nIt is possible to write tuple calculus expressions that generate infinite relations.
nFor example, { t | Ø t Î r } results in an infinite relation if the domain of any attribute of relation r is infinite
nTo guard against the problem, we restrict the set of allowable expressions to safe expressions.
nAn expression {t | P (t )} in the tuple relational calculus is safe if every component of t appears in one of the relations, tuples, or constants that appear in P
lNOTE: this is more than just a syntax condition.
4E.g. { t | t [A] = 5 Ú true } is not safe --- it defines an infinite set with attribute values that do not appear in any relation or tuples or constants in P.
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