[jira] [Commented] (ARROW-10121) [C++][Python] Variable dictionaries do not survive roundtrip to IPC stream

["Antoine Pitrou (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ARROW-10121?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17204119#comment-17204119 ] 

Antoine Pitrou commented on ARROW-10121:
----------------------------------------

AFAICT, this bug was already present in 0.15.1, and perhaps before.
The problem is basically this snippet in {{IpcFormatWriter::RecordBatch}}:
{code:c++}
    if (!wrote_dictionaries_) {
      RETURN_NOT_OK(WriteDictionaries(batch));
      wrote_dictionaries_ = true;
    }
{code}

The solution isn't entirely trivial: presumably we don't want to write dictionaries again when they are identical. We also don't want to compare them every time. Perhaps compare them by pointer?

> [C++][Python] Variable dictionaries do not survive roundtrip to IPC stream
> --------------------------------------------------------------------------
>
>                 Key: ARROW-10121
>                 URL: https://issues.apache.org/jira/browse/ARROW-10121
>             Project: Apache Arrow
>          Issue Type: Bug
>          Components: C++, Python
>            Reporter: Wes McKinney
>            Assignee: Antoine Pitrou
>            Priority: Blocker
>             Fix For: 2.0.0
>
>
> Failing test case (from dev@ https://lists.apache.org/thread.html/r338942b4e9f9316b48e87aab41ac49c7ffedd45733d4a6349523b7eb%40%3Cdev.arrow.apache.org%3E)
> {code}
> import pyarrow as pa
> from io import BytesIO
> pa.__version__
> schema = pa.schema([pa.field('foo', pa.int32()), pa.field('bar', pa.dictionary(pa.int32(), pa.string()))] )
> r1 = pa.record_batch(
>     [
>         [1, 2, 3, 4, 5],
>         pa.array(["a", "b", "c", "d", "e"]).dictionary_encode()
>     ],
>     schema
> )
> r1.validate()
> r2 = pa.record_batch(
>     [
>         [1, 2, 3, 4, 5],
>         pa.array(["c", "c", "e", "f", "g"]).dictionary_encode()
>     ],
>     schema
> )
> r2.validate()
> assert r1.column(1).dictionary != r2.column(1).dictionary
> sink =  pa.BufferOutputStream()
> writer = pa.RecordBatchStreamWriter(sink, schema)
> writer.write(r1)
> writer.write(r2)
> serialized = BytesIO(sink.getvalue().to_pybytes())
> stream = pa.ipc.open_stream(serialized)
> deserialized = []
> while True:
>     try:
>         deserialized.append(stream.read_next_batch())
>     except StopIteration:
>         break
> assert deserialized[1][1].to_pylist() == r2[1].to_pylist()
> {code}



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