Geographic Information Systems Asked by Alexandre Neto on April 5, 2021
I’m trying to use Postgis 2.0 new function <-> (Geometry Distance Centroid) in order to calculate, for each row of my table (cosn1), the distance to the nearest polygon of the same class.
I was trying to use the following code:
WITH index_query AS (
SELECT g1.gid As ref_gid, ST_Distance(g1.the_geom,g2.the_geom) As ENN
FROM "cosn1" As g1, "cosn1" As g2
WHERE g1.gid <> g2.gid AND g1.class = g2.class
ORDER BY g1.gid, g1.the_geom <-> g2.the_geom)
SELECT DISTINCT ON (ref_gid) ref_gid, ENN
FROM index_query
ORDER BY ref_gid, ENN;
But then I realize the warning:
Note: Index only kicks in if one of the geometries is a constant (not in a subquery/cte). e.g. ‘SRID=3005;POINT(1011102 450541)’::geometry instead of a.geom
Meaning that the Index wont be used at all, and the query will take almost the same time as before using:
SELECT DISTINCT ON(g1.gid) g1.gid As ref_gid, ST_Distance(g1.the_geom,g2.the_geom) As ENN
FROM "cosn1" As g1, "cosn1" As g2
WHERE g1.gid <> g2.gid AND g1.class = g2.class
ORDER BY g1.gid, ST_Distance(g1.the_geom,g2.the_geom)
Can anyone point me a workaround that allows me to improve performance of my query?
Thank you very much.
Doing some tests on my machine suggested this operator <-> is not working properly. I am not sure that is a bug but it reported zero distance on not overlapped geometries.
I tried the fair traditional SQL query optimizations. Since those unexpected results with <-> operator I replace it with st_centroid. Got much better results in speed.
Hope semantics with st_overlaps keep same. At least this was I understood from documentation about <->
From docs on Postigs <->
For other geometry types the distance between the floating point bounding box centroids is returned.
On my test data with ~5.5k polygons got speed up from ~1000 seconds to ~5 seconds without spatial indexing.
I see some people using DISTINCT ON to do grouping but not the group by exists to eliminate duplicates.
Your query with standard SQL optimizations without the st_centroid error introduced
select g1.gid, min( st_distance( g1.the_geom, g2.the_geom ) ) AS enn
FROM
"cosn1" AS g1, "cosn1" AS g2
WHERE
g1.gid <> g2.gid
AND g1.class = g2.class
AND g1.the_geom && g2.the_geom
GROUP BY
g1.gid
Answered by cavila on April 5, 2021
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