b-tree索引

索引其实就是一个二叉树的结构

 

包括根节点，分支节点，叶节点

 

索引的叶节点之间其实是一个链表的结构

 

查询索引键的最值可以在hint 中加 index_desc 或者index_asc

 

文档上的解释：index_asc 

The INDEX_ASC hint instructs the optimizer to use an index scan for the specified table. If the statement uses an index range scan, then Oracle Database scans the index entries in ascending order of their indexed values. Each parameter serves the same purpose as in "INDEX Hint".

The default behavior. for a range scan is to scan index entries in ascending order of their indexed values, or in descending order for a descending index. This hint does not change the default order of the index, and therefore does not specify anything more than the INDEX hint. However, you can use the INDEX_ASC hint to specify ascending range scans explicitly should the default behavior. change.

 

index_desc:

The INDEX_DESC hint instructs the optimizer to use a descending index scan for the specified table. If the statement uses an index range scan and the index is ascending, then Oracle scans the index entries in descending order of their indexed values. In a partitioned index, the results are in descending order within each partition. For a descending index, this hint effectively cancels out the descending order, resulting in a scan of the index entries in ascending order. Each parameter serves the same purpose as in "INDEX Hint".

 

 

 

 

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