Why we need to specify cardinality in cognos/hibernate but not in oracle

Question

When we specify a foregin key relationship in oracle, we do not need to say whether relationship is 1 to 1, 1 to many etc. But when we specify a relationship in the cognos framework manager, why we need to specify 1 to 1, 1 to many etc? Also, unrelated but just curious, the same is the behavior in a ORM tool like Hibernate. Is the specification only for optmisation purposes?

My other question is what is th effect of specifying a wrong cardinality relationship? i,e, I specify a 1 to 1 relationship where it is actually 1 to many. What is the effect i am risking?

Answer 1

You do declare the relationships in Oracle, the nature of the relationship is implicit in the way that it is declared to Oracle, e.g.:

CONSTRAINT fk FOREIGN KEY (id) REFERENCES parent_table (id)

implies that there is a 1:M relationship between the parent table and this table.

If, in addition, there was a unique constraint on the child table, e.g.:

CONSTRAINT uk UNIQUE (id)

implies that the relationship is 1:1.

Answer 2

From Cognos documentation:

IBM® Cognos® software uses the cardinality of a relationship in the following ways:

to avoid double-counting fact data
to support loop joins that are common in star schema models
to optimize access to the underlying data source system
to identify query subjects that behave as facts or dimensions

For further details, look here: Framework Manager Cardinality