Actually this is a side comment to the part the part below. I have always stored the most commonly used transforms in my table as a secondary column, but I was wondering how much it slows down a query if you don't select the fields you don't need. I imagine there must be some slow down. Otherwise column-oriented databases wouldn't be quite as useful compared to row-oriented databases as they are - http://www.databasecolumn.com/2007/09/stonebraker-comment-response.html ..
Has anyone done any benchmarks on using a view with index on transform vs. index plus realized transform? "If you want to squeeze a little bit more performance out of your query, you can cache the transform as a new column, so it's already precomputed.... but I would not store this in the same table. If the goal here is speed, then duplicating points in your table will double the table size, slowing your query time. Create a new table for this. The problem with this approach is that you now have duplicate data and you may have to write triggers to deal with modifications done to your original points. " Thanks, Regina -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Kevin Neufeld Sent: Friday, September 28, 2007 11:39 AM To: PostGIS Users Discussion Subject: Re: [postgis-users] Large point table Unfortunately Shane, I totally disagree. If one want to do transforming or grouping in a grid, it's far, far easier to do this in right in PostGIS. Steve, First, aggregating the points into a grid is a great approach. I've been working with a table of ~ 170 million points and using this technique, I've speed up my search queries significantly by grouping these points into a multipoints of about 150-200 points. As you've probably discovered, this can be done by creating a new table, and simply grouping your points using the snaptogrid function. CREATE TABLE new_pt_tbl AS SELECT collect(my_pt) AS my_multi_pt FROM pt_tbl GROUP BY ST_SnapToGrid(my_pt, <grid_size>); Second, a view is the same as a query. A view in postgresql is just a stored query. If you don't want to always type "ST_Transform...." in your query, then using a view is the way to do this. PostGIS is pretty good in doing transforms on the fly, so using a view is a good approach. Keep in mind that you may have to create a functional index on your multipoint geometry column if you want to use an index in Albers. CREATE INDEX new_pt_tbl_idx ON new_pt_tbl USING GIST ON (ST_Transform(my_multi_pt, <Albers SRID>)); If you want to squeeze a little bit more performance out of your query, you can cache the transform as a new column, so it's already precomputed.... but I would not store this in the same table. If the goal here is speed, then duplicating points in your table will double the table size, slowing your query time. Create a new table for this. The problem with this approach is that you now have duplicate data and you may have to write triggers to deal with modifications done to your original points. Hope this helps. Kevin ------------- Kevin Neufeld Software Developer Refractions Research Inc. 300-1207 Douglas St. Victoria, B.C., V8W 2E7 Phone: (250) 383-3022 Email: [EMAIL PROTECTED] Shane Spencer wrote: > stupid mouse.. > > As I was saying.... > > You would be better off pre-transforming them with ogr2ogr.. dump them > to a shapefile, write a quicky script to move the points around to > match your grid (if that is what you are indeed doing) then insert > them into the database as the right projection. > > It sounds like you will have a lot of duplicate points. I recommend > you only insert distinct points into a table, then make a reference > table with smaller data types and a primary key referencing the point > table. Python could handle that kind of insert situation pretty easily. > > On 9/25/07, *Shane Spencer* <[EMAIL PROTECTED] > <mailto:[EMAIL PROTECTED]>> wrote: > > you would be better off pre-transforming them with ogr2ogr.. dump > them to a shapefile, write a q > > > On 9/25/07, * Stephen Crawford* < [EMAIL PROTECTED] > <mailto:[EMAIL PROTECTED]>> wrote: > > All, > > I have a table of about 30 million point observations. The > geometries are > stored in lon/lat., with a gist index on the column. For > display and > analysis I want to use an albers projection....mostly I'm > aggregating the > points to a grid. Currently I just do the transform(geom) in > my queries. > Would it be better for me to perhaps create a view with the > tranformation > instead of in my query....or should I instead create another > geometry column > in the original table, with the tranformation and do another > gist index on > the new column? What's the best approach? > > Thanks, > Steve > > > Stephen Crawford > Center for Environmental Informatics > The Pennsylvania State University > > _______________________________________________ > postgis-users mailing list > [email protected] > <mailto:[email protected]> > http://postgis.refractions.net/mailman/listinfo/postgis-users > <http://postgis.refractions.net/mailman/listinfo/postgis-users> > > > > ------------------------------------------------------------------------ > > _______________________________________________ > postgis-users mailing list > [email protected] > http://postgis.refractions.net/mailman/listinfo/postgis-users > _______________________________________________ postgis-users mailing list [email protected] http://postgis.refractions.net/mailman/listinfo/postgis-users ----------------------------------------- The substance of this message, including any attachments, may be confidential, legally privileged and/or exempt from disclosure pursuant to Massachusetts law. 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