Philipp Moritz created ARROW-1692:
-------------------------------------
Summary: [Python, Java] UnionArray round trip not working
Key: ARROW-1692
URL: https://issues.apache.org/jira/browse/ARROW-1692
Project: Apache Arrow
Issue Type: Bug
Reporter: Philipp Moritz
Attachments: union_array.arrow
I'm currently working on making pyarrow.serialization data available from the
Java side, one problem I was running into is that it seems the Java
implementation cannot read UnionArrays generated from C++. To make this easily
reproducible I created a clean Python implementation for creating UnionArrays:
https://github.com/apache/arrow/pull/1216
The data is generated with the following script:
```
import pyarrow as pa
binary = pa.array([b'a', b'b', b'c', b'd'], type='binary')
int64 = pa.array([1, 2, 3], type='int64')
types = pa.array([0, 1, 0, 0, 1, 1, 0], type='int8')
value_offsets = pa.array([0, 0, 2, 1, 1, 2, 3], type='int32')
result = pa.UnionArray.from_arrays([binary, int64], types, value_offsets)
batch = pa.RecordBatch.from_arrays([result], ["test"])
sink = pa.BufferOutputStream()
writer = pa.RecordBatchStreamWriter(sink, batch.schema)
writer.write_batch(batch)
sink.close()
b = sink.get_result()
with open("union_array.arrow", "wb") as f:
f.write(b)
# Sanity check: Read the batch in again
with open("union_array.arrow", "rb") as f:
b = f.read()
reader = pa.RecordBatchStreamReader(pa.BufferReader(b))
batch = reader.read_next_batch()
print("union array is", batch.column(0))
```
I attached the file generated by that script. Then when I run the following
code in Java:
```
RootAllocator allocator = new RootAllocator(1000000000);
ByteArrayInputStream in = new
ByteArrayInputStream(Files.readAllBytes(Paths.get("union_array.arrow")));
ArrowStreamReader reader = new ArrowStreamReader(in, allocator);
reader.loadNextBatch()
```
I get the following error:
```
| java.lang.IllegalArgumentException thrown: Could not load buffers for field
test: Union(Sparse, [22, 5])<0: Binary, 1: Int(64, true)>. error message: can
not truncate buffer to a larger size 7: 0
| at VectorLoader.loadBuffers (VectorLoader.java:83)
| at VectorLoader.load (VectorLoader.java:62)
| at ArrowReader$1.visit (ArrowReader.java:125)
| at ArrowReader$1.visit (ArrowReader.java:111)
| at ArrowRecordBatch.accepts (ArrowRecordBatch.java:128)
| at ArrowReader.loadNextBatch (ArrowReader.java:137)
| at (#7:1)
```
It seems like Java is not picking up that the UnionArray is Dense instead of
Sparse. After changing the default in
java/vector/src/main/codegen/templates/UnionVector.java from Sparse to Dense, I
get this:
```
jshell> reader.getVectorSchemaRoot().getSchema()
$9 ==> Schema<list: Union(Dense, [0])<: Struct<list: List<item: Union(Dense,
[0])<: Int(64, true)>>>>>
```
but then reading doesn't work:
```
jshell> reader.loadNextBatch()
| java.lang.IllegalArgumentException thrown: Could not load buffers for field
list: Union(Dense, [1])<: Struct<list: List<$data$: Union(Dense, [5])<: Int(64,
true)>>>>. error message: can not truncate buffer to a larger size 1: 0
| at VectorLoader.loadBuffers (VectorLoader.java:83)
| at VectorLoader.load (VectorLoader.java:62)
| at ArrowReader$1.visit (ArrowReader.java:125)
| at ArrowReader$1.visit (ArrowReader.java:111)
| at ArrowRecordBatch.accepts (ArrowRecordBatch.java:128)
| at ArrowReader.loadNextBatch (ArrowReader.java:137)
| at (#8:1)
```
Any help with this is appreciated!
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