Safety of Expressions It is possible to write tuple calculus expressions that generate infinite relations. For example,{tter}results in an infinite relation if the domain of any attribute of relation ris infinite To guard against the problem,we restrict the set of allowable expressions to safe expressions. An 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 NOTE:this is more than just a syntax condition. E.g.{tt[A]=5 v 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. Database System Concepts-5th Edition,July 8,2005 5.11 @Silberschatz,Korth and SudarshanDatabase System Concepts - 5 5.11 ©Silberschatz, Korth and Sudarshan th Edition, July 8, 2005 Safety of Expressions It is possible to write tuple calculus expressions that generate infinite relations. For example, { t |  t  r } results in an infinite relation if the domain of any attribute of relation r is infinite To guard against the problem, we restrict the set of allowable expressions to safe expressions. An 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 NOTE: this is more than just a syntax condition.  E.g. { t | t [A] = 5  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