Обеспечение свободного ограничения в postgres между несколькими клиентами

Создайте gist ограничение исключения с типом timestamp range

create table request (
    entry_id bigserial primary key,
    site_id bigint not null,
    uuid varchar(256) not null,
    session_start timestamp not null,
    session_end timestamp not null,
    user_ip varchar(40) not null,
    user_agent varchar(256) not null,
    constraint session_overlap exclude using gist (
        site_id with =,
        uuid with =,
        user_ip with =,
        user_agent with =,
        tsrange(session_end, session_end + interval '3 hours', '[)') with &&
    )
);

Теперь эта вставка не работает:

insert into request (site_id, uuid, session_start, session_end, user_ip, user_agent)
select site_id, uuid, ts::timestamp, ts::timestamp, user_id, user_agent
from (values
    (1, '123', '2014-01-01T10:00:32', '123.123.123.123', 'Mozilla/Chrome'),
    (1, '123', '2014-01-01T10:00:33', '123.123.123.123', 'Mozilla/Chrome'),
    (1, '123', '2014-01-01T10:00:34', '123.123.123.123', 'Mozilla/Chrome')
) s(site_id, uuid, ts, user_id, user_agent)
;
ERROR:  conflicting key value violates exclusion constraint "session_overlap"
DETAIL:  Key (site_id, uuid, user_ip, user_agent, tsrange(session_end, session_end + '03:00:00'::interval, '[)'::text))=(1, 123, 123.123.123.123, Mozilla/Chrome, ["2014-01-01 10:00:33","2014-01-01 13:00:33")) conflicts with existing key (site_id, uuid, user_ip, user_agent, tsrange(session_end, session_end + '03:00:00'::interval, '[)'::text))=(1, 123, 123.123.123.123, Mozilla/Chrome, ["2014-01-01 10:00:32","2014-01-01 13:00:32")).

Возможно, вам потребуется установить расширение btree_gist как суперпользователь

create extension btree_gist;

http://www.postgresql.org/docs/current/interactive/btree-gist.html

Ознакомьтесь с рекомендациями по блокировкам.

SELECT pg_advisory_lock(key);
// INSERT OR UPDATE...
SELECT pg_advisory_unlock(key);

Или используя версию с блокировкой nob:

SELECT pg_try_advisory_lock(key) INTO :acquired;
// if (acquired) then INSERT OR UPDATE...
SELECT pg_advisory_unlock(key);