select * from json_populate_record(null::myrowtype, '{"a": 1, "b": ["2", "a b"], "c": {"d": 4, "e": "a b c"}}')
What is the null::myrowtype doing?
this function uses polymorphic parameters - its some like
create or replace function fx(in anyelement, json) returns anyelement ..
output polymorphic value requires minimally one input polymorphic value for type specification. Postgres has not possibility to pass only datatype - so pattern null:type is used instead.