排序改进了字对齐位图索引
Sorting improves word-aligned bitmap indexes
摘要：
必须压缩位图索引以降低输入/输出成本并最大限度地减少CPU使用。为了加速位图上的逻辑运算（AND、OR、XOR），我们使用基于行程长度编码（RLE）的技术，例如字对齐混合（WAH）压缩。这些技术对行的顺序很敏感：一个简单的字典序排序可以将索引大小除以9，使索引速度提高几倍。我们研究行重排算法。简单地排列表中的列可以将排序效率提高40%。次要贡献包括构造和聚合位图的有效算法。通过构造16位、32位和64位索引，还考察了字长的影响。使用64位CPU，我们发现64位索引比32位索引稍快，尽管它几乎是32位索引的两倍。
英文摘要：
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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