filter

create table historicos (
    id serial primary key,
    problemas float,
    total float,
    data timestamp
);

insert into historicos (problemas, total, data)
  values (25, 125, '2017-01-01 00:00:00');
insert into historicos (problemas, total, data)
  values (25, 125, '2017-01-10 00:00:00');
insert into historicos (problemas, total, data)
  values (25, 125, '2017-02-01 00:00:00');
insert into historicos (problemas, total, data)
  values (25, 125, '2017-03-01 00:00:00');
insert into historicos (problemas, total, data)
  values (25, 125, '2017-04-01 00:00:00');
insert into historicos (problemas, total, data)
  values (25, 125, '2017-06-01 00:00:00');
insert into historicos (problemas, total, data)
  values (25, 125, '2017-08-01 00:00:00');
insert into historicos (problemas, total, data)
  values (25, 125, '2017-08-21 00:00:00');
insert into historicos (problemas, total, data)
  values (25, 125, '2017-09-01 00:00:00');
insert into historicos (problemas, total, data)
  values (25, 125, '2017-10-01 00:00:00');
insert into historicos (problemas, total, data)
  values (25, 125, '2017-11-01 00:00:00');
insert into historicos (problemas, total, data)
  values (25, 125, '2017-12-01 00:00:00');

select
  sum(problemas) filter (where data between '2017-01-01' and '2017-02-28') as problemas_mes_1,
  sum(total) filter (where data between '2017-01-01' and '2017-02-28') as total_mes_1,
  sum(problemas) filter (where data between '2017-03-01' and '2017-04-30') as problemas_mes_2,
  sum(total) filter (where data between '2017-03-01' and '2017-04-30') as total_mes_2
from historicos;
problemas_mes_1 | total_mes_1 | problemas_mes_2 | total_mes_2
-----------------+-------------+-----------------+-------------
              75 |         375 |              50 |         250