I am trying to count different stores inside trade zone polygons in a sql-query. Each store is a different table in my postgis database.
It is possible to count points in polygons using the following code taken from https://gis.stackexchange.com/a/54680/19349:
SELECT grid.gid, count(kioskdhd3.geom) AS totale
FROM grid LEFT JOIN kioskdhd3
ON st_contains(grid.geom,kioskdhd3.geom)
GROUP BY grid.gid;
I modified the code to allow for multiple points:
SELECT grid.gid,
count(DISTINCT store1.geom) AS store1_count,
count(DISTINCT store2.geom) AS store2_count
FROM grid
LEFT JOIN store1
ON st_contains(grid.geom,store1.geom)
LEFT JOIN store2
ON st_contains(grid.geom,store2.geom)
GROUP BY grid.gid;
This worked, but surely there needs to be a better way, as continuous left joins will exponentially worsen the execution time.
Related
I have two polygon layers. I want to run st_intersection on them, to give the result of the areas where they overlap as a new layer. The new layer should contain the attributes from both input layers. I found this image which seems to illustrate my desired end results.
My two input layers are both polygons:
SELECT st_geometrytype(geom),
COUNT(*)
FROM a
GROUP BY st_geometrytype(geom)
-- Result is 1368 st_polygons
SELECT st_geometrytype(geom),
COUNT(*)
FROM b
GROUP BY st_geometrytype(geom)
-- Result is 539548 st_polygons
The query I run is as below:
SELECT a.*,
b.*,
st_intersection(a.geom, b.geom) as geom
FROM a,b
WHERE st_intersects(a.geom, b.geom)
However in the result I get not just polygons (which I expect), but lines, points, multipolygons and geometry collections. I guess because some of my input polygons share points but not true intersections perhaps?
Grateful for some advice please on how to deal with this, whether my query is correct, anything I can do to improve performance etc. Thanks.
ST_intersect returns several geometry types, depending on the relative topology.
For example, running ST_intersect on two adjacent polygons returns the common part of the shared boundary.
While it ouptuts a single table (as you can verify in pgadmin, for example), in the Browser swatch of QGIS it will be shown as multiple tables of different geometry types (for example: POLYGON, MULTIPOLY, LINE, and POINT) but (somewhat confusingly) with the same name.
Visually, you can tell them apart observing the accompaining icons on the left:
You can however select which type of geometry you want, for example by adding a WHERE filter with ST_Dimension:
SELECT a.*,
b.*,
st_intersection(a.geom, b.geom) as geom
FROM a,b
WHERE st_intersects(a.geom, b.geom)
AND ST_Dimension(st_intersects(a.geom, b.geom)) = 2;
or, for performance sake, re-write it in a fashion similar to:
SELECT clipped.*
FROM (
SELECT a.id, b."fieldName",
(ST_Dump(ST_Intersection(a.geom, b.geom))).geom AS geom
FROM "public"."table_A_name" AS a INNER JOIN "public"."table_B_name" AS b
ON ST_Intersects(a.geom, b.geom)
) AS clipped
WHERE ST_Dimension("clipped"."geom") = 2;
The latter solution creates an anonymous temporary table, which allows ST_Intersection to run only once.
You might have noticed thath the trick is in ST_Dimension("clipped"."geom") = 2.
ST_Dimensions which filters the outputs from ST_Intersection so as to keep only polygons (which have a topological dimension of 2).
I have table of polygons (thousands), and table of points (millions). Both tables have GIST indexes on geometry columns. Important this is, polygons do not overlap, so every point is contained by exactly one polygon. I want to generate table with this relation (polygon_id + point_id).
Trivial solution of course is
SELECT a.polygon_id, p.point_id
FROM my_polygons a
JOIN my_points p ON ST_Contains(a.geom, p.geom)
This works, but I think it is unnecessary slow, since it matches every polygon with every point - it does not know that every point can belong to one polygon only.
Is there any way to speed things up?
I tried looping for every polygon, selecting points by ST_Contains, but only those not already in the result table:
CREATE TABLE polygon2point (polygon_id uuid, point_id uuid);
DO $$DECLARE r record;
BEGIN
FOR r IN SELECT polygon_id, geom
FROM my_polygon
LOOP
INSERT INTO polygon2point (polygon_id, point_id)
SELECT r.polygon_id, p.point_id
FROM my_points p
LEFT JOIN polygon2point t ON p.point_id = t.point_id
WHERE t.point_id IS NULL AND ST_Contains(r.geom, p.geom);
END LOOP;
END$$;
This even slower than trivial JOIN approach. Any ideas?
A way to increase the speed is to subdivide the polygons into smaller ones.
You would create a new table (or a materialized view should the polygon change often), index it, and then run the query. If the subdivisions have 128 vertices or less, the data will, by default, be stored uncompressed on disk, making the queries even faster.
CREATE TABLE poly_subdivided AS
SELECT ST_SUBDIVIDE(a.geom, 128) AS geom , a.polygon_id
FROM poly;
CREATE INDEX poly_subdivided_geom_idx ON poly_subdivided USING gist(geom);
ANALYZE poly_subdivided;
SELECT a.polygon_id, p.point_id
FROM poly_subdivided a
JOIN my_points p ON ST_Contains(a.geom, p.geom)
Here is a great article on the topic.
