[SYSTEMDS-3779] Add ColGroupDDCLZW with LZW-compressed MapToData #2398
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…extending on APreAgg like ColGroupDDC for easier implementation. Idea: store only compressed version of _data vector and important metadata. If decompression is needed we reconstruct the _data vector using the metadata and the compressed _data vector. Decompression takes place at most once. This is just an idea and theres other ways of implementing.
…ng von Decompress
…and decompress and its used data structures compatible.
…lgorithms and try made them compatible.
…DC test for ColGroupDDCTest. Improved compress/decompress methods in LZW class.
…lemted from its Interface.
…mapping This commit adds an initial implementation of ColGroupDDCLZW, a new column group that stores the mapping vector in LZW-compressed form instead of materializing MapToData explicitly. The design focuses on enabling sequential access directly on the compressed representation, while complex access patterns are intended to fall back to DDC. No cache or lazy decompression mechanism is introduced at this stage.
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Summary
This PR introduces a new column group
ColGroupDDCLZWthat stores the mapping vector in LZW-compressed form.Key design points
MapToDatais not stored explicitly; only the compressed LZW representation is kept._dataLZWwithout full decompression.Current status
Feedback on design and integration is very welcome.