[postgis-users] Buffer Performance

[Alexander Pucher]

Hi all,

Martin, sounds clear to me, but what's the deal with the intersection? 
 From my experience, when I buffer 50 line segments, I get 50 buffers 
(i.e 50 "rows"). So no intersection is made between the several buffers, 
as far as I can see. Please correct me if I'm wrong.

reagrds,
alex.

Martin Davis wrote:

> The performance difference is probably due to the difference in the 
> buffer widths.  In general, larger buffer widths require longer to 
> compute than smaller ones.  This is due to the greater interaction 
> (intersections) between the offset curves computed to build the buffer 
> curve.
>  
>  
> Martin Davis, Senior Technical Architect
> *Vivid Solutions Inc.*
> /Suite #1A-2328 Government Street Victoria, B.C. V8T 5G5
> Phone: (250) 385 6040 - Local 308 Fax: (250) 385 6046/
>
>     -----Original Message-----
>     *From:* Alexander Pucher [mailto:pucher at atlas.gis.univie.ac.at]
>     *Sent:* Friday, March 12, 2004 3:39 AM
>     *To:* PostGIS Users Discussion
>     *Subject:* [postgis-users] Buffer Performance
>
>     Hi,
>
>     did some buffer performance testing with a table that holds about
>     2100 multilinestrings (river network of europe).
>
>     'select buffer(the_geom,5) from river;'
>
>     takes about 7 seconds to execute, whereas
>
>     'select buffer(the_geom,15000) from river'
>
>     takes a couple of minutes...
>
>     The 'river' table is not indexed, if this might be of importance ?!?
>
>     'explain select....'
>
>     shows similar output for both queries:
>
>     QUERY PLAN
>     -----------------------------------------------------------
>      Seq Scan on river  (cost=0.00..139.51 rows=2051 width=32)
>     (1 row)
>
>
>     What's the reason for the difference in execution time, depending
>     on the buffer size?
>     Is there a way to speed up execution time of such queries??
>
>     Best regards,
>     alex.
>
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