Sorting improves word-aligned bitmap indexes
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.
Keywords
Cite
@article{arxiv.0901.3751,
title = {Sorting improves word-aligned bitmap indexes},
author = {Daniel Lemire and Owen Kaser and Kamel Aouiche},
journal= {arXiv preprint arXiv:0901.3751},
year = {2016}
}