# 11. 索引（Index）

索引是增強資料庫效能的常用方法。索引允許資料庫伺服器比沒有索引時更快地查詢和檢索特定資料列。但索引也會增加整個資料庫系統的開銷，因此應該合理地使用它們。


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# Agent Instructions: 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:

```
GET https://docs.postgresql.tw/13/the-sql-language/index.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

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.