excuse my english. I have a question regarding database theory, particularly the area of the relational datamodel. As I am not from the field, I would be thankfull for search keywords or names of groups that fit to my questions. I do not expect long answers.

I understand that, given a join-query, a full reducer (that is independent from the initial relations) exists iff the query is acyclic (with respect to the associated hypergraph). here ends my database literature (ullman vol II).

I am searching for results regarding cyclic join queries, i.e.,
- dependent from the initial relations, when does a full reducer exists?
- are there any results on a fixed point with respect to a semijoin program in cyclic queries?
- Is there a generalized approach? (for example in set theory)

thank you! adebar