1. 本課程將討論服務(wù)器端的用戶(hù)定義函數(shù)（UDF）和用戶(hù)定義過(guò)程（UDS）的邏輯執(zhí)行。

2. UDFs是由應(yīng)用程序開(kāi)發(fā)人員編寫(xiě)的函數(shù)，用于擴(kuò)展數(shù)據(jù)庫(kù)系統(tǒng)的功能。

3. UDFs可以執(zhí)行各種計(jì)算，包括循環(huán)、條件語(yǔ)句和調(diào)用其他SQL語(yǔ)句和函數(shù)。

4. UDFs可以嵌入在SQL查詢(xún)中的任何位置，以提供更多的功能和靈活性。

5. 不同的數(shù)據(jù)庫(kù)系統(tǒng)支持不同的UDF語(yǔ)言，如SQL PSM、Transact SQL和PL/SQL等。

6. UDFs (User-Defined Functions) can provide modularity and code reuse in database systems.

7. UDFs written in procedural languages like PL/SQL can be difficult for the database system to optimize and parallelize.

8. UDFs can cause performance issues and unpredictable behavior in queries due to their black box nature.

9. The Freud project in SQL Server aims to convert imperative UDFs into relational algebra expressions for better optimization and inlining.

10. Freud allows UDFs to be converted into subqueries, which can be optimized by the query optimizer without requiring changes to the UDF code.

11. UDF（用戶(hù)定義函數(shù)）是類(lèi)似子查詢(xún)的概念，除非進(jìn)行內(nèi)聯(lián)或去相關(guān)子查詢(xún)的轉(zhuǎn)換，否則它將被視為函數(shù)調(diào)用，需要執(zhí)行嵌套查詢(xún)并返回結(jié)果。

12. 處理嵌套查詢(xún)的兩種方法是去相關(guān)或?qū)⑵渲貙?xiě)為連接，或者將嵌套查詢(xún)提取出來(lái)作為子計(jì)劃運(yùn)行，并將結(jié)果存儲(chǔ)在臨時(shí)表中進(jìn)行連接。

13. 使用Lateral Join（橫向連接）可以讓嵌套查詢(xún)?cè)谕粚蛹?jí)中引用其他表或查詢(xún)的屬性，避免了多層嵌套的問(wèn)題。

14. 使用Lateral Join（橫向連接）可以將多個(gè)子查詢(xún)放在FROM子句中，使它們可以相互引用，而不需要嵌套。

15. 將UDF內(nèi)聯(lián)到查詢(xún)中后，查詢(xún)優(yōu)化器可以對(duì)其進(jìn)行各種優(yōu)化，例如動(dòng)態(tài)切片和優(yōu)化冗余計(jì)算，從而提高查詢(xún)性能。

16. Freud is a technique that optimizes the performance of user-defined functions (UDFs) in SQL queries by inlining them and eliminating unnecessary computations.

17. Freud can significantly improve the performance of UDFs that involve joins or subqueries, but it cannot handle loops, exceptions, or cursors.

18. The Appfel technique is an alternative to Freud that converts UDFs into common table expressions (CTEs) and supports control flow constructs like loops.

19. Freud has been implemented in SQL Server and has shown significant performance gains in real-world workloads.

20. SQL Server's optimizer has limitations in decorrelating subqueries, while the Appfel technique provides a systematic approach to eliminate and decorrelate any subquery.