the correct number.

Describe the solution you'd like
Support was added to arrow-rs in #1552 for a RecordBatch with zero columns but non zero row count.

I'd like impl FromPyArrow for RecordBatch to use this functionality.

arrow-rs/arrow/src/pyarrow.rs

Lines 334 to 392 in b711f23

	impl FromPyArrow for RecordBatch {
	fn from_pyarrow_bound(value: &Bound<PyAny>) -> PyResult<Self> {
	// Newer versions of PyArrow as well as other libraries with Arrow data implement this
	// method, so prefer it over _export_to_c.
	// See https://arrow.apache.org/docs/format/CDataInterface/PyCapsuleInterface.html
	if value.hasattr("__arrow_c_array__")? {
	let tuple = value.getattr("__arrow_c_array__")?.call0()?;
	if !tuple.is_instance_of::<PyTuple>() {
	return Err(PyTypeError::new_err(
	"Expected __arrow_c_array__ to return a tuple.",
	));
	}
	let schema_capsule = tuple.get_item(0)?;
	let schema_capsule = schema_capsule.downcast::<PyCapsule>()?;
	let array_capsule = tuple.get_item(1)?;
	let array_capsule = array_capsule.downcast::<PyCapsule>()?;
	validate_pycapsule(schema_capsule, "arrow_schema")?;
	validate_pycapsule(array_capsule, "arrow_array")?;
	let schema_ptr = unsafe { schema_capsule.reference::<FFI_ArrowSchema>() };
	let ffi_array = unsafe { FFI_ArrowArray::from_raw(array_capsule.pointer() as _) };
	let array_data = unsafe { ffi::from_ffi(ffi_array, schema_ptr) }.map_err(to_py_err)?;
	if !matches!(array_data.data_type(), DataType::Struct(_)) {
	return Err(PyTypeError::new_err(
	"Expected Struct type from __arrow_c_array.",
	));
	}
	let array = StructArray::from(array_data);
	// StructArray does not embed metadata from schema. We need to override
	// the output schema with the schema from the capsule.
	let schema = Arc::new(Schema::try_from(schema_ptr).map_err(to_py_err)?);
	let (_fields, columns, nulls) = array.into_parts();
	assert_eq!(
	nulls.map(|n| n.null_count()).unwrap_or_default(),
	0,
	"Cannot convert nullable StructArray to RecordBatch, see StructArray documentation"
	);
	return RecordBatch::try_new(schema, columns).map_err(to_py_err);
	}
	validate_class("RecordBatch", value)?;
	// TODO(kszucs): implement the FFI conversions in arrow-rs for RecordBatches
	let schema = value.getattr("schema")?;
	let schema = Arc::new(Schema::from_pyarrow_bound(&schema)?);
	let arrays = value.getattr("columns")?;
	let arrays = arrays
	.downcast::<PyList>()?
	.iter()
	.map(|a| Ok(make_array(ArrayData::from_pyarrow_bound(&a)?)))
	.collect::<PyResult<_>>()?;
	let batch = RecordBatch::try_new(schema, arrays).map_err(to_py_err)?;
	Ok(batch)
	}
	}

Additional Context

datafusion-python issue: apache/datafusion-python#800

[alamb]

label_issue.py automatically added labels {'arrow'} from #6320