Dear Regina, Many thanks for this suggestion.
I ran the query in this form and for one raster it takes 3372s (~55 mins). I guess that is what I had anticipated from the single tile exercise I ran with the alternate algorithm. This still seems a little too long however and so I have started to explore ways to improve upon this by pre-processing some of the data. The cells of interest represent the ‘main channels’ in a drainage network. Although in future this network may change position from one output timestep to another (they are actually 1000l iterations of the model apart) in this particular variant the position of these cells is static. With that in mind I decided to create a binary raster where the 1’s represent the channel cells (I had to do this in arcgis as QGIS does not appear to have a Con function in the raster calculator!) and then exported this as a point layer. I then deleted the points coded zero and saved the shp file in QGIS. I imported the ntwork shp file into postgis (there are about 15500 15 points) and I am now running: CREATE TABLE mymodel.networkdep AS SELECT filename, gid, ST_Value(rast, geom) val FROM mymodel.deposition, mymodel.network WHERE ST_Intersects(rast, geom) ORDER BY gid, rid; I will sum by raster (i.e. filename) using the new table but will settle for just having the relevant data for now. This took 2057s(~35 minutes!) to complete! If this is the best way to do it I will explore the OGR library and try and hardcode the network point output directly into the model or, more realistically, write a short routine to extract this automatically from the flow accumulation output rasters without recourse to a GIS. I am learning a lot through this exercise, so thanks once again to all of you who have made suggestions, they are very much appreciated. Best wishes Darrel From: postgis-users [mailto:[email protected]] On Behalf Of Paragon Corporation Sent: 26 November 2015 05:08 To: 'PostGIS Users Discussion' <[email protected]> Subject: Re: [postgis-users] Help with SQL query? That timing seems much slower than I recall. FWIW expression based mapalgebra as I recall is slower than using the call back function approach. So you could try wrapping your CASE in a call back function. However I think something else might be going on here and postgres might be repeating work. I forgot under what conditions it decides to reevaluate a function call, I just remember being really surprised by it. To avoid that, you can try using a CTE, also you don't need that ST_Union call which for larger number of rasters is expensive, and you might even generate a raster that is too big to compute. I'm also guessing your rasts are all tiled the same, so you really don't need ST_Intersects, just use the same box operator So try this: WITH foo AS ( SELECT ST_SummaryStats( ST_MapAlgebra(deposition.rast, concentrated.rast, 'CASE WHEN [rast2] > 0.0 THEN [rast1] ELSE NULL END' ) ) As st FROM mymodel.deposition INNER JOIN mymodel.concentrated ON ( deposition.rast ~= concentrated.rast ) WHERE deposition.rid=1 ) SELECT SUM( (st).sum ) FROM foo; Hope that helps, Regina http://www.postgis.us http://postgis.net From: postgis-users [mailto:[email protected]] On Behalf Of Darrel Maddy Sent: Wednesday, November 25, 2015 5:06 PM To: PostGIS Users Discussion <[email protected]<mailto:[email protected]>>; Brent Wood <[email protected]<mailto:[email protected]>> Subject: Re: [postgis-users] Help with SQL query? Dear Brent, I must confess that my attempts to do this are so far proving very unsuccessful If I run the following query: SELECT (ST_SummaryStats(ST_Union(rast))).sum AS sum FROM (SELECT ST_MapAlgebra(deposition.rast, concentrated.rast, 'CASE WHEN [rast2] > 0.0 THEN [rast1] ELSE NULL END' ) As rast FROM mymodel.deposition, mymodel.concentrated WHERE ST_Intersects(deposition.rast, concentrated.rast) AND deposition.rid=1 ) foo ; It takes around 30 seconds to complete as I assume it is only looking at one tile(they are 256x256 pixels) i.e. rid 1. It is not easy to check the sum – for that I need one complete raster. For the record this was marginally faster than SELECT (ST_SummaryStats(ST_Union(rast))).sum AS sum FROM (SELECT ST_MapAlgebra(deposition.rast, concentrated.rast, 'CASE WHEN [rast2] > 0.0 THEN [rast1] ELSE NULL END' ) As rast FROM mymodel.deposition, mymodel.concentrated WHERE mymodel.deposition.filename='10_depo.tif' AND ST_UpperleftX(mymodel.deposition.rast) = ST_UpperleftX(mymodel.concentrated.rast) AND ST_UpperleftY(mymodel.deposition.rast) = ST_UpperleftY(mymodel.deposition.rast) ) foo ; Even after I built indexes for the clauses after the WHERE. Now there are 144 tiles in each of the rasters I want to perform this operation on. Logic would therefore suggest this should take ~4500s However when I perform the following query SELECT (ST_SummaryStats(ST_Union(rast))).sum AS sum FROM (SELECT ST_MapAlgebra(deposition.rast, concentrated.rast, 'CASE WHEN [rast2] > 0.0 THEN [rast1] ELSE NULL END' ) As rast FROM mymodel.deposition, mymodel.concentrated WHERE ST_Intersects(deposition.rast, concentrated.rast) AND deposition.filename='10_depo.tif' ) foo ; The query is still running after 18000s! I must therefore assume I have done something wrong but as you may have guessed the answer eludes me. Any further suggestions would be welcome but I will continue to try and find a solution as I have 135 rasters to perform this operations on now and potentially many thousands more in the future. Darrel . I From: postgis-users [mailto:[email protected]] On Behalf Of Darrel Maddy Sent: 24 November 2015 19:52 To: Brent Wood <[email protected]<mailto:[email protected]>>; [email protected]<mailto:[email protected]> Subject: Re: [postgis-users] Help with SQL query? Dear Brent, Many thanks. The data are tiled (256x256) hence the large number of rows from the original 135 tifs. I did not build any indexes however, so I will do some reading and see how best to approach that (the threads you listed look useful so thanks for that). I will run some additional mini queries limited to just one comparison and check using QGIS as you suggest – I probably should have done that first! My workstation has 64GB Ram and I would be surprised if it was significantly caching to disk. I also have a hexacore intel extreme processor so I would not expect this to be hardware limited. I must confess I expected it to finish within a couple of hours. Anyhow very many thanks. I will continue to explore and report back hopefully with positive news. Darrel From: Brent Wood [mailto:[email protected]] Sent: 24 November, 2015 7:36 PM To: Darrel Maddy <[email protected]<mailto:[email protected]>>; [email protected]<mailto:[email protected]> Subject: Re: [postgis-users] Help with SQL query? Indexing can improve performance by 100s of x, without them things can be slow. Also, did you tile the images when you imported them? If not, then each iteration is working through all the pixels in the image, rather than a small subset. Essentially with tiles, you have a deep (long) table rather than a wide one. RDBMSs work better with lots of small records than a few wide ones, especially when indexes are used. This might help: http://gis.stackexchange.com/questions/43053/how-to-speed-up-queries-for-raster-databases and see the raster tutorial they mention for the SRTM data, as to how that is loaded into Postgis: https://trac.osgeo.org/postgis/wiki/WKTRasterTutorial01 To test the logic (the syntax is correct or it wouldn't be working) you could add to the "where" clause an extra filter so that only a small subset of the entire dataset is included (like just one QGIS operation) then compare this with the QGIS result. That would be much faster that testing on the entire dataset. Once you know it is correct for the test case(s), then you can run it on the complete set. Note that some queries can build up large in-memory objects, so make sure your system is not swapping to disk, as that will also slow things down (hugely). Cheers Brent
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