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One-pass Algorithm for π

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Slideshow:

The projection (π) operator

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

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• Projection π
  • Projection operator:

    πattrs ( R )

  • Example projection:

    R = { ('John', 30000), ('Jane', 50000) } πfname ( employee ) = { ('John'), ('Jane') }

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  • One-pass algorithm:

    while ( R has more data blocks ) { read data blocks to fill available buffers; Extract the projection attrs for each tuple in buffers; Move projected tuples to output }

    Buffer utilization when there are M buffers available:

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• Cost Analysis for π
  • # 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