[postgis-users] Topological Exceptions

[Martin Davis]

Sounds good to me. 

If you wanted to see which geoms caused the problem, couldn't you just 
run another query looking only for ones which returned NULL from the 
operation, and then check the log?

The log msg could also give the first couple of points of the offending 
geoms - that would narrow it down pretty good.

Does ST_Contains (and the other predicates) ever result in 
TopologyExceptions?  They are generally pretty darn robust, to my 
knowledge.  It's the overlay ops which are the real bad boys.

Paul Ramsey wrote:
> On 3/12/08, Martin Davis <mbdavis at refractions.net> wrote:
>
>   
>>  Orginally the idea was that it was nice to report the actual nature of
>>  the exception, since it contains an indication of where the error
>>  occurred.   It would be nice to be able to continue to have access to
>>  this information.  One way to do this would be to have a parallel set of
>>  functions which would abort as they do now.  Or is there some other way
>>  to report this info - to a log somewhere perhaps?
>>     
>
> I prefer using something like NULL to trying to create a new geometry
> type to encompass the error... particularly if the error conditions
> aren't that interesting to people.  We can get into the log easily
> enough by using a WARNING log level, the only question then is how to
> identify which geometries caused the failure.  One way might be to
> hash the geometries, so that if people want to find their bad
> geometries from the log they can just select for the hash value.
>
> This means that the return values of
>
> ST_Contains(geom,geom) => true, false or null
>
> and
>
> ST_Intersection(geom,geom) => geometry or null
>
> I just checked, and null seems to be interpreted as false in a WHERE
> clause, so adding this behavior probably wouldn't wreck too many
> joins.
>
> Paul
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>   

-- 
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022