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https://issues.apache.org/jira/browse/ARROW-14026?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17418107#comment-17418107
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David Li edited comment on ARROW-14026 at 9/21/21, 6:51 PM:
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[~westonpace] how did we determine that there was no readahead? What I see is
that there are actually concurrent pulls to the row group generator (the
generator of generators):
(Ignore this; I was mistakenly testing with multiple files which would cover up
issues.)
was (Author: lidavidm):
[~westonpace] how did we determine that there was no readahead? What I see is
that there are actually concurrent pulls to the row group generator (the
generator of generators):
!image-2021-09-21-08-59-50-418.png!
> [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
> Assignee: David Li
> Priority: Major
> Labels: pull-request-available
> Attachments: image-2021-09-21-08-59-50-418.png
>
> Time Spent: 1.5h
> Remaining Estimate: 0h
>
> 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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