internal sealed class Person
{
    internal int Id { get; set; }
    internal string FirstName { get; set; }
    internal string LastName { get; set; }
    internal bool IsActive { get; set; }
}
internal sealed class Visit
{
    internal int Id { get; set; }
    internal int PersonId { get; set; }
    internal DateTime Date { get; set; }
    internal decimal Spent { get; set; }
}
// ...
internal Person[] persons = new Person[];
internal Visit[] visits = new Visit[];
// ...

create table public.visit
(
    id integer,
    person_id integer,
    visit_datetime timestamp without time zone,
    spent money
) tablespace pg_default;
create table public.person
(
    id integer,
    first_name character varying(100) COLLATE pg_catalog."default",
    last_name character varying(100) COLLATE pg_catalog."default",
    is_active boolean
) tablespace pg_default;

public decimal NestedLoop()
{
    decimal result = 0;
    var upperLimit = new DateTime(2020, 12, 31);
    foreach (var person in persons)
    {
        if (person.IsActive == false)
        {
            continue;
        }
        foreach (var visit in visits)
        {
            if (person.Id == visit.PersonId && visit.Date <= upperLimit)
            {
                result += visit.Spent;
            }
        }
    }
    return result;
}

select setseed(0.777);
delete from public.person;
insert into public.person(id, first_name, last_name, is_active)
select row_number() over () as id,
substr(md5(random()::text), 1, 10) as first_name,
substr(md5(random()::text), 1, 10) as last_name,
((row_number() over ()) % 5 = 0) as is_active
from generate_series(1, 5000);/*<-- 5000 это число клиентов*/
delete from public.visit;
insert into public.visit(id, person_id, visit_datetime, spent)
select row_number() over () as id,
(random()*5000)::integer as person_id, /*<-- 5000 это число клиентов*/
DATE '2020-01-01' + (random() * 500)::integer as visit_datetime,
(random()*10000)::integer as spent
from generate_series(1, 10000); /* 10000 - это общее число визитов в СТО*/

select sum(v.spent) from public.visit v
                    join public.person p on p.id = v.person_id
where v.visit_datetime <= '2020-12-31' and p.is_active = True

set enable_hashjoin to 'off';--Заставляем БД использовать Nested Loop
set enable_mergejoin to 'off';
set enable_material to 'off';
select sum(v.spent) from public.visit v
join public.person p on p.id = v.person_id
where v.visit_datetime <= '2020-12-31' and p.is_active = True

set enable_hashjoin to 'on';
set enable_mergejoin to 'on';
set enable_material to 'on';
select sum(v.spent) from public.visit v
join public.person p on p.id = v.person_id
where v.visit_datetime <= '2020-12-31' and p.is_active = True