[jira] [Commented] (ARROW-5916) [C++] Allow RecordBatch.length to be less than array lengths

["Wes McKinney (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ARROW-5916?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16884058#comment-16884058 ] 

Wes McKinney commented on ARROW-5916:
-------------------------------------

I think that the fields' length needs to be respected in {{GetFieldMetadata}} (since this can vary for nested fields). The appropriate fix is probably here:


> [C++] Allow RecordBatch.length to be less than array lengths
> ------------------------------------------------------------
>
>                 Key: ARROW-5916
>                 URL: https://issues.apache.org/jira/browse/ARROW-5916
>             Project: Apache Arrow
>          Issue Type: New Feature
>            Reporter: John Muehlhausen
>            Priority: Minor
>         Attachments: test.arrow_ipc
>
>
> 0.13 ignored RecordBatch.length.  0.14 requires that RecordBatch.length and array length be equal.  As per [https://lists.apache.org/thread.html/2692dd8fe09c92aa313bded2f4c2d4240b9ef75a8604ec214eb02571@%3Cdev.arrow.apache.org%3E] , we discussed changing this so that RecordBatch.length can be [0,array length].
>  If RecordBatch.length is less than array length, the reader should ignore the portion of the array(s) beyond RecordBatch.length.  This will allow partially populated batches to be read in scenarios identified in the above discussion.
> {code:c++}
>   Status GetFieldMetadata(int field_index, ArrayData* out) {
>     auto nodes = metadata_->nodes();
>     // pop off a field
>     if (field_index >= static_cast<int>(nodes->size())) {
>       return Status::Invalid("Ran out of field metadata, likely malformed");
>     }
>     const flatbuf::FieldNode* node = nodes->Get(field_index);
> *    //out->length = node->length();*
> *    out->length = metadata_->length();*
>     out->null_count = node->null_count();
>     out->offset = 0;
>     return Status::OK();
>   }
> {code}
> Attached is a test IPC File containing a batch with length 1, array length 3.



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