Guys -- I'd like to point out that the issue with ST_value performance I think is the same issue we have with ST_GeometryN and ST_PointN.
That being the memory copy. For each ST_Value call the whole raster tile needs to be copied into memory. Paul can correct me if I am mistaken, but tht is how he explained the issue to me. This I consider a very fundamental limitation / problem with the plpgsql / PostGIS C whatever interface which causes a lot of wasted time since some things are just faster programming wise to implement in plgsql than in C and its esier to take advantage of PostgreSQL SQL power in plpgsql /sql than it is in C. This is BTW why plpgsql for this kind of task is really slow. C doesn't have the same issue if you are doing the steps in one call since its dealing with the same pointer. Thanks, Regina http://www.postgis.us > -----Original Message----- > From: postgis-users-boun...@postgis.refractions.net > [mailto:postgis-users-boun...@postgis.refractions.net] On > Behalf Of Nordgren, Bryce > Sent: Wednesday, October 12, 2011 4:02 PM > To: PostGIS Users Discussion > Subject: Re: [postgis-users] Retrieving Data from PostGIS > Rasterusing ST_Value > > 1.44 million pixel accesses does not sound like something > that should take 1/2 hour. > > For better or worse, raster depends on Postgresql's > underlying caching mechanism to make data I/O efficient; and > the unit of caching is one "raster tile". If I read the SRTM3 > data description correctly, each pixel is a 16 bit int, > making for a 2.88 Mb "raster tile" (if you loaded it all into > one row.) I'm a little out of my depth here, but your tile > may be too big for postgresql to cache. Does performance > improve if you load the SRTM data into many raster tiles? > > Bryce > > > -----Original Message----- > From: postgis-users-boun...@postgis.refractions.net > [mailto:postgis-users-boun...@postgis.refractions.net] On > Behalf Of elliott > Sent: Wednesday, October 12, 2011 8:28 PM > To: PostGIS Users Discussion > Subject: Re: [postgis-users] Retrieving Data from PostGIS > Raster using ST_Value > > It is just one SRTM3 tile which is 1201 x 1201. The data is > used by scientific codes to determine the elevation at a > given latitude and longitude. > > _______________________________________________ > 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
