[postgis-users] Help with SQL query?

Wed Nov 25 21:08:02 PST 2015

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:postgis-users-bounces at lists.osgeo.org] On Behalf Of Darrel Maddy
Sent: Wednesday, November 25, 2015 5:06 PM
To: PostGIS Users Discussion <postgis-users at lists.osgeo.org>; Brent Wood <pcreso at yahoo.com>
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:postgis-users-bounces at lists.osgeo.org] On Behalf Of Darrel Maddy
Sent: 24 November 2015 19:52
To: Brent Wood <pcreso at yahoo.com <mailto:pcreso at yahoo.com> >; postgis-users at lists.osgeo.org <mailto:postgis-users at lists.osgeo.org> 
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:pcreso at yahoo.com] 
Sent: 24 November, 2015 7:36 PM
To: Darrel Maddy <darrel.maddy at newcastle.ac.uk <mailto:darrel.maddy at newcastle.ac.uk> >; postgis-users at lists.osgeo.org <mailto:postgis-users at lists.osgeo.org> 
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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