Use a materialized view in a subquery

You can create a materialized view for optimizing a subquery.

In this task, you create a materialized view and use it in a subquery to return the number of destination-origin pairs. Suppose the data resides in a table named flights_hdfs that has the following data:


c_id	dest	origin
1	Chicago	Hyderabad
2	London	Moscow
...

Create a table schema definition named flights_hdfs for destination and origin data.
```
CREATE TABLE flights_hdfs(
  c_id INT,
  dest VARCHAR(256),
  origin VARCHAR(256));
```

Create a materialized view that counts destinations and origins.

CREATE MATERIALIZED VIEW mv1
AS
  SELECT dest, origin, count(*)
  FROM flights_hdfs
  GROUP BY dest, origin;

Use the materialized view in a subquery to return the number of destination-origin pairs.
```
SELECT count(*)/2
FROM(
  SELECT dest, origin, count(*)
  FROM flights_hdfs
  GROUP BY dest, origin
) AS t;
```