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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