DECLARE
  r matable1;
  opt pg_catalog.json;
  text_var1 text;
  text_var2 text;
  text_var3 text;
BEGIN
    opt := '{...........}'; --тут вставляем реальный объект JSON
    select * from json_populate_record(null::matable1, opt) into r;
    insert into matable1 VALUES(r);
    ......
    EXCEPTION WHEN OTHERS THEN 
    GET STACKED DIAGNOSTICS text_var1 = MESSAGE_TEXT,
                          text_var2 = PG_EXCEPTION_DETAIL,
                          text_var3 = PG_EXCEPTION_HINT;
    RAISE NOTICE '%', text_var1;
END

ERROR:  control reached end of function without RETURN
NOTICE:  column "id" is of type integer but expression is of type mytable1

CREATE TABLE core.test (
  id SERIAL,
  name VARCHAR(50) NOT NULL,
  system BOOLEAN DEFAULT false NOT NULL,
  created_on TIMESTAMP(0) WITH TIME ZONE DEFAULT clock_timestamp() NOT NULL,
  CONSTRAINT test_pkey PRIMARY KEY(id)
) 
WITH (oids = false);


CREATE OR REPLACE FUNCTION core.test (
)
RETURNS pg_catalog.json AS
$body$
DECLARE
opt pg_catalog.json = '{' ||
   '"name": "Вася"' ||
  '}';
  r core.test;
  text_var1 text;
  text_var2 text;
  text_var3 text;
  
  begin
  	select * from json_populate_record(null::core.test, opt) into r;
--        return row_to_json(r);
    insert into core.test (id, name, system, created_on)
		 VALUES(r);
    return row_to_json(r);
    EXCEPTION WHEN OTHERS THEN 
    GET STACKED DIAGNOSTICS text_var1 = MESSAGE_TEXT,
                          text_var2 = PG_EXCEPTION_DETAIL,
                          text_var3 = PG_EXCEPTION_HINT;
    RAISE NOTICE '%', text_var1;
  end
$body$
LANGUAGE 'plpgsql'
VOLATILE
CALLED ON NULL INPUT
SECURITY INVOKER
COST 100;

insert into core.test VALUES(r.*);

insert into core.test VALUES('Петя')

insert into core.test VALUES(DEFAULT, 'Петя')

opt pg_catalog.json = '{' ||
   '"name": "Вася"' ||
  '}';
.......
select * from json_populate_record(null::core.test, opt) into r;
return row_to_json(r);

{"id":null,"name":"Вася","system":null,"created_on":null}

opt pg_catalog.json = '{' ||
   '"name": "Вася"' ||
  '}';
r core.test;
.......
select * from json_populate_record(r, opt) into r;
return row_to_json(r);

{"id":null,"name":"Вася","system":null,"created_on":null}

SELEСT (null::core.test).*;

null::core.test