Logical (Heuristic) Query Optimization

1. Break up conjunctive selection predicates

2. Push selections down

3. Introduce joins by combining selections and cross products

4. Determine join order

   Heuristic: execute joins with input from selections before executing other joins

5. Introduce and push down projections

   Not always useful

Rules of thumb

• Perform selections as early as possible
• Perform projections as early as possible

The optimization process

• Generate initial query plan from SQL statement
• Transform query plan into more efficient query plan via a series of modifications, each of which hopefully reducing execution time

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Last update: June 1, 2020