I would like to add a temporal contrib module. The most important piece
would be adding a period data type and some support functions. Jeff
Davis and I both have temporal projects on pgFoundry, and we've been
collaborating for a while. But there are some areas we'd like to get
some advice on.
Disk format - A period can be represented as [closed-closed],
(open-open), [closed-open) or (open-closed] intervals. Right now we
convert these to the most common form, closed-open and store as two
timestamptz's.
Nulls - A common use case for periods is for modeling valid time. Often
the end point is not known. For instance, you know when an employee has
been hired but the termination time typically wouldn't be known ahead of
time. We can either represent these with a null end time or with
infinity. But I'm not sure how to deal with them. Obviously we can test
for containment and overlap. But what about length or set operations?
Non-contiguous Sets - A period defines a contiguous set of time. But
many times we need to work with non-contiguous sets (work shifts in a
week, bus schedules, etc). Right now, I'm using period arrays. But
period arrays can contain overlapping and adjacent periods. And we have
no way to indicate that a period array has been coalesced into a
non-contiguous set. And what indexing strategies could be used with
non-contiguous sets?
Temporal Keys - We need two types of temporal keys. A primary key,
exclusion type prevents overlap so someone isn't at two places at the
same time. And a foreign key, inclusion type so we can check that the
valid time of a child is contained with in the valid time of the parent.
Jeff is working on the former, but there is no easy way to do the latter.
There is actually a lot of theory out there but very few
implementations. Although not an official standard, we try to follow the
TSQL2 spec pretty closely. Further reading:
Developing Time-Oriented Database Applications - Snodgrass
http://www.cs.arizona.edu/~rts/tdbbook.pdf
TSQL2 spec ftp://ftp.cs.arizona.edu/tsql/tsql2/spec.pdf
Temporal Data and the Relational Model - Date et al
http://books.google.com/books?isbn=1558608559
Dozens of publications
http://timecenter.cs.aau.dk/pub.htm
Regards,
Scott Bailey
