Help with trigger

(Postgres 8.4.2)

I have two small tables:

dimensions (id, height, width, depth, weight);
components (id, dimension references dimensions(id));

When dimensions are changed, a new row is inserted in dimensions-table
and the id is updated in the components-table. This means there can be
several dimension-ids that are "orphans" (not referenced by
components). Instead of purging them manually, I read about triggers
and tried to use them.

I did this:

ecdb=> CREATE OR REPLACE FUNCTION dimension_purge() RETURNS "trigger"
AS 'BEGIN DELETE FROM dimensions WHERE id NOT IN (SELECT dimensions
FROM components WHERE dimensions IS NOT NULL); END;' LANGUAGE
'plpgsql';
CREATE FUNCTION
ecdb=> CREATE TRIGGER dimensions_trigger AFTER INSERT OR UPDATE OR
DELETE ON dimensions EXECUTE PROCEDURE dimension_purge();
CREATE TRIGGER

So far so good. I try to insert a dimension to test the trigger:

ecdb=> insert into dimensions (weight, height, depth, width) values
((100*random())::smallint,(100*random())::smallint,(100*random())::smallint,(100*random())::smallint);

here, Postgres hangs (99.9% cpu according to top(1)). I press ctrl+c
and see "Cancel request sent". After a minute or two, the following
appears:

ERROR: stack depth limit exceeded
HINT: Increase the configuration parameter "max_stack_depth", after
ensuring the platform's stack depth limit is adequate.
CONTEXT: SQL statement "DELETE FROM dimensions WHERE id NOT IN
(SELECT dimensions FROM components WHERE dimensions IS NOT NULL)"
PL/pgSQL function "dimension_purge" line 1 at SQL statement
SQL statement "DELETE FROM dimensions WHERE id NOT IN (SELECT
dimensions FROM components WHERE dimensions IS NOT NULL)"
PL/pgSQL function "dimension_purge" line 1 at SQL statement
SQL statement "DELETE FROM dimensions WHERE id NOT IN (SELECT
dimensions FROM components WHERE dimensions IS NOT NULL)"
PL/pgSQL function "dimension_purge" line 1 at SQL statement
SQL statement "DELETE FROM dimensions WHERE id NOT IN (SELECT
dimensions FROM components WHERE dimensions IS NOT NULL)"
PL/pgSQL function "dimension_purge" line 1 at SQL statement
[...]

If I manually run "DELETE FROM dimensions WHERE id NOT IN (SELECT
dimensions FROM components WHERE dimensions IS NOT NULL);" it works
fine - all unused id:s are purged - so this cannot be the error.

Can anyone tell me what I'm doing wrong?

--
- Rikard