Hi Michael,
assume you have the following relations
R1 with characteristics A, B
R2 with characteristics B, C
These relations share the characteristic B.
How does the CREATE method (characteristic relationship) work? It takes the selection and all requested characteristics and creates all valid combination contained in the restriction. So it calls CREATE for relation R1 and R2 and computes the inner-join of the two 'tables' (join by B).
CREATE for A makes no sense, CREATE only has a meaning for characteristics A, B (relation R1),
B,C (relation R2 or A,B,C (both relations R1, R2).
Assume you have set 'Access Type for Result Values' based on characteristic relationships in a Query containing characteristics A,B,C only for characteristic A. What should happen in this case?
The relation R1 contains no information about A alone. Thus the system build a characteristic relationship hul (CR-hul):
- since A is requested relation R1 is involved
- since B is in R1 and in R2 also R2 is involved
Thus the CR-hul(A) = {A,B,C} and the system will call CREATE for A,B,C.
Observe that similar things happen in case you have a chain of derivation relations; this applies to the method DERIVE called in the delta or after image buffer.
Back to CREATE, consider the case that you may have a restriction for characteristic C, but not for A,B. The via the CR-hul mechanism one will get an effect on the admissible combinations (a,b,c) via the inner-join.
Regards,
Gregor