I once wrote my on categorization thing in a normalized fashion and I found that once I moved queries away from Class::DBI and into SQL JOINs, the results became much faster. This of course meant adding code like the following to the classes:

__PACKAGE__->set_sql(related_items => <<SQL)
SELECT __PRIMARY__ from __TABLE__
INNER JOIN relations ON child = __TABLE__.__PRIMARY__
WHERE relations.parent = ?
SQL

sub related_items {
  my ($self) = @_;
  return $self->search_related_items($self->id);
};
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and much uglier JOINs if you are constructing nested sets or junctive queries. I have, for the time being, moved to a denormalized database and found writing the queries much nicer. Performance is acceptable, that is, interactive, for the single user of the system (me).

If you'll be going the route of JOINing the relation tables, consider writing an extension to SQL::Abstract - I wrote me an (yet unreleased) extension to handle GROUP BY and HAVING clauses, and it worked well for me.