[jira] [Commented] (ARROW-14026) [C++] Batch readahead not working correctly in Parquet scanner

["David Li (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ARROW-14026?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17416653#comment-17416653 ] 

David Li commented on ARROW-14026:
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Though maybe with the way the Parquet reader works, the ChunkedArrays will always have a single chunk? We'd have to dig more into the Parquet reader.

> [C++] Batch readahead not working correctly in Parquet scanner
> --------------------------------------------------------------
>
>                 Key: ARROW-14026
>                 URL: https://issues.apache.org/jira/browse/ARROW-14026
>             Project: Apache Arrow
>          Issue Type: Bug
>          Components: C++
>            Reporter: Weston Pace
>            Priority: Major
>
> The parquet scanner implements batch readahead by applying a readahead generator to the generator returned by parquet::arrow::FileReader::GetRecordBatchGenerator.  However, that generator is constructed with MakeConcatenatedGenerator which, regrettably, has this comment:
> > This generator is async-reentrant but will never pull from source reentrantly and will never pull from any subscription reentrantly.
> This effectively prevents any batch readahead from happening and the file is always read one batch at a time.  Part of the problem seems to be that ReadOneRowGroup in reader.cc returns a RecordBatchGenerator when it seems it should be able to return a RecordBatch.  For the testing I am doing I changed this to return a single record batch which allowed me to get rid of the concatenated generator and batch readahead appeared to work properly but I didn't fully confirm the correctness of this.



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