Physical Query Plan Optimization

query-processing-3rd.png

Translating SQL to RA expression is the first step in Query Processing Pipeline

  • Input: Optimized Logical Query plan - expression in Extended Relational Algebra
  • Output: Optimized Physical Query Plan - expression in Relational algebra with each node assigned some physical algorithm


Cost-Based Plan Selection

We need to select the optimal plan based on

This is an Optimization Problem


Greedy Algorithm

At each step we choose the operation with least local cost

Bottom-up approach:

  • first assign physical operators to leaves
  • then to parents of the leaves
  • then to their parents
  • and so on
  • at each step choose an operator that gives the lowest cost
  • for join operators use a gredy algorithm for join ordering

Limitations

Doesn't take into account the properties of the output of an operator

Consider this example:

  • plan-selection-bad.png
  • suppose optimized sort-merge join is not available - not enough memory
  • so the Greedy algorithm decides to use hash-based join (recall that its output is not sorted)
  • we could've taken slightly more expensive sort-merge join
    • but instead of 3-pass duplicate elimination we'd just need one pass for $\delta$
    • which would give us overall lower total cost


Exercises

Main Article: Query Plan Selection Exercises


See also

Sources

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