Variable word length word-aligned hybrid compression

Abstract

The Word-Aligned Hybrid (WAH) compression is a prominent example of a lightweight compression scheme for bitmap indices that considers the word size of the underlying architecture. This is a compromise toward commodity CPUs, where operations below the word granularity perform poorly. With the emergence of novel hardware classes, such compromises may no longer be appropriate. Field-programmable gate arrays (FPGAs) do not even have any meaningful "word size". In this work, we reconsider strategies for bitmap compression in the light of modern hardware architectures. Rather than tuning compression toward a fixed word size, we propose to tune the word size toward optimal compression. The resulting compression scheme, Variable Word Length Word-Aligned Hybrid (VWLWAH), improves compression rates by almost 75% while maintaining line rate performance on FPGAs.