也就是说,我们在使用ABC时,必须优先使用A再使用B最后是C,并且都要合理运用。 例如select * from student where name = "张三" and class="三班"这时我们只使用了name一个索引,其余的都没有使用,我们可以尝试上面explain自己测试一下,算一下ken_len的值。 我们来说几个特殊的情况,select * from student wher...
B+树总是会保持平衡。但是为了保持平衡对于新插入的键值可能需要做大量的拆分页(split)操作;部分情况下可以通过B+树的旋转来替代拆分页操作,进而达到平衡效果。 B+树的删除 B+树使用填充因子(fill factor)来控制树的删除变化,50%是填充因子可设的最小值。B+树的删除操作同样必须保证删除后叶子节点中的记...
该章节涉及关于MySQL的B+Tree索引结构的相关知识,具体可见code-Wu:深入理解MySQL索引底层数据结构与算法 2.1 什么是Explain 使用EXPLAIN关键字可以模拟优化器执行SQL语句,分析你的查询语句或是结构的性能瓶颈 在 select 语句之前增加 explain 关键字,MySQL 会在查询上设置一个标记,执行查询会返回执行计划的信息,而不是执...
> condition, which you'll find on indexes with INCLUDE-d columns or > filters on non-index columns. Why not combine them? And both call them Filter? In a sense this filtering acts very similar to INCLUDE based filtering (for btrees at least). Although I might be wrong about that, bec...
your patch. I would like to resolve the tension between the two patches, but I'm not sure how to do that. If you look at the example query that I included in my introductory email on the skip scan thread (the query against the sales_mdam_paper ...
EXPLAIN 作为 MySQL 的性能分析神器,读懂其结果是很有必要的,然而我在各种搜索引擎上竟然找不到特别完整的解读。都是只有重点,没有细节(例如 type 的取值不全、Extra 缺乏完整的介绍等)。 所以,我肝了将近一个星期,整理了一下。这应该是全网最全面、最细致的 EXPLAIN
test表bnet_id不是索引,test2表aid为索引列 ref_or_null:类似ref,但是添加了可以专门搜索null值的行 mysql> explain select * from test where bnet_id=1 or bnet_id is null; +---+---+---+---+---+---+---+---+---+---+ | id | select_type | table | type | possible_keys |...
The diagram below shows typical API designs with a shopping cart example. Note that API design is not just URL path design. Most of the time, we need to choose the proper resource names, identifiers, and path patterns. It is equally important to design proper HTTP header fields or to desi...
An attempt to explain the distribution of the tree species composing the riparian forests of Lake Duparquet, southern boreal region of Quebec, Canada. Canadian Journal of Botany, 77: 1744-1755.DENNELER B.; Y. BERGERON; Y. BEGIN (1999) An attempt to explain the distribution of the tree ...
Also, I thought it would be better to change the way bound quals are reported to align with the "Filter". I think it would be better to modify it so that it does not output when the bound quals are the same as the index conditions. ...