Here’s a good article on tagging Database schemas:
http://howto.philippkeller.com/2005/04/24/Tags-Database-schemas/
along with performance tests:
http://howto.philippkeller.com/2005/06/19/Tagsystems-performance-tests/
Note that the conclusions there are very specific to MySQL, which (at least in 2005 at the time that was written) had very poor full text indexing characteristics.