Grouping connected linestrings in PostGIS?

I have a table of streets I've selected based on a set of attributes (let's say it's speed_limit < 25). There are groups of streets that are locally contiguous; I'd like to group these sets of connected linestrings into GeometryCollections. In the image below, there would be two GeometryCollections: one with the red lines and one with the blue lines.

I tried running a couple of "dissolve, deaggregate" queries along the lines of:

SELECT (ST_Dump(st_union)).geom
FROM 
    (SELECT ST_Union(geom) FROM roads) sq

With everything I've tried, I either end up with a single feature (ST_Union) or my original geometry (ST_Dump of ST_Union).

Maybe it's possible to do this with some kind of WITH RECURSIVE magic?

Answer

So, by example. Here's a simple table with two connected groups of edges:

drop table lines;
create table lines ( id integer primary key, geom geometry(linestring) );
insert into lines (id, geom) values ( 1, 'LINESTRING(0 0, 0 1)');
insert into lines (id, geom) values ( 2, 'LINESTRING(0 1, 1 1)');
insert into lines (id, geom) values ( 3, 'LINESTRING(1 1, 1 2)');
insert into lines (id, geom) values ( 4, 'LINESTRING(1 2, 2 2)');
insert into lines (id, geom) values ( 11, 'LINESTRING(10 10, 10 11)');
insert into lines (id, geom) values ( 12, 'LINESTRING(10 11, 11 11)');

insert into lines (id, geom) values ( 13, 'LINESTRING(11 11, 11 12)');
insert into lines (id, geom) values ( 14, 'LINESTRING(11 12, 12 12)');
create index lines_gix on lines using gist(geom);

Now, here's a recursive function that, given the id of an edge, accumulates all the edges that touch:

CREATE OR REPLACE FUNCTION find_connected(integer) returns integer[] AS
$$
WITH RECURSIVE lines_r AS (
  SELECT ARRAY[id] AS idlist, geom, id
  FROM lines 

  WHERE id = $1
  UNION ALL
  SELECT array_append(lines_r.idlist, lines.id) AS idlist, 
         lines.geom AS geom, 
         lines.id AS id
  FROM lines, lines_r
  WHERE ST_Touches(lines.geom, lines_r.geom)
  AND NOT lines_r.idlist @> ARRAY[lines.id]
)
SELECT 

  array_agg(id) AS idlist
  FROM lines_r
$$ 
LANGUAGE 'sql';

That just leaves us needing to find, after each group is accumulated, the id of an edge that is not already part of a group. Which, tragically, requires a second recursive query.

WITH RECURSIVE groups_r AS (
  (SELECT find_connected(id) AS idlist, 
          find_connected(id) AS grouplist, 
          id FROM lines WHERE id = 1)

  UNION ALL
  (SELECT array_cat(groups_r.idlist,find_connected(lines.id)) AS idlist,
         find_connected(lines.id) AS grouplist,
         lines.id
  FROM lines, groups_r
  WHERE NOT idlist @> ARRAY[lines.id]
  LIMIT 1)
)
SELECT id, grouplist
FROM groups_r;   

Which taken together return a nice set with the seed id and each group it accumulated. I leave it as an exercise to the reader to turn the arrays of id back into a query to create geometry for mapping.

 id |   grouplist   
----+---------------
  1 | {1,2,3,4}
 11 | {11,12,13,14}
(2 rows)