三、覆盖索引(covering indexes)的使用覆盖索引是指那些索引项中包含查寻所需要的全部信息的非聚簇索引，这种索引之所以比较快也正是因为索引页中包含了查寻所必...
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覆盖索引（covering index ）就是用来解决这个问题的。覆盖索引帮助我们在非聚集索引的叶子层增加非主键列，最小可能增加B树结构的深度。
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索引覆盖 Index Covering ; Index Coverage
电视覆盖搜索引擎 TVCE ; TV Coverage Engine
银河舰队覆盖图标索引 Galactic Fleet Override Icon Index
索引覆盖查询 index-covered query
为最常用和最重要的查询创建覆盖索引。
Create a covering index for often-used, high-impact queries.
注意编写7个不等式作为5个不必有序的决策变量的和太过繁琐了，因为在某些约束中，索引可能会覆盖索引值7。
Note that it would be too boring to write seven inequalities as a sum of five decision variables that are not necessarily in order, because in some constraints, the index may overlap the index 7.
因为索引会带来额外的数据更新开销，所以应该争取创建少量索引来覆盖尽可能多的查询。
Because each index incurs extra data updating overhead, the goal should be to create a small number of indexes to cover as many queries as possible.
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