[GitHub] [arrow-rs] yordan-pavlov commented on issue #1111: ArrowArrayReader Reads Too Many Values From Bit-Packed Runs

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yordan-pavlov commented on issue #1111:
URL: https://github.com/apache/arrow-rs/issues/1111#issuecomment-1003176898


   @tustvold 's work on making the "old" arrow reader faster looks promising, plus I've hit a wall in making the `ArrowArrayReader` faster in some cases, so I agree that it doesn't make sense to try to fix the `ArrowArrayReader` and instead it makes more sense to focus on finishing the work in #1082 .
   
   `ArrowArrayReader` does use def levels to calculate how many values to read, see here https://github.com/apache/arrow-rs/blob/master/parquet/src/arrow/arrow_array_reader.rs#L576 , but reading def levels is done independently of reading of values (one of the main characteristics of `ArrowArrayReader`) and so if the value of `num_values` field from `Page` is incorrect and does indeed include NULL values, then the entire idea of `ArrowArrayReader` falls apart and it won't work correctly.
   
   One reason why I implemented `ArrowArrayReader` in a new struct so that it can easily be swapped out if was necessary, so I guess that time has come. Yes - it will result in a significant reduction in performance of reading string arrays, but correctness is more important than speed and also it would be only until #1082 is finished.
   
   


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