Re: Overlapping timestamptz ranges with priority

On 27/06/2021 23:41, Ray O'Donnell wrote:
> Hi all,
>
> I'm playing with timestamptz ranges for a hobby project. I have a
> table with a tstzrange column, in which the timestamps can overlap;
> where they do, rows with a higher priority (derived from a bigint
> primary key column) should be picked.
>
> What I'd like to do is present a view which shows timestamp ranges at
> the front of the queue, as it were; where ranges overlap, these may
> be segments of a range from a particular row. I'm having trouble with
> this and would appreciate suggestions.

I've come up with a way of doing it using a function... it's not going
to be very efficient if the number of rows gets large, due to nested
loops, but as the system generally keeps only a limited number of
bookings (no more that a few hundred), I think it'll do - certainly as a
first run at it.

Firstly, the table structure (as it now stands) on which the function
will operate:

CREATE TABLE bookings
(
booking_id bigint NOT NULL GENERATED ALWAYS AS IDENTITY,
aircraft_reg text NOT NULL,
type_code text NOT NULL,
booking_time tstzrange NOT NULL,
owner_uid text NOT NULL,
owner_name text NOT NULL,

CONSTRAINT bookings_pk PRIMARY KEY (booking_id),

(... foregin keys etc...)
);

And here's the function:

create or replace function get_visible_bookings()
returns setof bookings
language plpgsql
as
$$
declare
m_rec bookings;
m_overlapping record;
m_visible_time tstzrange;
begin
-- Loop through all bookings on the system, ordered on booking ID.
-- The booking ID also give the queue priority of the booking:
-- bookings with a lower ID have a higher priority.
for m_rec in
select * from bookings order by booking_id
loop
m_visible_time := m_rec.booking_time;

-- For each booking, check whether there are any with
-- a higher priority and whose times overlap it.
for m_overlapping in
select booking_id, booking_time from bookings
where booking_id < m_rec.booking_id
and booking_time && m_rec.booking_time
loop
-- Snip away any overlapping (obscured) time.
m_visible_time := m_visible_time - m_overlapping.booking_time;
end loop;

-- If any of the current booking's time is still visible,
-- then return the row with what's left of the time.
if not isempty(m_visible_time) then
return next row(m_rec.booking_id, m_rec.aircraft_reg,
m_rec.type_code, m_visible_time,
m_rec.owner_uid, m_rec.owner_name);
end if;
end loop;

return;
end;
$$;

--
Raymond O'Donnell // Galway // Ireland
ray(at)rodonnell(dot)ie