Unir dos tablas en PostgreSQL

si no le importa eliminar datos de Table1 y luego insertar nuevos datos, puede hacer esto:

with cte1 as (
    delete from Table1
    returning * 
), cte2 as (
    select c1, c2, c3, c4, c5, cnt from cte1
    union all
    select c1, c2, c3, c4, c5, cnt from Table2
)
insert into Table1
select c1, c2, c3, c4, c5, sum(cnt)
from cte2
group by c1, c2, c3, c4, c5;

demostración de violín de sql

si realmente desea actualizar/insertar datos, puede hacer esto:

with cte_upsert as (
    update Table1 as T1 set
        cnt = T1.cnt + T2.cnt
    from Table2 as T2
    where
        T1.c1 = T2.c1 and T1.c2 = T2.c2 and
        T1.c3 = T2.c3 and T1.c4 = T2.c4 and 
        T1.c5 = T2.c5
    returning T1.*
)
insert into Table1
select T2.c1, T2.c2, T2.c3, T2.c4, T2.c5, T2.cnt
from Table2 as T2
where
  not exists (
      select *
      from cte_upsert as T1
      where
          T1.c1 = T2.c1 and T1.c2 = T2.c2 and
          T1.c3 = T2.c3 and T1.c4 = T2.c4 and 
          T1.c5 = T2.c5
);

demostración de violín de sql

o puedes hacer la más obvia:

update Table1 as T1 set
    cnt = T1.cnt + T2.cnt
from Table2 as T2
where
    T1.c1 = T2.c1 and T1.c2 = T2.c2 and
    T1.c3 = T2.c3 and T1.c4 = T2.c4 and 
    T1.c5 = T2.c5;

insert into Table1
select T2.c1, T2.c2, T2.c3, T2.c4, T2.c5, T2.cnt
from Table2 as T2
where
  not exists (
      select *
      from Table1 as T1
      where
          T1.c1 = T2.c1 and T1.c2 = T2.c2 and
          T1.c3 = T2.c3 and T1.c4 = T2.c4 and 
          T1.c5 = T2.c5
);

demostración de violín de sql

Sé que no es una solución optimizada, pero resolverá su problema

select 
    a.c1 as c1 , a.c2 as c2, a.c3 as c3, a.c4 as c4, a.c5 as c5, a.count + b.count as count
from
    t1 a
        join
    t2 b ON (a.c1 = b.c1 and a.c2 = b.c2
        and a.c3 = b.c3
        and a.c4 = b.c4
        and a.c5 = b.c5)
union all
(select 
    a.c1 as c1 , a.c2 as c2, a.c3 as c3, a.c4 as c4, a.c5 as c5, a.count  as count
from
    t1 a
        LEFT join
    t2 b ON (a.c1 = b.c1 and a.c2 = b.c2
        and a.c3 = b.c3
        and a.c4 = b.c4
        and a.c5 = b.c5)
WHERE b.c1 is null)
UNION all
select 
    a.c1 as c1 , a.c2 as c2, a.c3 as c3, a.c4 as c4, a.c5 as c5, a.count  as count
from
    t2 a
        LEFT join
    t1 b ON (a.c1 = b.c1 and a.c2 = b.c2
        and a.c3 = b.c3
        and a.c4 = b.c4
        and a.c5 = b.c5)
WHERE b.c1 is null

No está claro si el "conteo" es la función agregada de una consulta anterior o un campo simple, sin embargo, si es un campo simple, como en los siguientes datos:

CREATE TABLE table1(
  c1 integer,
  c2 integer,
  c3 integer,
  c4 integer,
  c5 integer,
  count integer
);

INSERT INTO table1 VALUES (1,2,3,4,5,3);
INSERT INTO table1 VALUES (2,3,4,5,6,2);
INSERT INTO table1 VALUES (2,3,5,6,7,2);

CREATE TABLE table2(
  c1 integer,
  c2 integer,
  c3 integer,
  c4 integer,
  c5 integer,
  count integer
);
INSERT INTO table2 VALUES (1,3,4,5,6,3);
INSERT INTO table2 VALUES (1,2,3,4,5,2);

Puedes obtener tus datos de esta forma:

SELECT c1, c1,c2, c3, c4,c5,SUM(count) AS count
FROM (
  SELECT * FROM table1
  UNION ALL
  SELECT * FROM table2) AS foo
GROUP BY c1, c2, c3, c4, c5
ORDER BY c1, c2, c3, c4, c5

espero que esto pueda ayudarte