[GitHub] [arrow-rs] tustvold commented on issue #4118: `ParquetRecordBatchReader` reads overlapping byte ranges

["tustvold (via GitHub)" <gi...@apache.org>]

tustvold commented on issue #4118:
URL: https://github.com/apache/arrow-rs/issues/4118#issuecomment-1520536549

   This is a consequence of #2464 which causes ChunkReader to be created per page, instead of per row group. This change was made to enable page-level predicate push down. We should definitely improve the documentation around ChunkReader, and its implicit assumptions regarding buffering at the application and/or OS level. I will add this to my list.
   
   The reason for the overlapping byte ranges, is that if the `OffsetIndex` isn't read, the reader doesn't know where the pages are located or even how many there are, only the end position of the column chunk. It therefore has to assume a given page may run to the end of the range. If you enable reading the [PageIndex](https://docs.rs/parquet/latest/parquet/arrow/arrow_reader/struct.ArrowReaderOptions.html#method.with_page_index) it shouldn't perform overlapping reads (although it will now need to perform IO to read the page index).
   
   Taking a step back I wonder if you've considered using the [async_reader](https://docs.rs/parquet/latest/parquet/arrow/async_reader/index.html). Not only does this provide a native async interface, but the [AsyncFileReader](https://docs.rs/parquet/latest/parquet/arrow/async_reader/trait.AsyncFileReader.html) interface naturally lends itself to IO pre-fetching. There is even out of the box integration with [object_store](https://docs.rs/parquet/latest/parquet/arrow/async_reader/struct.ParquetObjectReader.html). 
   
   
   
   
   
   


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