CREATE OR REPLACE FUNCTION my_trigger_func()
RETURNS "trigger" AS
$BODY$
begin
if new.p3!=old.p3 then
INSERT INTO table2 (p2) VALUES (new.p3);
end if;
end;
$BODY$
LANGUAGE 'plpgsql' VOLATILE;

CREATE TRIGGER tg_my
AFTER UPDATE
ON table1
FOR EACH ROW
EXECUTE PROCEDURE my_trigger_func();

ERROR:  control reached end of trigger procedure without RETURN
КОНТЕКСТ:  PL/pgSQL function "my_trigger_func"

CREATE OR REPLACE FUNCTION my_trigger_func()
RETURNS "trigger" AS
$BODY$
begin
if new.p3!=old.p3 then 
   if new.p4='значение1' then
     INSERT INTO table2 (p2,p5) VALUES (new.p3,'1');
   end if;
   
   if new.p4='значение2' then
     INSERT INTO table2 (p2,p5) VALUES (new.p3,'2');
   end if;
RETURN NEW;
end if;
end;
$BODY$
LANGUAGE 'plpgsql' VOLATILE;

CREATE TRIGGER tg_my
AFTER UPDATE
ON table1
FOR EACH ROW
EXECUTE PROCEDURE my_trigger_func();