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Posted to jira@arrow.apache.org by "David Li (Jira)" <ji...@apache.org> on 2021/09/17 18:15:00 UTC
[jira] [Assigned] (ARROW-14026) [C++] Batch readahead not working
correctly in Parquet scanner
[ https://issues.apache.org/jira/browse/ARROW-14026?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]
David Li reassigned ARROW-14026:
--------------------------------
Assignee: David Li
> [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
>
> 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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