ERROR: type "myrowtype" does not exist

[Arup Rakshit]

I was reading the json_populate_record function from the official doc 
https://www.postgresql.org/docs/10/static/functions-json.html#FUNCTIONS-JSON-PROCESSING-TABLE
 
<https://www.postgresql.org/docs/10/static/functions-json.html#FUNCTIONS-JSON-PROCESSING-TABLE>.
 The doc mentioned examples are giving me errors

psql (11.0, server 10.5)
Type "help" for help.

aruprakshit=# select * from json_populate_record(null::extype, '{"x":"foo", 
"y":43, "q": true}');
ERROR:  type "extype" does not exist
LINE 1: select * from json_populate_record(null::extype, '{"x":"foo"...
                                                 ^
aruprakshit=# select * from json_populate_record(null::myrowtype, '{"a": 1, 
"b": ["2", "a b"], "c": {"d": 4, "e": "a b c"}}');
ERROR:  type "myrowtype" does not exist
LINE 1: select * from json_populate_record(null::myrowtype, '{"a": 1…

What does null::extype, null::myrowtype mean here ?


Thanks,

Arup Rakshit
a...@zeit.io