Sugiro escrever a consulta como:
select sum(i.amount) as total
from incomes i
where i.date >= '2019-07-01' and
i.date < '2019-08-01' and
i.deleted_at is null;
Esta consulta pode tirar proveito de um índice em
incomes(deleted_at, date, amount) :create index idx_incomes_deleted_at_date_amount on incomes(deleted_at, date, amount)
