It will be _very_ slow. So much that you may prefer to make the topology persistent.
Another approach I'd like to see implemented would be to expose a tolerance to the GEOS implemented functions. Basically exposing the GEOS PrecisionModel. By doing that you could specify a tolerance in the overlay operations and the result should never have gaps bigger than the given tolerance. --strk; On Thu, Oct 25, 2012 at 10:28:19AM -0700, pcr...@pcreso.com wrote: > My impression is that working topologies is slow, especially with large > datasets - lots of computation involved. > > How effective would this be with hundreds of thousands or millions or small > polygons? > > Just running ST_Union() takes hours to days. > > Thanks > > Brent Wood > > --- On Fri, 10/26/12, Sandro Santilli <s...@keybit.net> wrote: > > From: Sandro Santilli <s...@keybit.net> > Subject: Re: [postgis-users] How to eliminate small gaps produced by ST_union? > To: "PostGIS Users Discussion" <postgis-users@postgis.refractions.net> > Date: Friday, October 26, 2012, 6:04 AM > > You may try to construct a topology within PostGIS-2.0, using > a small tolerance, and then get the geometries back. > > --strk; > > On Thu, Oct 25, 2012 at 05:54:05PM +0100, Alexandre Neto wrote: > > I'm trying to Aggregate\Dissolve a set of adjacent, but not touching > > polygons in Postgis. The geometries were created editing the Postgis table > > in QGIS created, with snapping and avoid overlap in the layer. > > > > I used ST_Union like this: > > > > SELECT ST_Union(the_geom) > > FROM table; > > > > Small gaps similar to lines appear in the result. This happens when the > > adjacent polygons do not share the exact same vertices. > > > > [image: Inline image 2] > > Is there a way to eliminate this "gaps" or "small angles" within Postgis? > > > > I know that v.clean in GRASS can eliminate small angles, and correct > > topology, but that would mean I needed to copy the features to GRASS, > > correct them and copy it back to Postgis. > > > > Thanks in advance, > > > > Alexandre Neto > > > _______________________________________________ > postgis-users mailing list > postgis-users@postgis.refractions.net > http://postgis.refractions.net/mailman/listinfo/postgis-users > _______________________________________________ > postgis-users mailing list > postgis-users@postgis.refractions.net > http://postgis.refractions.net/mailman/listinfo/postgis-users -- http://www.cartodb.com - Map, analyze and build applications with your data ~~ http://strk.keybit.net _______________________________________________ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users