[jira] [Created] (IMPALA-6658) Parquet RLE encoding can waste space with small repeated runs

["Csaba Ringhofer (JIRA)" <ji...@apache.org>]

Csaba Ringhofer created IMPALA-6658:
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             Summary: Parquet RLE encoding can waste space with small repeated runs
                 Key: IMPALA-6658
                 URL: https://issues.apache.org/jira/browse/IMPALA-6658
             Project: IMPALA
          Issue Type: Improvement
          Components: Backend
            Reporter: Csaba Ringhofer


Currently RleEncoder creates repeated runs from 8 repeated values, which can be less space efficient than bit-packed if bit width is 1 or 2. In the worst case, the whole data page can be ~2X larger if bit width is 1, and ~1.25X larger if bit is 2 compared to bit-packing.

A comment in rle_encoding.h writes different numbers, but it probably does not calculate with the overhead of splitting long runs into smaller ones (every run adds +1 byte for its length): https://github.com/apache/impala/blob/8079cd9d2a87051f81a41910b74fab15e35f36ea/be/src/util/rle-encoding.h#L62

Note that if the data page is compressed, this size difference probably disappears, but the larger uncompressed buffer size can still affect  performance.

Parquet RLE encoding is described here: https://github.com/apache/parquet-
format/blob/master/Encodings.md#run-length-encoding--bit-packing-hybrid-rle--3



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