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the benefits of compression come from the fact that a properly compressed index uses a smaller number of leaf blocks - which tends to mean that less i/o is involved when the index is used, there is a reduced amount of buffer cache flushing, and the optimizer is likely to calculate a lower cost for using that index for range scans. (there is a tiny chance that the number of branch blocks, and the index height might be reduced, too, but that is a little unlikely).
压缩的优势来自一个恰当压缩的索引使用更少的叶块——这样当用到索引时涉及更少的i/o,buffer cache清洗量减小,优化器对index range scan代价的计算可能更低.（甚至有机会分支块数与索引高度也会减少,但这不太可能）. 【程序编程相关:（原创）飘浮广告显示脚本类（VBS，JS】 【推荐阅读:在C#中应用哈希表(Hashtable)】 【扩展信息:VB连接数据库时出现“ActiveX部件】but compressing indexes, especially compressing the wrong number of columns, can have negative impact on your performance. if you compress more columns than you should, the ´compressed´ index may be larger than the uncompressed index. use the validate option on the index, and check view index_stats to find out the optimum compression count. how did i know that i should compress just the first two columns of the t1_ci_1 index ? (apart from knowing the data, that is):
但压缩索引,特别是压缩烈数不正确时,会对性能产生负面影响.如果压缩了过多的列,“压缩”了的索引可能比未压缩的索引更大.对索引使用validate选项,然后检查视图index_stats找到最优的压缩数.... 下一页