Re: Finding first free time from reservations table

On 11/14/2012 01:02 PM, Andrus wrote:
> I’m looking for a way to find first free time in reservations table.
> Reservation table contains reservations start dates, start hours and
> durations.
> Start hour is by half hour increments in working hours 8:00 .. 18:00
> in work days.
> Duration is also by half hour increments in day.
> CREATE TABLE reservation (
> id serial primary key,
> startdate date not null, -- start date
> starthour numeric(4,1) not null , -- start hour 8 8.5 9 9.5 ..
> 16.5 17 17.5
> duration Numeric(3,1) not null -- duration by hours 0.5 1 1.5
> .. 9 9.5 10
> );
> table structure can changed if required.

I'm not sure if it will work well for your specific use-case and it
requires an up-to-date version (9.2+??) but I would recommend
investigating range types which have some characteristics that are
useful for reservation and calendaring applications including the
ability to have a "non-overlapping" constraint that prevents creating a
record with a range that overlaps an existing range in the table.

Instead of having three columns (startdate, starthour and duration) you
would have a single column of type tsrange which includes the starting-
and ending-times of each reservation.

Here's the info on range types:
http://www.postgresql.org/docs/9.2/static/rangetypes.html

If you want to limit reservations to start/end at half-hours and/or to
certain times of the day you will probably want to include those
constraints in your table definition.

You asked about finding a free half-hour but since you show durations
that exceed a half-hour, you may want to include the capability to
search for the first available occurrence of X free-time.

Range-types are new and I'm not experienced with them - others may have
better ideas - but the method of finding the first occurrence that
springs to mind is to make a query that uses generate_series to create a
list of "candidate" reservation periods of the desired duration and
select the first one that doesn't overlap an existing reservation. This
should work fine as long as you are looking a limited time in the future
(there are fewer than 20 possible start-times in a day so even looking
100-days ahead is only 2000 candidates) however generating a series of
ranges may involve a sub-query - I don't know if you can generate a
series of ranges directly.

Hope this helps.

Cheers,
Steve