Bitmap indexes must be compressed to reduce input/output costs and minimizeCPU usage. To accelerate logical operations (AND, OR, XOR) over bitmaps, we usetechniques based on run-length encoding (RLE), such as Word-Aligned Hybrid(WAH) compression. These techniques are sensitive to the order of the rows: asimple lexicographical sort can divide the index size by 9 and make indexesseveral times faster. We investigate row-reordering heuristics. Simplypermuting the columns of the table can increase the sorting efficiency by 40%.Secondary contributions include efficient algorithms to construct and aggregatebitmaps. 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 areslightly faster than 32-bit indexes despite being nearly twice as large.
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