[postgis-users] Determining clusters of points

[Sébastien Lorion]

Hello Kevin,

I like the elegance of your query :) I am comparing it with the buffer/merge
approach explained by Emilie to see which one scales better and will post my
finding when done.

Thank you!

Sébastien

On Wed, Dec 8, 2010 at 00:20, Kevin Neufeld <kneufeld.ca at gmail.com> wrote:

> On 12/7/2010 5:15 PM, pcreso at pcreso.com wrote:
>
>> You might look at the newish (v8.4+ ?) RECURSIVE/WITH capabilities of
>> Postgres.
>>
>> Given Postgis allows you to create arbitrary buffer zones around your
>> points, the RECURSIVE query capability allows you to write an SQL that will
>> recursively track points which can interact with each other
>>
>
> So, just playing around a bit, I came up with this RECURSIVE query
> statement that will find all points in a particular "cluster".  Every time I
> run the UPDATE statement, another cluster of points is identified and
> updated to have the same group id.  I have no idea how this would perform
> over a table of a million points ...
>
>
> CREATE TABLE pts (id serial, grp int, geom geometry);
> CREATE SEQUENCE pts_grp_seq; -- to label cluster groups
>
> -- Sample data similiar to Sébastien's original post
> INSERT INTO pts (geom)
> SELECT (ST_Dump(
> 'MULTIPOINT (
>    6 129,
>    21 200,
>    30 66,
>    33 109,
>    41 137,
>    48 191,
>    76 119,
>    90 197,
>    175 212,
>    178 230,
>    182 53,
>    196 209,
>    259 124,
>    259 148
> )')).geom;
>
> -- Simple wrapper around an UPDATE statement that identifies a
> -- point cluster everytime its run
> CREATE OR REPLACE FUNCTION cluster_pts() RETURNS text AS
> $body$
> DECLARE
>   results int;
> BEGIN
>   LOOP
>      -- Assign all records in the same cluster, the same group id
>      UPDATE pts
>      SET grp = f.grp
>      FROM (
>         WITH RECURSIVE t(id, geom, grp, path, cycle) AS (
>             -- Grab the first record that doesn't have a group id
>            (SELECT id, geom, nextval('pts_grp_seq'), ARRAY[id], false
>             FROM pts
>             WHERE grp IS NULL
>             LIMIT 1
>             )
>           UNION ALL
>             -- Find all the other records that are iteratively within some
>             -- distance to the starting point.
>             -- This includes A to B to C paths, but not A to B to A cyclic
> paths
>             SELECT a.id, a.geom, b.grp, path || ARRAY[a.id], a.id =
> ANY(path)
>             FROM pts a, t b
>             WHERE a.id != b.id
>             AND ST_DWithin(a.geom, b.geom, 80)
>             AND NOT cycle
>         )
>         SELECT DISTINCT id, grp FROM t
>      ) AS f
>      WHERE pts.id = f.id;
>
>      -- repeat until no more updates are performed.
>      GET DIAGNOSTICS results = ROW_COUNT;
>      EXIT WHEN results = 0;
>   END LOOP;
>
>   RETURN 'done.';
> END;
> $body$ LANGUAGE plpgsql;
>
> SELECT cluster_pts();
>
>
>
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