HGDB视图和物化视图的区别

视图和物化视图区别
HGDB中的视图是在查询的时候进行扫描子表的操作，而物化视图则是实实在在地将数据存成一张表。
物化视图是在HGDB 3.0.0之后才有的功能。


比较下视图和物化视图的性能
创建两个表


CREATE TABLE teacher (
    id int NOT NULL,
    sname varchar(100)
);

CREATE TABLE student (
    sid int NOT NULL,
    teacher_id int NOT NULL DEFAULT 0,
    tname varchar(100)
);
创建一个视图
CREATE OR REPLACE VIEW student_view AS
SELECT  *
   FROM student
   LEFT JOIN teacher 
   ON student.teacher_id = teacher.id;
创建一个物化视图


CREATE MATERIALIZED VIEW student_view_m AS
SELECT  *
   FROM student
   LEFT JOIN teacher 
   ON student.teacher_id = teacher.id;
进行查询explain:


highgo=#  explain select * from student_view;
                               QUERY PLAN                               
------------------------------------------------------------------------
 Hash Left Join  (cost=17.20..49.12 rows=512 width=448)
   Hash Cond: (student.teacher_id = teacher.id)
   ->  Seq Scan on student  (cost=0.00..13.20 rows=320 width=226)
   ->  Hash  (cost=13.20..13.20 rows=320 width=222)
         ->  Seq Scan on teacher  (cost=0.00..13.20 rows=320 width=222)
(5 rows)


highgo=# explain select * from student_view_m;
                            QUERY PLAN                             
-------------------------------------------------------------------
 Seq Scan on student_view_m  (cost=0.00..11.70 rows=170 width=448)
(1 row)
可以看出，student_view去每个表中进行查询，而student_view_m 直接去视图表查询，而物化视图的查询效率确确实实高于视图不少。


物化视图的数据填充
物化视图既然是一个实实在在存在的表，它就需要有数据填充过程，数据填充的命令是REFRESH MATERIALIZED VIEW


highgo=#  \h REFRESH
Command:     REFRESH MATERIALIZED VIEW
Description: replace the contents of a materialized view
Syntax:
REFRESH MATERIALIZED VIEW [ CONCURRENTLY ] name
    [ WITH [ NO ] DATA ]