create table webhook_details (
  currency_rate_event text references currency_rate_events not null,
  url text not null
);
comment on table webhook_details is '
  describe subscribed action and corresponding url of a webhook
';

comment on column webhook_details.currency_rate_event is '
  enum value, currency rate event that this webhook subscribe to
';
comment on column webhook_details.url is '
  where POST request to when subscribed event happens
';