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One-pass Algorithm for σ

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Slideshow:

The selection (σ) operator (a.k.a "filter")

The one-pass selection (σ) algorithm
   
Schematically:
   
Note: typically, you will use 1 buffer to execute the selection (σ) algorithm
Cost and constraint of the one-pass selection (σ) algorithm
   
 

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• Selection σ
  • Selection operator:

    σcond ( R )

  • Example selection:

    R = { ('John', 30000), ('Jane', 50000) } σsalary > 40000 ( employee ) = { ('Jane', 50000) }

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  • One-pass algorithm:

    while ( R has more data blocks ) { read data blocks to fill available buffers; check condition for each tuple in the buffers; Move qualifying tuples to output }

    Buffer utilization when there are M buffers available:

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• Cost Analysis for σ
  • # disk I/O used:

    B(R) --- if the relation R is clustered T(R) --- if the relation R is UNclustered

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  • Memory requirement:

    M ≥ 1 buffer