[jira] [Commented] (ARROW-809) C++: Writing sliced record batch to IPC writes the entire array

["Itai Incze (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ARROW-809?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15965731#comment-15965731 ] 

Itai Incze commented on ARROW-809:
----------------------------------

The reason for the bug seems to be that sliced arrays are tested with the condition {{(array.offset() != 0)}} throughout {{arrow/ipc/writer.cc}}, which doesn't account for a {{\[0:x\]}} slice. 

To test for 0-based slices there's a need to know the number of elements in the original array, which is easily computed in cases like fixed-width array types but could be harder in others.

Another approach could be adding a class member to mark the slices. This could be either by a simple boolean, the original dimensions or a reference to the original array.


> C++: Writing sliced record batch to IPC writes the entire array
> ---------------------------------------------------------------
>
>                 Key: ARROW-809
>                 URL: https://issues.apache.org/jira/browse/ARROW-809
>             Project: Apache Arrow
>          Issue Type: Bug
>          Components: C++
>            Reporter: Itai Incze
>            Priority: Minor
>
> The bug can be triggered through python:
> {code}
> import pyarrow.parquet
> array = pyarrow.array.from_pylist([1] * 1000000)
> rb = pyarrow.RecordBatch.from_arrays([array], ['a'])
> rb2 = rb.slice(0,2)
> with open('/tmp/t.arrow', 'wb') as f:
>   w = pyarrow.ipc.FileWriter(f, rb.schema)
>   w.write_batch(rb2)
>   w.close()
> {code}
> which will result in a big file:
> {code}
> $ ll /tmp/t2.arrow 
> -rw-rw-r-- 1 itai itai 800618 Apr 12 13:22 /tmp/t.arrow
> {code}



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