Reading row groups in parallel

[Pyroluk]

Issue description

Hi!

I would like to read parquet files in parallel to significantly speed up a processing pipeline.

using (var parquetReader = await ParquetReader.CreateAsync(File.OpenRead(filePath)))
{
    for (int currentRowGroupIndex = 0; currentRowGroupIndex < parquetReader.RowGroupCount; currentRowGroupIndex++)
    { ... }
}

Works as expected.

using (var parquetReader = await ParquetReader.CreateAsync(File.OpenRead(filePath)))
{
    Parallel.For(0, parquetReader.RowGroupCount, async currentRowGroupIndex =>
    { ... } );
}

But reading the row groups in parallel with Parallel.For or Parallel.ForAsync fails with "random" error messages like System.InvalidOperationException: "don't know how to skip type Double", System.InvalidOperationException: "don't know how to skip type 14", ...

Is there a thread safe way for reading row groups in parallel already implemented?

Cheers!

[aloneguid]

File stream are generally not compatible with parallel processing. You can, however, open file stream per prallel thread i.e. your Parallel.For should perform file opening operation. Or you can introduce a lock on file read, depends on what works better for you. I might state the obvious here, but asynchronous and parallel are not the same thing.

[Pyroluk]

Thanks a lot for the hint.

This works flawlessly:

var rowGroupCount = (await ParquetReader.CreateAsync(filePath)).RowGroupCount;
await Parallel.ForAsync(0, rowGroupCount, async (currentRowGroupIndex, cancellationToken) =>
{
    using (var parquetReader = await ParquetReader.CreateAsync(predictionFilePath))
    using (var rowGroupReader = parquetReader.OpenRowGroupReader(currentRowGroupIndex))
    { ... }
});

I think a short section about reading row groups in parallel would be cool in the documentation: https://aloneguid.github.io/parquet-dotnet/reading.html
When passing a filePath (string) to ParquetReader.CreateAsync(), it might also be possible to handle the concurrency within the library itself. This would be a little bit more elegant, sparing the need to figure out the row group count beforehand with another separate ParquetReader.CreateAsync() call.

Tangent problem: Is there any way to write row groups in parallel?