[postgis-devel] [PostGIS] #2024: raster: ST_Union is buggy

PostGIS trac at osgeo.org
Fri Oct 5 11:02:05 PDT 2012

#2024: raster: ST_Union is buggy
  Reporter:  robe     |       Owner:  dustymugs    
      Type:  defect   |      Status:  closed       
  Priority:  blocker  |   Milestone:  PostGIS 2.1.0
 Component:  raster   |     Version:  trunk        
Resolution:  fixed    |    Keywords:               

Comment(by robe):


 I'll have to dig up the example I saw.  In most cases I would say at least
 98% of cases 2.1 is faster, however there are cases I have run into where
 the 2.0 (with clipping) is faster than 2.1 with clipping.  Sorry I threw
 it out.  I have to verify the answer is correct in 2.0 in the ones I saw.

 So might be an issue that 2.1 is more sensitive to difference in extents
 than 2.0 is or 2.0 was just wrong.

 The reason I use clipping is because in 2.0, Clipping before unioning was
 almost ALWAYS faster than trying to do union alone and in 2.1 this does
 not seem to be the case.

 I also see it as the most common use case.  Why would I want to just union
 tiles and be relegated to the size of my tiles that happen to have my area
 of interest. In fact if you were going to fix an extent -- I would suggest
 you allow the user to pass the extent in as an argument to Union allowing
 the union to create the size and throw out anything that doesn't fit.
 That would save having to compute in most cases and probably satisfy a lot
 of user needs.

 I see the common case -- as doing small analysis of a region say a 500x500
 area and clipping to that region, which is what the browser would expect.
 The browser only wants to see the region that was asked for. But my
 perspective might be different since I'm coming from a web-developer POV
 where I see the user specifying the area and I give them just that region
 that fits the area they requested.  The unioning tons and tons of huge
 tiles seems useful if you are outputting large coverages as a single file,
 which quite frankly puts me to sleep.

Ticket URL: <https://trac.osgeo.org/postgis/ticket/2024#comment:26>
PostGIS <http://trac.osgeo.org/postgis/>
The PostGIS Trac is used for bug, enhancement & task tracking, a user and developer wiki, and a view into the subversion code repository of PostGIS project.

More information about the postgis-devel mailing list