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GH-47752: [C++][Python][Parquet] Add minimum compression savings for V2 pages - #52580
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Add min_space_savings to control how much space compression must save. V2 pages below the threshold are written uncompressed. Keep the default behavior and V1 pages unchanged.
Add an option to choose how much space compression must save when writing Parquet files from Python. Add tests as well.
Dataset writing ignored min_space_savings. Move the argument to the end of the call so it reaches the writer. Add a test to check that the setting changes the file size.
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Fixes #47752.
Rationale for this change
Parquet's V2 pages currently keep compression whenever the compressed values are smaller than the original values. Even if it saves very little space, it still does it. For instance, it can compress 100 KB to 99 KB and save it. Even though that's 1% storage efficient, it might not be worth the decompression overhead while reading the file.
What changes are included in this PR?
In the PR, I added an optional
min_space_savingssetting between0.0and1.0. It gives users an option to select the minimum threshold to compress V2 page data values. For instance, if the original file is100 KBand the user hasmin_space_savings = 0.1, it means the compressed file needs to save at least 10% space. As a result, unless the file is<= 90 KB, it won't get saved.I keep the current behavior when the user does not set
min_space_savings.Added C++ and Python implementation.
Are these changes tested?
Are there any user-facing changes?
C++ and Python users can set
min_space_savingswhen writing V2 pages. For example,0.1requires at least 10% space savings before keeping compression.Was AI used for this PR?
ChatGPT helped to set up the build for PyArrow.
Codex helped find a bug in dataset_writing which ignored min_space_savings.
PR code and description written by:
Reviewed before submission by: