Views
This document describes how to create, use, and manage views in YSQL.
Overview
Regular views allow you to present data in YugabyteDB tables by using a different variety of named queries. In essence, a view is a proxy for a complex query to which you assign a name. In YSQL, views do not store data. However, YSQL also supports materialized views which do store the results of the query.
Creating Views
You create views based on the following syntax:
CREATE VIEW view_name AS query_definition;
query_definition can be a simple SELECT statement or a SELECT statement with joins.
Suppose you work with a database that includes the following table populated with data:
CREATE TABLE employees (
employee_no integer PRIMARY KEY,
name text,
address text,
department text
);
INSERT INTO employees VALUES
(1221, 'John Smith', '1 Main Street', 'Marketing'),
(1222, 'Bette Davis', '2 Second Avenue', 'Sales'),
(1223, 'Lucille Ball', '3 Third Avenue', 'Operations'),
(1224, 'John Zimmerman', '4 Fourth Avenue', 'Sales');
The following very simplified example creates a view based on only one table and selects two of its columns:
CREATE VIEW employees_view AS
SELECT employee_no, name FROM employees;
The following example shows how to query employees_view:
SELECT * FROM employees_view;
The preceding query produces the following output:
employee_no | name
------------+---------------------------
1223 | Lucille Ball
1224 | John Zimmerman
1221 | John Smith
1222 | Bette Davis
If you create a view based on multiple tables with joins, using this view in your queries would significantly simplify the process.
Modifying Views
You can modify the query based on which a view was created by combining the CREATE VIEW statement with OR REPLACE, as demonstrated by the following syntax:
CREATE OR REPLACE VIEW view_name AS query_definition;
YSQL does not allow you to remove existing columns from the view (your query has to produce the same columns that were produced when you created the view), but it allows you to append more columns, as shown in the following example:
CREATE OR REPLACE VIEW employees_view AS
SELECT employee_no, name, department FROM employees;
The following example shows how to query employees_view to verify that the department column was appended to the end of the columns list:
SELECT * FROM employees_view;
The preceding query produces the following output:
employee_no | name | department
-------------+------------------+----------------
1223 | Lucille Ball | Operations
1224 | John Zimmerman | Sales
1221 | John Smith | Marketing
1222 | Bette Davis | Sales
Deleting Views
You can remove (drop) an existing view by using the DROP VIEW statement, as demonstrated by the following syntax:
DROP VIEW [ IF EXISTS ] view_name;
It is recommended to add the IF EXISTS option to the DROP VIEW statement: if you omit this option and attempt to drop the view, an error will occur.
The following example shows how to remove a view from the database:
DROP VIEW IF EXISTS employees_view;
You can also remove more than one view by providing a comma-separated list of view names.
Using Updatable Views
Some YSQL views are updatable. The defining query of such views (1) must have only one entry (either a table or another updatable view) in its FROM clause; and (2) cannot contain DISTINCT, GROUP BY, HAVING, EXCEPT, INTERSECT, or LIMIT clauses at the top level. In addition, the view's selection list cannot contain window functions, set-returning or aggregate functions.
The following example shows how to update the employees table with a new row via the employees_view defined in Creating Views:
INSERT INTO employees_view (employee_no, name)
VALUES (1227, 'Lee Bo');
If you select everything from the employees table by executing SELECT * FROM employees; , you should expect the following output:
employee_no | name | address | department
-------------+------------------+----------------+--------------
1227 | Lee Bo | |
1223 | Lucille Ball | 3 Third Avenue | Operations
1224 | John Zimmerman | 4 Fourth Avenue| Sales
1221 | John Smith | 1 Main Street | Marketing
1222 | Bette Davis | 2 Second Avenue| Sales
Executing INSERT, UPDATE, or DELETE on an updatable view converts that statement into the corresponding statement of the base table.
If the defining query of a view contains a WHERE clause, you are allowed to modify the rows that are not visible via the view.
An updatable view can contain a combination of updatable and non-updatable columns. An attempt to update the latter results in an error.
To be able to update a view, you need to have the required privilege on this view (but not necessarily on the base table).
The following example shows how use the employees_view to delete the row that has been added to the employees table:
DELETE FROM employees_view
WHERE employee_no = 1227;
Materialized Views
Materialized views are relations that persist the results of a query. They can be created using the CREATE MATERIALIZED VIEW command, and their contents can be updated using the REFRESH MATERIALIZED VIEW command.
The following very simplified example creates a materialized view based on only one table and selects two of its columns:
CREATE MATERIALIZED VIEW employees_mview AS
SELECT employee_no, name FROM employees;
The following example shows how to query employees_mview:
SELECT * FROM employees_mview;
The preceding query produces the following output:
employee_no | name
------------+---------------------------
1223 | Lucille Ball
1224 | John Zimmerman
1221 | John Smith
1222 | Bette Davis
INSERT INTO employees VALUES
(1225, 'Jane Doe', '4 Fifth Street', 'Accounting');
After inserting values into the base relation (employees), we will have to REFRESH the materialized view to update its contents.
REFRESH MATERIALIZED VIEW employees;
SELECT * FROM employees_mview;
The preceding query produces the following output:
employee_no | name
------------+---------------------------
1223 | Lucille Ball
1224 | John Zimmerman
1221 | John Smith
1222 | Bette Davis
For detailed documentation on materialized views please refer to the following links: