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A commonly used query optimization technique involving σ: push selection down the parse tree

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• Simple query optimization
  • Fact:

    The running time of database operations depends on: The size of the input relations (operands)

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  • Therefore:

    It is always beneficial (for running time) to reduce the size of the input relation(s)

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• Reducing the size of input relation using σ
  • Fact:

    The selection operator σ can reduce the size of the input relation of some operators

    Example:

    Observe that:

    The input relation of ⋈ in the second case: σbirthday LIKE '%1960'(StarsIn) can be much smaller than the input relation: StarsIn

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• A simple query optimization technique
  • One of the many query optimization techniques used by the DBMS is:

    Execute a σC as soon as possible.

  • In terms of a query tree, it means that:

    The σC operation is push as far down the logical query tree as possible

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  • Example:

    This is same as applying the following Relation Algebra law: