[postgis-users] Topology: cannot delete slivers (or gaps)

[Rémi Cura]

This is suprising indeed.
Im my test using the v.out.postgis was still way faster than constructing
with postgis topology (about x3 I think).

Are you using v.out.postgis -l, with grass 7?


Grass is a topology that is more node centered, opposite to postgis
topology which is more edge centered.
It is possible to output a node centered topology with grass as text (*v.info
<http://v.info> -t*),
but I found it unpractical to try to convert this into a propoer postgis
topology with pure SQL.
Now I think I should have used a python way from within grass (grass offers
full binding, so it should be doable to loop trough grass internal topology
and convert it to postgis topology)

I tried to reach grass dev several time to try to improve the postgis
export, I had no answer.

My intuition is that the slowness comes from the fact that grass tries to
write  edge by edge , and each time the edge_data trigger are triggered.
The correct way would be to compute full topology structure, then
deactivate trigger, then fill table , then reactivate trigger.

Cheers,
Rémi-C

2014-11-18 19:48 GMT+01:00 Sandro Santilli <strk at keybit.net>:

> On Wed, Nov 12, 2014 at 10:51:34AM +0100, Rémi Cura wrote:
> > Note you could have directly exported into postgis topology, which is
> slow
> > but very convenient (stil faster than postgis topology conversion).
> > I'm not a grass user,
> > from what I understood you have to use the "-l" switch in the command "
> > *v.out.postgis*"
>
> I'm trying this out today, and indeed is very slow, much slower than
> building the topology, which is surprising to me.
>
> A look at database activity shows that the command (v.out.postgis -l)
> is writing _simple_ features in the output layer.  Do you have an idea
> about how to skip that part ? (TopoGeometry would be enough for PostGIS
> to reconstruct the SFS on demand).
>
> --strk;
>
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