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Posted to issues@arrow.apache.org by "Wes McKinney (Jira)" <ji...@apache.org> on 2019/08/21 16:48:00 UTC

[jira] [Commented] (ARROW-2296) [C++] Add num_rows to file footer

    [ https://issues.apache.org/jira/browse/ARROW-2296?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16912507#comment-16912507 ] 

Wes McKinney commented on ARROW-2296:
-------------------------------------

At minimum having a method in C++ to provide this information (without computing it yourself) seems useful. We don't need to change the file format

> [C++] Add num_rows to file footer
> ---------------------------------
>
>                 Key: ARROW-2296
>                 URL: https://issues.apache.org/jira/browse/ARROW-2296
>             Project: Apache Arrow
>          Issue Type: Improvement
>          Components: C++, Format
>            Reporter: Lawrence Chan
>            Priority: Minor
>             Fix For: 0.15.0
>
>
> Maybe I'm overlooking something, but I don't see something on the API surface to get the number of rows in a arrow file without reading all the record batches. This is useful when we want to read into contiguous buffers, because it allows us to allocate the right sizes up front.
> I'd like to propose that we add `num_rows` as a field in the file footer so it's easy to query without reading the whole file.
> Meanwhile, before we get that added to the official format fbs, it would be nice to have a method that iterates over the record batch headers and sums up the lengths without reading the actual record batch body.



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