Rich, I figured since people ask me this question a lot, I would blog about it. Hope my explanation below helps a bit. http://www.bostongis.com/blog/index.php?/archives/37-Explain-Analyze-Geo metry-Relation-Operators-and-Joins-Except-Where.html Hope that helps, Regina
________________________________ From: Richard Heimann [C] [mailto:[EMAIL PROTECTED] Sent: Wednesday, October 24, 2007 7:01 PM To: Obe, Regina; 'PostGIS Users Discussion' Subject: RE: [postgis-users] ST_Difference Again, thank you both Paul and Regina... Regina, perhaps you can explain the LEFT JOIN. Nevertheless, it works with one disclaimer. I am left with the following message, perhaps you can demystify it? "NOTICE: LWGEOM_gist_joinsel called with incorrect join type Query returned successfully with no result in 812 ms." Paul, I also received a message from your sql though that was abit more inauspicious. I attempted to hack your sql though unsuccessfully. The first step is obvious; you are selecting all points that are not completely contained within country boundaries. And although the first query provides a list of all IDs it will not tell if the rest have the correct ids assigned to them. So, maybe the second query performs a data integrity check? Message below. Also, strangely, the output was a polygon feature. "row number -1 is out of range 0..-1 Total query runtime: 703 ms. 0 rows retrieved." Rich ________________________________ From: Obe, Regina [mailto:[EMAIL PROTECTED] Sent: Wednesday, October 24, 2007 5:23 PM To: PostGIS Users Discussion; [EMAIL PROTECTED] Cc: PostGIS Users Discussion Subject: RE: [postgis-users] ST_Difference Oops even easier SELECT pd.* FROM pandatestdata p LEFT JOIN world2_12nm w On st_contains(w.the_geom, p.the_geom) WHERE w.id IS NULL Here I am assuing w.id is the id of world2_12nm ________________________________ From: [EMAIL PROTECTED] on behalf of Paul Ramsey Sent: Wed 10/24/2007 5:01 PM To: [EMAIL PROTECTED] Cc: 'PostGIS Users Discussion' Subject: Re: [postgis-users] ST_Difference Oh, yeah, oops, I know. It's the join logic, its finding the things not contained for each world feature, not for the full world set. select pd.* from pandatestdata pd, (select p.id from pandatestdata p, world2_12nm w where st_contains(w.the_geom, p.the_geom)) as wd where pd.id <> wd.id; Ugly, but probably effective: find all the things contained, then just strip those out. The trouble is that disjointness is not an easily spatially indexable operation. If you're going to take this into operation with large data volumes, a more effective data set would be ocean polygons, cut up into smallish regular grid squares, then you can test containment with relatively good index selectivity. P. On 24-Oct-07, at 1:53 PM, Richard Heimann [C] wrote: > Thanks Paul for the response. I hadn't thought of this logic, it > didn't work > however. Strangely, it increased the number of features by eight > times. Any > thoughts? > > Vr > Rich > > > > -----Original Message----- > From: Paul Ramsey [mailto:[EMAIL PROTECTED] > Sent: Wednesday, October 24, 2007 4:07 PM > To: [EMAIL PROTECTED]; PostGIS Users Discussion > Subject: Re: [postgis-users] ST_Difference > > select p.* > from pandatestdata p, world2_12nm w > where not st_contains(w.the_geom, p.the_geom); > > On 24-Oct-07, at 11:45 AM, Richard Heimann [C] wrote: > >> Thanks for your response W. I should have posted the sql query >> earlier. >> Anyway...its below. >> >> My goal is to drop all features (points) that fall within my >> polygon. In >> this case, I have ship track data and want to filter it with world >> country >> buffers. >> >> World2_12nm - my world country 12nm buffered polygon >> Pandatestdata - track data (point) >> Panda_diff2 - new table >> >> >> CREATE TABLE panda_diff2 AS >> SELECT AsText(Difference(world2_12nm.the_geom, >> pandatestdata.the_geom)) FROM >> world2_12nm, pandatestdata >> >> Also attached are the astext versions of my data as well as a csv >> of the >> output from the above sql. Please debunk the mystery... >> >> Thanks again >> Rich >> >> -----Original Message----- >> From: Webb Sprague [mailto:[EMAIL PROTECTED] >> Sent: Wednesday, October 24, 2007 12:00 PM >> To: [EMAIL PROTECTED]; PostGIS Users Discussion >> Subject: Re: [postgis-users] ST_Difference >> >> You might get more useful help if you post an "astext()" version of >> your data, the query, the result, and a *desired* result. >> >> I know the difference functions can be tricky and are sometiimes >> defined in ways you might not expect. >> >> Thx >> W >> >> On 10/24/07, Richard Heimann [C] <[EMAIL PROTECTED]> wrote: >>> >>> >>> >>> >>> All, >>> >>> >>> >>> I would appreciate help in constructing an SQL statement that >>> performs a >>> difference function on two datasets. My intersect function works but >> cannot >>> work out the kinks with st_difference. Thanks for your help.Im >>> new to >>> Postgis. >>> >>> >>> >>> Rich >>> >>> >>> >>> >>> _______________________________________________ >>> 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 > > <panda_diff2.csv> _______________________________________________ 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. It is intended solely for the addressee. 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