Apply Selections as soon as you have the relevant columns:
![[Database/attachments/CleanShot 2023-03-02 at 20.19.59@2x.png]]
The second relation algebra is better than the first one.
Keep only the columns you need to evaluate downstream operators
![[Database/attachments/CleanShot 2023-03-02 at 20.41.32@2x.png]]
The second relation algebra is better than the first one.
Avoid Cartesian products(笛卡尔积)
Given a choice, do theta-joins rather than cross-products:
Favor (R ⋈ S) ⋈ T over (R X T) ⋈ S
![[Database/attachments/CleanShot 2023-03-02 at 20.50.47@2x.png]]
