create or replace function t_part() returns trigger as $$
declare
   Cnt Integer;
begin
   select count(*) into Cnt
   from pg_catalog.pg_tables
   where tablename = 't_part_' || New.Part_Id;

   if Cnt=0 then
      execute 'create table t_part_' || New.Part_Id
         || ' (id integer not null primary key, '
         || '  part_id integer, '
         || '  val Varchar, '
         || '  constraint ch_t_part_' || New.Part_Id || ' check(part_id=' || New.Part_Id || ')) ';
   end if;

   return new;
end;
$$ language plpgsql;

create trigger tg_part instead of insert on t
    for each row execute procedure t_part();

postgres=> \d t_part_5
Отношение "t_part_5" не найдено.
postgres=> insert into t(id, part_id, val) values (5, 5, '5');
INSERT 0 1
postgres=> \d t_part_5
           Таблица "crm.t_part_5"
 Колонка |        Тип        | Модификаторы 
---------+-------------------+--------------
 id      | integer           | NOT NULL
 part_id | integer           | 
 val     | character varying | 
Индексы:
    "t_part_5_pkey" PRIMARY KEY, btree (id)
Ограничения-проверки:
    "ch_t_part_5" CHECK (part_id = 5)

postgres=> 

ОШИБКА:  нельзя выполнить CREATE OR REPLACE VIEW "t", так как этот объект используется активными запросами в данном сеансе

create or replace view t_ins as
select id, part_id, val from t_part_1 where 0=1;

create or replace function t_part() returns trigger as $$
declare
   Cnt Integer;
   v_Sql Varchar := '';
   v_TblName Varchar;
begin
   select count(*) into Cnt
   from pg_catalog.pg_tables
   where tablename = 't_part_' || New.Part_Id;

   if Cnt=0 then
      execute 'create table t_part_' || New.Part_Id
         || ' (id integer not null primary key, '
         || '  part_id integer, '
         || '  val Varchar, '
         || '  constraint ch_t_part_' || New.Part_Id || ' check(part_id=' || New.Part_Id || ')) ';

      for v_TblName in (select tablename from pg_catalog.pg_tables
                        where tablename like 't|_part|_%' escape '|') 
      loop
         if (v_Sql<>'') then
            v_Sql := v_Sql || ' union all ';
         end if;
         v_Sql := v_Sql || 'select id, part_id, val from ' || v_TblName;
      end loop;

      execute 'create or replace view t as ' || v_Sql;
   end if;

   return new;
end;
$$ language plpgsql;

create trigger tg_ins instead of insert on t_ins
    for each row execute procedure t_part();

postgres=> explain select * from t where part_id=7;
                QUERY PLAN                
------------------------------------------
 Result  (cost=0.00..0.01 rows=1 width=0)
   One-Time Filter: false
(2 строки)

postgres=> insert into t_ins(id, part_id, val) values (7, 7, '7');
INSERT 0 1
postgres=> explain select * from t where part_id=7;
                           QUERY PLAN                           
----------------------------------------------------------------
 Append  (cost=0.00..24.50 rows=6 width=40)
   ->  Seq Scan on t_part_7  (cost=0.00..24.50 rows=6 width=40)
         Filter: (part_id = 7)
(3 строки)

postgres=> 

postgres=> begin transaction ;
BEGIN
postgres=> select * from t;
 id | part_id | val 
----+---------+-----
(0 строк)

postgres=> insert into t_ins(id, part_id, val) values (9, 9, '9');