Le 23/01/2020 à 18:16, John Muehlhausen a écrit : > Perhaps related to this thread, are there any current or proposed tools to > transform columns for fixed-length data types according to a "shuffle?" > For precedent see the implementation of the shuffle filter in hdf5. > https://support.hdfgroup.org/ftp/HDF5//documentation/doc1.6/TechNotes/shuffling-algorithm-report.pdf > > For example, the column (length 3) would store bytes 00 00 00 00 00 00 00 > 00 00 01 02 03 to represent the three 32-bit numbers 00 00 00 01 00 00 00 > 02 00 00 00 03 (I'm writing big-endian even if that is not actually the > case). > > Value(1) would return 00 00 00 02 by referring to some metadata flag that > the column is shuffled, stitching the bytes back together at call time. > > Thus if the column pages were backed by a memory map to something like > zfs/gzip-9 (my actual use-case), one would expect approx 30% savings in > underlying disk usage due to better run lengths. > > It would enable a space/time tradeoff that could be useful? The filesystem > itself cannot easily do this particular compression transform since it > benefits from knowing the shape of the data.
For the record, there's a pull request adding this encoding to the Parquet C++ specification. Regards Antoine.
