Make DELTA_LENGTH_BYTE_ARRAY default encoding for binary values
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Assessment
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Newbie friendliness
- 48/100
- Issue type
- Feature
- Clarity
- Mostly clear
- Activity status
- Stale
- Tech stack
- java
- Domain
- data-engineering
Research direction
No source file or test is named in the issue. Start by locating the V1 and V2 default encoding selection in parquet-java's Core implementation, then inspect the existing encoding tests. Done means BYTE_ARRAY columns use DELTA_LENGTH_BYTE_ARRAY by default while other encoding behavior remains unchanged.
Written by the indexing model from the issue text.
Description
Describe the enhancement requested
The current default for V1 pages is PLAIN encoding. This encoding mixes string length with string data. This is inefficient for for skipping N values, as the encoding does not allow random access. It's also slow to decode as the interleaving of lengths with data does not allow efficient batched implementations and forces most implementations to make copies of the data to fit the usual representation of separate offsets and data for strings.
DELTA_LENGTH_BYTE_ARRAY has none of the above problems as it separates offsets and data. The parquet-format spec also seems to recommend this
https://github.com/apache/parquet-format/blob/c70281359087dfaee8bd43bed9748675f4aabe11/Encodings.md?plain=1#L299
### Delta-length byte array: (DELTA_LENGTH_BYTE_ARRAY = 6)
Supported Types: BYTE_ARRAY
This encoding is always preferred over PLAIN for byte array columns.
V2 pages use DELTA_BYTE_ARRAY as the default encoding, this is an improvement over PLAIN but adds complexity which makes it slower to decode than DELTA_LENGTH_BYTE_ARRAY with the potential benefit of lower storage requirements.
JMH benchmarks in Trino's parquet reader at io.trino.parquet.reader.BenchmarkBinaryColumnReader showed that DELTA_LENGTH_BYTE_ARRAY can be decoded at over 5X speed and DELTA_BYTE_ARRAY at over 2X the speed of decoding PLAIN encoding.
Given the above recommendation of parquet-format spec and significant performance difference, the reference implementation here should be updated to use DELTA_LENGTH_BYTE_ARRAY by default.
Component(s)
Core
- Dominant language
- Java
- Stars
- 3.1k
- Forks
- 1.6k
- Avg merge
- 6d 16h
- Merged PRs (30d)
- 36
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