关系代数，关系数据库基本概念

Data Model：1）structure  2) constraints 3) operation

hierarchy/Treelike(give me my children)， table， graph， stream（give me next n bytes)

数据库 key idea

physical data independence : interface with tables instead of file and pointers

optional : logical data independence: views

关系代数（Relational Algebra)

closure: 输入是表，输出也是表，

操作：select, project, join, cross product 加上集合的几种操作，union, difference, intersect

Relational Algebra Expression

optimizer:

logical optimization:  SQL -> a best RA expression (Tree). SQL just specifies the result, RA expression include evaluation order, choose the best one.

physical optimization:  e.g., how to do a join ? (nested loop join, hash join, merge join) , depending on the data size,  index available or not.

view只是一个命名的 query, 会和user query 一起组成一个大Query交给Optimizer。也就是把query中的view替换成定义它的查询，变成一个子查询，效果是一样的。

view 一般是不materialize的。