[postgis-users] Buffer bug ?

[strk at refractions.net]

On Wed, Sep 07, 2005 at 08:19:05PM +0200, Daniel wrote:
> Thank u very much. We really appreciate all you do for PostGIS users. 
> I'll try your suggested update as soon as possible.
> 
> Nevertheless, we have had our application working quite well "before", 
> and I really don't see why it doesn't work anymore. As a PostGIS expert, 
> could you see a way to optimize/split our "huge query" ?

Try collecting buffered geometries, rather then buffer collected ones,
this might reduce complexity of the graph operations.

Anyway, if you keep the discussion on the mailing list there'll be
more minds connected, and other users might find these informations
useful ... 

--strk;

> 
> Yours,
> 
> dF
> 
> 
> strk at refractions.net wrote:
> 
> >Ok, I downloaded your test case and tried comparing JTS,
> >GEOS-2.1.4 and GEOS-1.0.0.
> >
> >I used PostGIS version 1.1.0CVS as test interface.
> >Nothing changed in collect() and npoints() since PostGIS-1.0.3,
> >so results should be about the same in the two versions.
> >
> >The query is:
> >
> >	-- ### is Number of features
> >	SELECT npoints(buffer(collect(geom), 1000))
> >	FROM ( select lga_geom as geom from lg_annee limit ### ) as foo;
> >
> >Number of features: 200 (1106 input points in collected MultiPolygon)
> >
> >	[JTS-1.7]    output_points 1754
> >		real    0m8.705s
> >
> >	[GEOS-2.1.4] output_points 1816
> >		real    0m5.594s
> >
> >	[GEOS-1.0.0] output_points 1735
> >		real    0m16.450s
> >
> >Number of features: 400 (2320 input points in collected MultiPolygon)
> >
> >	[JTS-1.7]    output_points 1897
> >		real    1m3.761s
> >
> >	[GEOS-2.1.4] output_points 1959
> >		real    0m43.020s
> >
> >	[GEOS-1.0.0] output_points 1912
> >		real    6m50.783s
> >
> >Number of features: 1000 (5992 input points in collected MultiPolygon)
> >
> >	[JTS-1.7]    output_points 2598
> >		real    9m20.873s
> >
> >	[GEOS-2.1.4] output_points 2618
> >		real    10m54.414s
> >
> >	[GEOS-1.0.0] 
> >		KILLED AFTER:
> >		real    94m14.421s
> >
> >
> >I don't really understand why number of output points change, anyway
> >the test shows that GEOS-2.1.4 is *much* faster then GEOS-1.0.0
> >(and sometimes faster then JTS-1.7). For 1000 features GEOS-1.0.0
> >is completely stuck after 94 Minutes !!
> >
> >So, to conclude, I could not verify the reported bug.
> >
> >I suggest you update GEOS to version 2.1.4 and PostGIS
> >to the upcoming 1.0.4 release. Then, if you can reproduce the
> >OLD vs. NEW performance loss problem yourself, please provide a
> >*simple* test case.
> >
> >As for killing GEOS during operation this is an old and known
> >problem we might approach soon or late, but it is not first
> >priority at the moment.
> >
> >--strk;
> >
> >
> >On Mon, Aug 29, 2005 at 11:28:13AM +0200, Daniel wrote:
> > 
> >
> >>Yes: it's harder for the whole community when you're away ! ;-)
> >>
> >>My dataset is about 5 Mo, so I would not post it on a public list ... 
> >>Can I upload it on a server and give you a private access ?
> >>
> >>I tried tests on reduced datasets, too: it works, but very slowly. Far 
> >>much slowly than with earlier versions. The execution time is 
> >>exponential. For example:
> >>100 features: 303 ms
> >>200               : 2478 ms
> >>500               : 34690 ms
> >>
> >>With our first implementation (firsts postgis versions with geos), we 
> >>got about 25 seconds for 7500 features.
> >>
> >>Sincerely yours,
> >>
> >>dF
> >>
> >>
> >>
> >>strk at refractions.net wrote:
> >>
> >>   
> >>
> >>>On Fri, Aug 26, 2005 at 04:33:27PM +0100, Mark Cave-Ayland wrote:
> >>>...
> >>>
> >>>
> >>>     
> >>>
> >>>>Unfortunately I'm now away from work for at least a week, but I hope 
> >>>>that
> >>>>someone else on the list will be able to give you some help in the 
> >>>>meantime.
> >>>> 
> >>>>
> >>>>       
> >>>>
> >>>Hello guys, it looks like I'm missing something here...
> >>>Daniel, would you post a test case for me to check out ?
> >>>A reduced case working on your old setup and hanging
> >>>on the new one ?
> >>>
> >>>--strk;
> >>>_______________________________________________
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> >>>postgis-users at postgis.refractions.net
> >>>http://postgis.refractions.net/mailman/listinfo/postgis-users
> >>>
> >>>
> >>>     
> >>>
> >
> >
> > 
> >
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> >>   
> >>
> >
> > 
> >
>