ST_ClusterDBSCAN gives all features a cluster

Question

I am using ST_ClusterDBSCAN to help cluster some building polygons. I am using the following SQL:

CREATE TABLE clusters AS SELECT ogc_fid, cid, wkb_geometry AS geom FROM ( 
    SELECT ogc_fid, ST_ClusterDBSCAN(wkb_geometry, eps := 5, minpoints := 1) over () AS cid, wkb_geometry 
    FROM building) sq; 

When I check the results every feature has a cid even though there are no other features with the same cid, i.e a building is not within 5 metres of another.

Reading the docs, ST_ClusterDBSCAN should give a feature not within a cluster the value of null.

So I am not sure why this is happening.

Answer 1

The docs for ST_ClusterDBSCAN are incorrect. The docs state that

A "core" geometry is within eps distance of at least minpoints other input geometries

The word "other" is incorrect here; each geometry is considered to be within eps distance of itself.

A correct definition is found on Wikipedia:

A point p is a core point if at least minPts points are within distance ε(ε is the maximum radius of the neighborhood from p) of it (including p).

If you want to use minpoints := 1 and remove clusters that have only a single geometry, you can add another layer to your query as follows:

CREATE TABLE clusters AS 
SELECT
  ogc_fid, 
  cid, 
  wkb_geometry AS geom
 FROM 
    SELECT
       ogc_fid,
       cid,
       count(1) OVER (PARTITION BY cid) AS num_geoms,
       wkb_geometry 
    FROM ( 
      SELECT
        ogc_fid, 
        ST_ClusterDBSCAN(wkb_geometry, eps := 5, minpoints := 1) over () AS cid, 
        wkb_geometry 
      FROM building
    ) sq
) ssq
WHERE num_geoms > 1;