Hi Peter et al,
On 9/1/23 12:56, Paul Jungwirth wrote:
On 9/1/23 11:30, Peter Eisentraut wrote:
I think the WITHOUT OVERLAPS clause should be per-column, so that
something like UNIQUE (a WITHOUT OVERLAPS, b, c WITHOUT OVERLAPS)
would be possible. Then the WITHOUT OVERLAPS clause would directly
correspond to the choice between equality or overlaps operator per
column.
I think allowing multiple uses of `WITHOUT OVERLAPS` (and in any
position) is a great recommendation that enables a lot of new
functionality.
I've been working on implementing this, but I've come to think it is the
wrong way to go.
If we support this in primary key and unique constraints, then we must
also support it for foreign keys and UPDATE/DELETE FOR PORTION OF. But
implementing that logic is pretty tricky. For example take a foreign key
on (id, PERIOD valid_at, PERIOD asserted_at). We need to ensure the
referenced two-dimensional time space `contains` the referencing
two-dimensional space. You can visualize a rectangle in two-dimensional
space for each referencing record (which we validate one at a time). The
referenced records must be aggregated and so form a polygon (of all
right angles). For example the referencing record may be (1, [0,2),
[0,2)) with referenced records of (1, [0,2), [0,1)) and (1, [0,1),
[1,2)). (I'm using intranges since they're easier to read, but you could
imagine these as dateranges like [2000-01-01,2002-01-01).) Now the
range_agg of their valid_ats is [0,2) and of their asserted_ats is
[0,2). But the referenced 2d space still doesn't contain the referencing
space. It's got one corner missing. This is a well-known problem among
game developers. We're lucky not to have arbitrary polygons, but it's
still a tough issue.
Besides `contains` we also need to compute `overlaps` and `intersects`
to support these temporal features. Implementing that for 2d, 3d, etc
looks very complicated, for something that is far outside the normal use
case and also not part of the standard. It will cost a little
performance for the normal 1d use case too.
I think a better approach (which I want to attempt as an add-on patch,
not in this main series) is to support not just range types, but any
type with the necessary operators. Then you could have an mdrange
(multi-dimensional range) or potentially even an arbitrary n-dimensional
polygon. (PostGIS has something like this, but its `contains` operator
compares (non-concave) *bounding boxes*, so it would not work for the
example above. Still the similarity between temporal and spatial data is
striking. I'm going to see if I can get some input from PostGIS folks
about how useful any of this is to them.) This approach would also let
us use multiranges: not for multiple dimensions, but for non-contiguous
time spans stored in a single row. This puts the complexity in the types
themselves (which seems more appropriate) and is ultimately more
flexible (supporting not just mdrange but also multirange, and other
things too).
This approach also means that instead of storing a mask/list of which
columns use WITHOUT OVERLAPS, I can just store one attnum. Again, this
saves the common use-case from paying a performance penalty to support a
much rarer one.
I've still got my multi-WITHOUT OVERLAPS work, but I'm going to switch
gears to what I've described here. Please let me know if you disagree!
Yours,
--
Paul ~{:-)
p...@illuminatedcomputing.com
