apache / apache/arrow-java

[Java] Unexpected RecordBatch length when saving empty table to file with compression

Aperta
#194 0 commenti 0 reazioni 0 assegnatari Vedi su GitHub
Type: bug
Lingua principale
Java
Stelle
94
Fork
152
Merge medio
3g 16h
PR unite (30g)
11

Descrizione

### Describe the bug, including details regarding any error messages, version, and platform.

This might be more of a usage question since I couldn't find anything in the format docs on how to set the length field with compression.

The issue is that if I try to read an empty table with the [Julia extension](https://github.com/apache/arrow-julia) it just hangs. The reason for this seems to be that it [only checks](https://github.com/apache/arrow-julia/blob/e893c327f177f5a4d5efeab831df0fe93ab4ec5b/src/table.jl#L518-L529) the length field in the RecordBatch when deciding whether to attempt to decode and not the length read from the first 8 bytes of the data.

The file created by the code below is readable by both pyarrow and the java implementation, so chances are that the Julia implementation is doing it wrong (I will open an issue there as well). Is there some reference to how one shall interpret the length field in RecordBatch when using compression?

Code to create an empty table in case I'm doing something wrong

```java
public static void main(String[] args) {
try (BufferAllocator allocator = new RootAllocator()) {
Field name = new Field("name", FieldType.nullable(new ArrowType.Utf8()), null);
Field age = new Field("age", FieldType.nullable(new ArrowType.Int(32, true)), null);
Schema schemaPerson = new Schema(asList(name, age));
try(
VectorSchemaRoot vectorSchemaRoot = VectorSchemaRoot.create(schemaPerson, allocator)
){
vectorSchemaRoot.allocateNew(); // Needed?
vectorSchemaRoot.setRowCount(0); // Needed?
File file = new File("randon_access_to_file.arrow");
try (
FileOutputStream fileOutputStream = new FileOutputStream(file);
ArrowFileWriter writer = new ArrowFileWriter(vectorSchemaRoot, null, fileOutputStream.getChannel(),
null, IpcOption.DEFAULT,
CommonsCompressionFactory.INSTANCE, CompressionUtil.CodecType.ZSTD)
) {
writer.start();
writer.writeBatch();
writer.end();
System.out.println("Record batches written: " + writer.getRecordBlocks().size() + ". Number of rows written: " + vectorSchemaRoot.getRowCount());
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
```

When I tried saving a compressed empty table using pyarrow I got 0 as the length field and the Julia implementation could read the table without hanging.

Disclaimer: I don't have a working python installation so I did this though PythonCall. Hopefully I managed to remove all the Julia-isms so that it runs in python:
```python
schema = pa.schema([pa.field('nums', pa.int32())])

with pa.OSFile('bigfile.arrow', 'wb') as sink:
with pa.ipc.new_file(sink, schema, options=pa.ipc.IpcWriteOptions(compression='zstd'))) as writer:
batch = pa.record_batch([pa.array([], type=pa.int32())], schema)
writer.write(batch)
```

### Component(s)

Java

Guida per i contributori

Apri la guida per i contributori

Direzione di ricerca

Inizia ispezionando ArrowFileWriter di Java e la gestione della compressione IPC utilizzata dall'esempio, quindi confronta il suo RecordBatch compresso vuoto con l'output di pyarrow e la logica di table.jl del reader Julia. Conferma l'interpretazione prevista del campo di lunghezza e aggiungi un test di regressione che copra un batch compresso vuoto; il lavoro è completato quando il file generato è interoperabile senza che il reader rimanga bloccato.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Valutazione

Stack tecnologico
java
Ambito
data-engineering
Tipo di issue
Bug
Difficoltà
4/5
Tempo stimato
3-5 giorni
Stato di attività
Ferma
Chiarezza
Abbastanza chiara
Idoneità per principianti
35/100

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