PostGIS ST_ClusterDBSCAN returns NULL

Question

I have problems with the ST_ClusterDBSCAN function in PostGIS. I try to group points derived from lines with a maximum distance of 15 meters from each other and a minimum cluster size of 4. This worked fine in the past but while using the function now, it returns in most cases of my dataset NULL as CLUSTER_ID even points are in near proximity.

Here is one example:

   WITH sample_points AS(
    SELECT 
        (ST_Dump(
            ST_GeomFromText(
                'MULTIPOINT(0 0,10 10,20 20,30 30,50 50,60 60,100 0,110 0,120 0,130 0,140 0)', 3763)
            )
        ).geom 
)
SELECT ST_ClusterDBSCAN(geom, eps:=15, minpoints :=4)  over() AS cluster_id
FROM sample_points;

I get as result only NULL values.

cluster_id
NULL
NULL
NULL
NULL
[...]

This happens on my dataset, using above code or even in QGIS running DBSCAN clustering tool on manually created point layer. Am I getting something wrong about the function or why does it not work properly?

Running Windows10, PostgreSQL 15.1, PostGIS 3.3.2.3.3.2, QGIS 3.28.0

Answer 1

DBSCAN is a 'density-first' (my own words) algorithm, meaning that it requires a base density of minpoints (but including itself) within eps distance of each other to declare a cluster of core points - and only then border points may be added to existing clusters, if their distance is within eps of core points.

In your example there is no point that has 3 nearest neighbors within eps distance, which would be required to form an initial cluster. Simple as that.