Periodically rebuild a materialized view

Using materialized views can enhance query performance. You need to refresh materialized view contents when new data is added to the underlying table. Instead of rebuilding the materialized view manually, you can schedule this task. Rebuilding occurs periodically and transparently to users.

In this task, you create a schema for storing employee information. Imagine that you add data for a number of employees to the table. Assume many users of your database issue queries to access to data about the employees hired during last year including the department they belong to. You create a materialized view of the table to address these queries. Imagine new employees are hired and you add their records to the table. These changes render the materialized view contents outdated. You need to refresh its contents. You create a scheduled query to perform this task. The scheduled rebuilding will not occur unless there are changes to the input tables. You test the scheduled query by bypassing the schedule and executing the schedule immediately. Finally, you change the schedule to rebuild less often.

Create a database schema for employee data.

CREATE TABLE emps (
  empid INTEGER,
  deptno INTEGER,
  name VARCHAR(256),
  salary FLOAT,
  hire_date TIMESTAMP);
CREATE TABLE depts (
  deptno INTEGER,
  deptname VARCHAR(256),
  locationid INTEGER);

To handle many queries to access recently hired employee and departmental data, create a materialized view.

CREATE MATERIALIZED VIEW mv_recently_hired AS
  SELECT empid, name, deptname, hire_date FROM emps
  JOIN depts ON (emps.deptno = depts.deptno)
  WHERE hire_date >= '2020-01-01 00:00:00';

Use the materialized view by querying the employee data.

SELECT empid, name FROM emps
JOIN depts  ON (emps.deptno = depts.deptno)
WHERE hire_date >= '2020-03-01 00:00:00' AND deptname = 'finance';

Assuming new hiring occurred and you added new records to the emps table, rebuild the materialized view.
```
ALTER MATERIALIZED VIEW mv_recently_hired REBUILD; 
```
The rebuilding refreshes the contents of the materialized view.
Create a scheduled query to invoke the rebuild statement every 10 minutes.
```
CREATE SCHEDULED QUERY scheduled_rebuild
EVERY 10 MINUTES AS
ALTER MATERIALIZED VIEW mv_recently_hired REBUILD; 
```
A rebuild executes every 10 minutes unless there are no changes to the emps table. If a materialized view can be rebuilt incrementally, the scheduled rebuild does not occur unless there are changes to the input tables.
To test the schedule, execute a scheduled query immediately.
```
ALTER SCHEDULED QUERY scheduled_rebuild EXECUTE;
```

Change the frequency of the rebuilding.

ALTER SCHEDULED QUERY scheduled_rebuild EVERY 20 MINUTES;