Convert JSON to table type

The function json_populate_record can be used to cast a JSON object to a table type:

CREATE TABLE foo(bar int);

SELECT  *
FROM    json_populate_record(NULL::foo, '{"bar": 42}')

+-------+
| bar   |
|-------|
| 42    |
+-------+

This can also be used to insert into the table:

INSERT INTO foo
SELECT *
FROM   json_populate_record(NULL::foo, '{"bar": 42}')