[jira] [Commented] (ARROW-6895) [C++][Parquet] parquet::arrow::ColumnReader: ByteArrayDictionaryRecordReader repeats returned values when calling `NextBatch()`

["Adam Hooper (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ARROW-6895?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17039184#comment-17039184 ] 

Adam Hooper commented on ARROW-6895:
------------------------------------

I just uploaded {{01-fix-arrow-6895.diff}}, which I _imagine_ contains the final pieces needed to make my code snippet work on my uploaded file. I haven't submitted a unit test.

> [C++][Parquet] parquet::arrow::ColumnReader: ByteArrayDictionaryRecordReader repeats returned values when calling `NextBatch()`
> -------------------------------------------------------------------------------------------------------------------------------
>
>                 Key: ARROW-6895
>                 URL: https://issues.apache.org/jira/browse/ARROW-6895
>             Project: Apache Arrow
>          Issue Type: Bug
>          Components: C++
>    Affects Versions: 0.15.0
>         Environment: Linux 5.2.17-200.fc30.x86_64 (Docker)
>            Reporter: Adam Hooper
>            Assignee: Francois Saint-Jacques
>            Priority: Critical
>              Labels: pull-request-available
>             Fix For: 0.16.0
>
>         Attachments: 01-fix-arrow-6895.diff, bad.parquet, reset-dictionary-on-read.diff, works.parquet
>
>          Time Spent: 40m
>  Remaining Estimate: 0h
>
> Given most columns, I can run a loop like:
> {code:cpp}
> std::unique_ptr<parquet::arrow::ColumnReader> columnReader(/*...*/);
> while (nRowsRemaining > 0) {
>     int n = std::min(100, nRowsRemaining);
>     std::shared_ptr<arrow::ChunkedArray> chunkedArray;
>     auto status = columnReader->NextBatch(n, &chunkedArray);
>     // ... and then use `chunkedArray`
>     nRowsRemaining -= n;
> }
> {code}
> (The context is: "convert Parquet to CSV/JSON, with small memory footprint." Used in https://github.com/CJWorkbench/parquet-to-arrow)
> Normally, the first {{NextBatch()}} return value looks like {{val0...val99}}; the second return value looks like {{val100...val199}}; and so on.
> ... but with a {{ByteArrayDictionaryRecordReader}}, that isn't the case. The first {{NextBatch()}} return value looks like {{val0...val100}}; the second return value looks like {{val0...val99, val100...val199}} (ChunkedArray with two arrays); the third return value looks like {{val0...val99, val100...val199, val200...val299}} (ChunkedArray with three arrays); and so on. The returned arrays are never cleared.
> In sum: {{NextBatch()}} on a dictionary column reader returns the wrong values.
> I've attached a minimal Parquet file that presents this problem with the above code; and I've written a patch that fixes this one case, to illustrate where things are wrong. I don't think I understand enough edge cases to decree that my patch is a correct fix.



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