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arXiv · 0901.3751

Sorting improves word-aligned bitmap indexes

Daniel Lemire·Owen Kaser·Kamel Aouiche
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Abstract

Bitmap indexes must be compressed to reduce input/output costs and minimize CPU usage. To accelerate logical operations (AND, OR, XOR) over bitmaps, we use techniques based on run-length encoding (RLE), such as Word-Aligned Hybrid (WAH) compression. These techniques are sensitive to the order of the rows: a simple lexicographical sort can divide the index size by 9 and make indexes several times faster. We investigate row-reordering heuristics. Simply permuting the columns of the table can increase the sorting efficiency by 40%. Secondary contributions include efficient algorithms to construct and aggregate bitmaps. The effect of word length is also reviewed by constructing 16-bit, 32-bit and 64-bit indexes. Using 64-bit CPUs, we find that 64-bit indexes are slightly faster than 32-bit indexes despite being nearly twice as large.

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Daniel Lemire, Owen Kaser, Kamel Aouiche. 2016-07-29. Sorting improves word-aligned bitmap indexes. https://doi.org/10.1016/j.datak.2009.08.006

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