SQL - VIEWS

A view is a database object that is of a stored query. A view can be accessed as a virtual table in PostgreSQL. In other words, a PostgreSQL view is a logical table that represents data of one or more underlying tables through a SELECT statement.
CREATE OR REPLACE VIEW is similar, but if a view of the same name already exists, it is replaced. The new query must generate the same columns that were generated by the existing view query (that is, the same column names in the same order and with the same data types), but it may add additional columns to the end of the list.

    create table company(id int, name varchar, place varchar);
    postgres=# Insert into company values(1,'babu','chennai');
    postgres=# Insert into company values(2,'pappu','avadi');
    postgres=# Insert into company values(3,'rosh','chepauk');
    postgres=# Insert into company values(4,'div','chepauk');
    postgres=# Insert into company values(5,'divya','chetpet');

    postgres=# select * from company;
    id | name  |  place   
    ----+-------+----------
    1 | babu  | chennai
    1 | pappu | mylapore 
    2 | pappu | avadi
    3 | rosh  | chepauk
    4 | div   | chepauk
    5 | divya | chetpet
    (6 rows)
    postgres=# create table emp(id int, name varchar,sal int);
    postgres=# Insert into emp values(1,'babu',2000)
    postgres=# Insert into emp values(2,'pappu',4000);
    postgres=# Insert into emp values(3,'rosh',5000);
    postgres=# Insert into emp values(4,'div',6000);
    postgres=# Insert into emp values(5,'divya',8000);

    postgres=# select * from emp;
    id | name  | sal  
    ----+-------+------
    1 | babu  | 2000
    2 | pappu | 4000
    3 | rosh  | 5000
    4 | div   | 6000
    5 | vidya | 8000

    postgres=# create temporary view v1_tmp as select a.id, a.name, b.place from emp a inner join company b on a.id=b.id;
    CREATE VIEW
    postgres=# select * from v1_tmp;
    id | name  |  place   
    ----+-------+----------
    1 | babu  | chennai
    1 | babu  | mylapore
    2 | pappu | avadi
    3 | rosh  | chepauk
    4 | div   | chepauk
    5 | vidya | chetpet
    (6 rows)

        postgres=# \dv
            List of relations
    Schema   |  Name  | Type |  Owner   
    -----------+--------+------+----------
    pg_temp_3 | v1_tmp | view | postgres

Simple views are automatically updatable: the system will allow INSERT, UPDATE and DELETE statements to be used on the view in the same way as on a regular table.
A view is automatically updatable if it satisfies all of the following conditions:

1. The view must have exactly one entry in its FROM list, which must be a table or another updatable view.
2. The view definition must not contain WITH, DISTINCT, GROUP BY, HAVING, LIMIT, or OFFSET clauses at the top level.
3. The view definition must not contain set operations (UNION, INTERSECT or EXCEPT) at the top level.
4. The view's select list must not contain any aggregates, window functions or set-returning functions.

	 
    postgres=# create view v1 as select a.id, a.name, b.place from emp a inner join company b on a.id=b.id;

CREATE VIEW


    postgres=# \dv
            List of relations
    Schema   |  Name  | Type |  Owner   
    -----------+--------+------+----------
    public    | v1     | view | postgres
    (2 rows)

    postgres=# select * from v1;
    id | name  |  place   
    ----+-------+----------
    1 | babu  | chennai
    1 | babu  | mylapore
    2 | pappu | avadi
    3 | rosh  | chepauk
    4 | div   | chepauk
    5 | vidya | chetpet
    (6 rows)
    postgres=# select * from pg_views where schemaname='public';
    schemaname     | viewname      | viewowner            |  definition                
    ------------+----------+-----------+------------------------------------------
    public     | v1       | postgres  |  SELECT a.id, a.name,  b.place   FROM (emp a  JOIN company b ON                         
                                                        ((a.id = b.id)));

Update the table will automatically update view :


    postgres=# update company set place ='madurai' where id=1;
    UPDATE 2
    postgres=# select * from company;
    id | name  |  place  
    ----+-------+---------
    2 | pappu | avadi
    3 | rosh  | chepauk
    4 | div   | chepauk
    5 | divya | chetpet
    1 | babu  | madurai
    1 | pappu | madurai
    (6 rows)

    postgres=# select * from v1;
    id | name  |  place  
    ----+-------+---------
    1 | babu  | madurai
    1 | babu  | madurai
    2 | pappu | avadi
    3 | rosh  | chepauk
    4 | div   | chepauk
    5 | vidya | chetpet
    (6 rows)

Drop view :


    postgres=# drop view v1;
    DROP VIEW
    postgres=# select * from pg_views where schemaname='public';
    schemaname | viewname | viewowner | definition 
    ------------+----------+-----------+------------
    (0 rows)

Materialized Views :

1. Materialized views are disc-stored views that can be refreshed. It can be used like a regular table, for example, you can add indexes or primary key on it, it supports VACUUM and ANALYZE commands
2. It is used when you want to combine rows from the same table based on a related column or condition

        postgres=# create materialized  view v1 as select a.id, a.name, b.place from emp a inner join company b on a.id=b.id;
    SELECT 6
    postgres=# select * from v1;
    id | name  |  place  
    ----+-------+---------
    1 | babu  | madurai
    1 | babu  | madurai
    2 | pappu | avadi
    3 | rosh  | chepauk
    4 | div   | chepauk
    5 | vidya | chetpet
    (6 rows)

    postgres=# 
    postgres=# select * from pg_matviews;
    schemaname | matviewname | matviewowner | tablespace | hasindexes | ispopulated |                definition                
    ------------+-------------+--------------+------------+------------+-------------+------------------------------------------
    public     | v1          | postgres     |            | f          | t           |  SELECT a.id,                           +
                |             |              |            |            |             |     a.name,                             +
                |             |              |            |            |             |     b.place                             +
                |             |              |            |            |             |    FROM (emp a                          +
                |             |              |            |            |             |      JOIN company b ON ((a.id = b.id)));

    postgres=# update company set place ='kollam' where id=5;

Update :


    postgres=# select * from company;
    id | name  |  place  
    ----+-------+---------
    2 | pappu | avadi
    3 | rosh  | chepauk
    4 | div   | chepauk
    1 | babu  | madurai
    1 | pappu | madurai
    5 | divya | kollam
    (6 rows)

    postgres=# select * from v1;
    id | name  |  place  
    ----+-------+---------
    1 | babu  | madurai
    1 | babu  | madurai
    2 | pappu | avadi
    3 | rosh  | chepauk
    4 | div   | chepauk
    5 | vidya | chetpet
    (6 rows)

    postgres=# refresh materialized view v1;
    REFRESH MATERIALIZED VIEW
    postgres=# select * from v1;
    id | name  |  place  
    ----+-------+---------
    1 | babu  | madurai
    1 | babu  | madurai
    2 | pappu | avadi
    3 | rosh  | chepauk
    4 | div   | chepauk
    5 | vidya | kollam
    (6 rows)
    postgres=# vacuum analyze v1;

Vacuum :


    postgres=# drop materialized view v1;
    DROP MATERIALIZED VIEW
    postgres=# \dm+
    Did not find any relations.
    postgres=# select * from pg_matviews;
    schemaname | matviewname | matviewowner | tablespace | hasindexes | ispopulated | definition 
    ------------+-------------+--------------+------------+------------+-------------+------------
    (0 rows)




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