[postgis-users] Only allowing valid geometries to be created

[David Blasby]

We are soon going to have functions like intersect(g1,g2), xor(g1,g2), 
union (g1,g2) - these functions pretty much require valid geometries.

The results of a function like union(g1,g2) where g1 or g2 are invalid 
could cause (1) errors (2) invalid results.

I've been thinking about:

1. Only allow valid geometries to be created.  Basically the creation 
would look like:
        a) Create Geometry  like now (does very small amount of validity 
checking)
        b) run the isvalid() function on it.  If it fails, throw an error
I.e.:
select 'LINESTRING(0 0, 0 0)'::geometry;
ERROR: invalid geometry

In this way (a) tables will only ever have valid geometries (b) 
functions will always have valid geometries as inputs (c) Only have to 
run the isValid() function on a geometry once (at creation).

Unfortunately, the main disadvantage to this is sometimes people want to 
have invalid geometries.  People might have multipolygns that overlap, 
or GPS tracks that have repeated points, etc...  Unfortunately, they 
would never be able to get these geometries inside PostGIS - leading to 
quite a bit of frustration during dataloading or querying.

Also, it would require geos to be installed.  As a work around to this, 
we could just ignore the validity test -  postgis would behave exactly 
as it does now.  This will not be a problem because all the 
sensitive-to-invalid-geometry-functions are in GEOS and, therefore, 
never callable.

2. Add a flag to Geometry that is either VALID, INVALID, UNKNOWN.
       a) Create Geometry  like now (does very small amount of validity 
checking)
       b) run the isvalid() function on it.  
                INVALID - flag geometry
                VALID     - flag geometry
                geos not installed - flag geometry  UNKNOWN

Then the GEOS functions could throw an error if the flag is INVALID. 
 Bad-dog geometries could still be put into the system, but be unuseable 
with GEOS .
The big problem with this is that these flagged geometries would be 
binary-incompatible with the current geometries - a dump and restore 
would be require before you could upgrade.

3. Keep things as they are  - do a isvalid() whenever you do a GEOS 
function.  This would work fine, but will be slow due to always running 
isvalid().

I'm leaning towards #2.

What think?
dave