sch.activity_log 
    activity_log_id   integer NOT NULL
    user_id           integer NOT NULL
    is_online         boolean
    insert_date       timestamp with time zone NOT NULL
    online_app        integer
    is_online_mobile  boolean NOT NULL
    last_seen         integer NOT NULL (unix epoch)

select x.* from ( select *, row_number() over (partition by user_id order by last_seen desc) as rn from sch.activity_log) x
where rn = 1

CREATE INDEX "IX_activity_log_last_seen"
    ON activity_log USING btree
    (last_seen DESC NULLS LAST)
    TABLESPACE pg_default;

CREATE INDEX "IX_activity_log_user_id"
    ON activity_log USING btree
    (user_id DESC NULLS LAST)
    TABLESPACE pg_default;

CREATE INDEX "IX_activity_log_user_id_last_seen"
    ON vk.activity_log USING btree
    (user_id, last_seen DESC NULLS LAST)
    TABLESPACE pg_default;