Creating a View with "clipped" data

Question

Hi all,
I'm wondering if someone might know if it's possible to have clipped data in a view without clipping the original data that the view is drawing from.
The problem we have is that we have a full dataset that we need to keep in a complete state (as a table in Oracle). We want to be able to create a View that will display a subset of data from this table, however, some of the objects in the full dataset cross a boundary and for these objects, we want the View to only show the part of the object within that boundary.
I know we can clip the original layer but we ideally need to keep that layer untouched and do the clipping on-the-fly in the View. Is this possible?
Thanks for any help you can give,
Cheers, Paul

Answer 1

Do you want to:
create view my_clip
as
select a.id,
a.attribute,
mdsys.sdo_intersection(a.geometry,b.geometry,0.005) as geometry
from mycliparea b,
mytable a
where sdo_anyinteract(a.geometry,b.geometry) = 'TRUE'
Now this will work but to get performance you will have to do one of two things:
1. Create a function based index. See doco or other postings in this forum for examples
of how to do this.
2. Change the view from an ordinary view to a materialized view. And then create
a normal spatial index on the computed shape. (I recommend the use of
these as against ordinary views.)
However, Bryan is right, the problem with SDO_INTERSECTION is that it can return:
1. Polygons;
2. Lines;
3. Points;
4. Compound objects containing one or more of the above.
If you want polygons only, then you need to extract them.
<selfpromtion>
I have written a set of extraction functions that do this which are available for free.
Contact me at "spatialdbadvisor at netspace dot net dot au" or "simon at spatialdbadvisor dot com"
</selfpromotion>
The correct view definition using one of my functions would be:
create view my_clip
as
select a.id,
a.attribute,
geom.ExtractPolygon(
mdsys.sdo_intersection(a.geometry,b.geometry,0.005))
as geometry
from mycliparea b,
mytable a
where sdo_anyinteract(a.geometry,b.geometry) = 'TRUE'
regards
S