8' Safe Query Languages

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8. Safe Query Languages

• Safe program its semantics can be at
  least partially
• computed on any
  valid database input.
• Safety is tied to program verification,
• because only safe programs are correct.
• Safety is a concern in any programming
  language.
• It is undecidable in general (Halting
  problem).
• Is it possible to verify in certain cases
  that a
• query is safe?

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• 8.1 Safety Levels
• Closed form output database contains same type
  of
• constraints as the input
  database.
• Evaluation output database may contain more
  complex
• constraints than the input
  database.
• Recognition any tuple can be tested to be in
  the output
• ?-Recognition for any tuple a it can be
  tested whether
• the output contains a
  tuple b such that
• a i b i lt ?
  for all i.
• Approximation lower, upper bounds can be given
  for output

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• 8.2 Restriction
• Restriction only allow certain combinations of
  
• (constraints, queries) to
  guarantee safety.
• Advantage easy to check
• Disadvantage limits flexibility and
  expressiveness

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• 8.4 Safe Refinement and Projection Queries
• Refinement queries
• always safe because they only add
  constraints.
• Projection queries
• closed-form evaluable if the UOR contains
  only
• Linear constraints
• Addition constraints
• Polynomial constraints