> For the complete documentation index, see [llms.txt](https://wtifs.gitbook.io/diva-notes/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://wtifs.gitbook.io/diva-notes/db/questions.md).

# Questions

## 聚簇 VS 非聚簇

聚簇索引的优点：

1. 能够提高读取速度，因为不需要回表。每个聚簇索引唯一指向一行数据，可以直接得到需要查询的数据。
2. 相邻的行数据在页内连续存储。因此，范围查找、排序时，后续索引值的行在物理相邻，可以连续读取页上的行，效率更高。
3. 节约存储空间，因为聚簇索引不需要额外的索引结构，因此占用的空间较小。

非聚簇索引的优点：

1. 聚簇索引只能有一个，非聚簇可以有多个
2. 聚集索引存储记录是物理上连续存在，物理存储按照索引排序，而非聚集索引是逻辑上的连续。因次聚簇索引的插入、更新、删除操作可能较慢，因此需要涉及到数据的重新排序和移动。

## B+树 VS 哈希索引

B+树索引的优点：

1. 支持区间查找，也就是说在查找某个范围内的数据时，哈希索引不支持。
2. 可以通过 B+树的叶子节点顺序进行顺序遍历（ORDER BY），哈希索引不支持。
3. 支持单索引的最左匹配、联合索引最左匹配（LIKE '%'），哈希不支持。

哈希索引的优点：

1. 哈希索引的查询效率非常高，O(1) 的查询复杂度可以保证极快的查找速度。
2. 插入、更新或删除数据时，不会有节点分裂的问题。


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# Agent Instructions
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## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter:

```
GET https://wtifs.gitbook.io/diva-notes/db/questions.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

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Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.