[postgis-users] Fast spatial join point to raster

[David Haynes]

Ok thanks

I aligned my query to the functional indices and it is much faster.

inner join mn_dasymetric r on ST_intersects(st_envelope(r.rast),
ST_Transform(p.geom, 26915))

On Sun, Apr 18, 2021 at 1:21 PM Paul Ramsey <pramsey at cleverelephant.ca>
wrote:

> The indexing of rasters is (and remains a pain in the butt) (maybe
> something I should look at shortly) because the index is bound to geometry
> and almost always declraed functionally. You've built functional indexes,
>
> GIST (ST_Transform(ST_Envelope(rast),26915))
>
> But you aren't calling them with queries that use the SAME functional
> signature. The rule of functional indexes:
>
> * An index like this:  CREATE INDEX foo_x ON (function(bar))
> * must be called like this: SELECT * FROM foo WHERE function(bar) = 'baz'
>
> Note that function(bar) appears in the CREATE and in the SELECT.
>
> P
>
> > On Apr 18, 2021, at 10:53 AM, David Haynes <haynesd2 at gmail.com> wrote:
> >
> > Hello,
> >
> > I'm following up on this blog post to see if there is a fast way to join
> points to rasters.
> > http://blog.cleverelephant.ca/2018/09/postgis-external-storage.html
> >
> > I'm attempting to do a dasymetric mapping process, which is assigning
> areal unit values to raster pixels. For this example, I have created two
> tables. Table 1, mn_smokers has are geom (x,y) coordinates and spatial
> indices on 4326 and 26915. The resulting raster will be stored in
> mn_dasymetric and it also has indices at 4326 and 26915. When running the
> explain query it seems that index isn't really being used.
> >
> >
> > CREATE INDEX mn_smokers_gist_4326  ON mn_smokers  USING GIST (geom);
> > CREATE INDEX mn_smokers_gist_26915  ON mn_smokers  USING GIST
> (ST_Transform(geom,26915));
> > CREATE INDEX mn_dasymetric_gist_4326 ON mn_dasymetric  USING GIST
> (ST_Transform(ST_Envelope(rast),4326));
> > CREATE INDEX mn_dasymetric_gist_26915  ON mn_dasymetric  USING GIST
> (ST_Transform(ST_Envelope(rast),26915));
> >
> > Type: Nested Loop (Inner); ; Cost: 6.22 - 1594516.11
> > Type: Limit; ; Cost: 5.11 - 6.22
> > Type: Nested Loop (Inner); ; Cost: 5.11 - 471.95
> > Type: Seq Scan; Rel: minnesota_counties ; Cost: 0.00 - 34.09
> > Type: Bitmap Heap Scan; Rel: mn_smokers ; Cost: 5.11 - 437.50
> > Type: Bitmap Index Scan; Rel: mn_smokers_gist_4326 ; Cost: 0.00 - 5.10
> > Type: CTE Scan; Rel: points ; Cost: 0.00 - 4.00
> > Type: Materialize; ; Cost: 0.00 - 59.57
> > Type: Seq Scan; Rel: mn_dasymetric ; Cost: 0.00 - 47.38
> >
> >
> > with points as
> > (
> > select sp_id as point_id, smoker as point_value, ms.geom
> > from mn_smokers ms
> > inner join minnesota_counties mc on ST_Intersects(ms.geom, mc.geom)
> > )
> > , point_raster_agg as
> > (
> > select geom, count(point_id) as pixel_point_value
> > from points
> > group by geom
> > )
> > , output_rast as
> > (
> > select r.rid, pixel_point_value,  ST_SetValue(rast, ST_Transform(p.geom,
> 26915), pixel_point_value ) as setrast
> > ,rast, geom
> > from point_raster_agg p
> > inner join mn_dasymetric r on ST_intersects(r.rast, ST_Transform(p.geom,
> 26915))
> > _______________________________________________
> > postgis-users mailing list
> > postgis-users at lists.osgeo.org
> > https://lists.osgeo.org/mailman/listinfo/postgis-users
>
> _______________________________________________
> postgis-users mailing list
> postgis-users at lists.osgeo.org
> https://lists.osgeo.org/mailman/listinfo/postgis-users
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/postgis-users/attachments/20210418/913ba0c7/attachment.html>