I 've run tests on 8i and 9i and the CBO is not clever enough to determine
such "certainties " with FKs even if there 's a check constraint on the parent
column(s), such as a check range constraints so that logically a FK value
can 't exist (I can post some demos if anyone 's interested although it 's easy
enough to play with this oneself).
[JPL] Just as an aside - I 've got a test case, though, where a
[JPL] check constraint on one table gets forwarded by transitive
[JPL] closure to become a predicate on a second table - 9.2.0.4
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