Details
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Bug
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Status: Resolved
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Critical
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Resolution: Fixed
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0.15.0
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Linux 5.2.17-200.fc30.x86_64 (Docker)
Description
Given most columns, I can run a loop like:
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; }
(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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Attachments
Issue Links
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