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https://issues.apache.org/jira/browse/ARROW-2296?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16394295#comment-16394295
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Wes McKinney commented on ARROW-2296:
-------------------------------------

We could pretty easily add a "total length" field to the file footer, though, 
which would be more convenient 
https://github.com/apache/arrow/blob/master/format/File.fbs#L33

> [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
>
> 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 haveĀ 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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