数据库调优

1.树结构的索引（Tree-Structured Indexing）
the root of this tree will always be in main memory, while the leaves will be located on disk（树的根节点在main memory,叶子节点在disk中）

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2.Indexed sequential access method (ISAM)
Cons（缺点）：losing sequentiality and balance.
Prons（优点）：No need to lock non-leaf index pages since we never modify them, which is one of important advantages of ISAM over B+tree.

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3.B+ tree
A B+-Tree is a balanced tree whose leaves contain a sequence of key-pointer pairs. It is the Most Widely Used Index.
B+ 树是一种树数据结构，是一个n叉排序树，每个节点通常有多个孩子，一棵B+树包含根节点、内部节点和叶子节点。
B+ 树通常用于数据库和操作系统的文件系统中。NTFS, ReiserFS, NSS, XFS, JFS, ReFS 和BFS等文件系统都在使用B+树作为元数据索引。B+ 树的特点是能够保持数据稳定有序，其插入与修改拥有较稳定的对数时间复杂度。B+ 树元素自底向上插入。