I have two tables stored in PostGIS:
1. a multipolygon vector with about 590000 rows (layerA) and
2. a single multipart (1 row) vector layer (layerB)
and I want to find the area of the intersection between each polygon's buffer in layerA and layerB. My query so far is
SELECT ST_Area(ST_Intersection(a.geom, b.geom)) AS myarea, a.gid AS mygid FROM
(SELECT ST_Buffer(geom, 500) AS geom, gid FROM layerA) AS a,
layerB AS b
So far, I can see my query working but I calculate that it needs 17 hours to be completed (with my PC). Is there another way to execute this query more efficiently and faster?
What if you check intersects of overlapping area before intersection and area calculation, it might lower time.
SELECT ST_Area(ST_Intersection(a.geom, b.geom)) AS myarea, a.gid AS mygid FROM
(SELECT ST_Buffer(geom, 500) AS geom, gid FROM layerA) AS a,
layerB AS b WHERE ST_intersects(a.geom, b.geom)
You would probably get more answers to this at gis.stackexchange.com.
Therea are several things you can do.
You should make sure you get that first filtering of polygons actually intersecting with help of index.
Put a gist index on the table with many geometries and use st_dwithin(geom,500) instead of st_intersects on the buffered geometries. That is because the buffered geometries cannot use the index calculated on the unbuffered geometries.
Also, you say you have multi polygons. If there actually is more than 1 polygon in each multipolygon you might get a lot more speed if you first split the polygons to single polygons before building the index. That will make the.index doing a much bigger part of the job.
There is actually a function in postgis to split even single polygons into smaller pieces for the same reason.
ST_SubDivide
So first use ST_Dump to get single polygons:
CREATE table a_singles AS
SELECT id, (ST_Dump(geom)).geom geom FROM a;
Then create index:
CREATE INDEX idx_a_s_geom
ON a_singles
USING gist(geom);
At last the query, something like
SELECT ST_Area(ST_Intersection(ST_Buffer(a_s.geom,500), b.geom))
FROM a_singles AS a_s
INNER JOIN b
on ST_DWithin(a_s.geom,b.geom,500);
If that still is slow you can start playing with ST_SubDivide.
One more thing. If the single multipolygon in table b contains many geometries, also split them and put an index also there.
It might be slow also after all those things. That depends on how many vertex points there is in the splitted polygons that actually intersect (and for st_dwithin also on how many vertexpoints there is in polygons with overlapping bounding boxes)
But now you don't have any index helping you so this should make it quite a lot faster.
I want to calculate in Postgis the total area of 'a' polygons, that intersects with others 'b'.
SELECT DISTINCT a.fk_sites,
SUM(ST_Area(a.the_geom)/100) as area
FROM parcelles a, sites b
WHERE st_intersects(a.the_geom,b.the_geom)
GROUP BY a.fk_sites
I need to do a SELECT DISTINCT because 'a' polygons may intersect with several 'b' polygons, so that the returned 'a' polygons appear a few times.
This works fine, I just have the problem, that not all areas are calculated correctly. A few seam to ignore the DISTINCT case, so that the calculated area reflects the SUM of all, even the duplicated 'a' records (even if they should be eliminated).
When I do a query without the SUM function, I get the correct number of 'a' polygons and while adding their area I get the right value.
SELECT DISTINCT a.fk_sites,
ST_Area(a.the_geom)/100 as area
FROM parcelles a, sites b
WHERE st_intersects(a.the_geom,b.the_geom)
ORDER BY a.fk_sites
Is the combination of SELECT DISTINCT and the SUM / GROUP BY not correct?
This may have something to do with you fk_sites column because the query itself should be ok, although doing a DISTINCT on a double precision value is never a good thing.
You can solve this by identifying the distinct rows from a in a sub-query, then sum() in the main query:
SELECT fk_sites, sum(ST_Area(the_geom)/100) AS area
FROM (
SELECT a.fk_sites, a.the_geom
FROM parcelles a
JOIN sites b ON ST_Intersects(a.the_geom, b.the_geom)
) sub
GROUP BY fk_sites
ORDER BY fk_sites;
I was trying to join pole(point) and customer(point) layers using spatial join. I created a 20 meter buffer around pole then wrote below query.It takes too much time
SELECT distinct h.gid, d.polecode FROM
buildings as h left join
pole_buffer_20 as d
on
ST_Intersects(d.the_geom,h.the_geom)
Help me with a query without using buffering
For checking if points are contained in polygons you should use the the function ST_DWithin, which is faster:
SELECT distinct h.gid, d.polecode FROM
buildings as h left join
pole_buffer_20 as d
on
ST_DWithin(d.the_geom,h.the_geom, 0);
ST_DWithin accepts a distance parameter too, so, depending of your needs, you could avoid the step with the buffer table and use directly you original table:
SELECT distinct h.gid, d.polecode FROM
buildings as h left join
pole as d
on
ST_DWithin(d.the_geom,h.the_geom, 20);
The distance parameter is in unit of the geometry. If you want a search radius of 20 meters, you need geometries which are expressed in meter (or use geography type)