create table t_part_1 (id integer not null primary key, part_id integer not null, val Varchar);
create table t_part_2 (id integer not null primary key, part_id integer not null, val Varchar);
create table t_part_3 (id integer not null primary key, part_id integer not null, val Varchar);

create or replace view t as 
select id, part_id, val from t_part_1
union all
select id, part_id, val from t_part_2
union all
select id, part_id, val from t_part_3;

postgres=> explain select * from t where part_id=7;
                           QUERY PLAN                           
----------------------------------------------------------------
 Append  (cost=0.00..73.50 rows=18 width=40)
   ->  Seq Scan on t_part_1  (cost=0.00..24.50 rows=6 width=40)
         Filter: (part_id = 7)
   ->  Seq Scan on t_part_2  (cost=0.00..24.50 rows=6 width=40)
         Filter: (part_id = 7)
   ->  Seq Scan on t_part_3  (cost=0.00..24.50 rows=6 width=40)
         Filter: (part_id = 7)
(7 строк)

postgres=> alter table t_part_1 add constraint ch_t_part_1 check(part_id=1);
ALTER TABLE
postgres=> alter table t_part_2 add constraint ch_t_part_2 check(part_id=2);
ALTER TABLE
postgres=> alter table t_part_3 add constraint ch_t_part_3 check(part_id=3);
ALTER TABLE
postgres=> 

postgres=> explain select * from t where part_id=1;
                           QUERY PLAN                           
----------------------------------------------------------------
 Append  (cost=0.00..24.50 rows=6 width=40)
   ->  Seq Scan on t_part_1  (cost=0.00..24.50 rows=6 width=40)
         Filter: (part_id = 1)
(3 строки)

postgres=> explain select * from t where part_id=2;
                           QUERY PLAN                           
----------------------------------------------------------------
 Append  (cost=0.00..24.50 rows=6 width=40)
   ->  Seq Scan on t_part_2  (cost=0.00..24.50 rows=6 width=40)
         Filter: (part_id = 2)
(3 строки)

postgres=> explain select * from t where part_id=3;
                           QUERY PLAN                           
----------------------------------------------------------------
 Append  (cost=0.00..24.50 rows=6 width=40)
   ->  Seq Scan on t_part_3  (cost=0.00..24.50 rows=6 width=40)
         Filter: (part_id = 3)
(3 строки)

postgres=> explain select * from t where part_id=4;
                QUERY PLAN                
------------------------------------------
 Result  (cost=0.00..0.01 rows=1 width=0)
   One-Time Filter: false
(2 строки)

with t as (
   select id ...
    where id=1
  union all
   select id ...
    where id=2
)
select * from t where id>1