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Questions and Answers
What is the primary difference between Exhaustive Search Optimization and Heuristic Based Optimization?
In Heuristic Based Optimization, why are cross-product operations avoided?
Which transformation allows combining selection operations into a sequence of individual selections in relational algebra?
When more than one projection operation is used in an expression, which projection operation is required?
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Under what condition do selection operations distribute over theta join operations?
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What is the main heuristic used in optimizing query operations?
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Which data structure corresponds to a relational algebra expression and indicates an order of operation execution?
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What does a query graph represent in the context of query optimization?
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In heuristic optimization of query trees, what is the objective?
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What is the purpose of cost-based query optimization?
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Study Notes
Query Optimization Techniques
- Exhaustive Search Optimization generates all possible query plans and selects the best one, providing the best solution.
- Heuristic Based Optimization uses rule-based optimization approaches, performing select and project operations before join operations to reduce the number of tuples available for join.
Properties of Relational Algebra Expressions
- Two relational algebra expressions are equivalent if they generate the same set of tuples.
- Combined selection operation can be divided into a sequence of individual selections (cascade of σ).
- Selection operations are commutative.
- Only the outer projection operation is required if multiple projection operations are used in an expression.
- Selection operation can be joined with Cartesian product and theta join.
- Theta operations are commutative, and selection operation distributes over theta join operation under certain conditions.
Heuristic Optimization Process
- The parser generates an initial internal representation of the query.
- Heuristics rules are applied to optimize the internal representation.
- A query execution plan is generated based on the access paths available on the files involved in the query.
Key Heuristics
- Apply first the operations that reduce the size of intermediate results.
Query Representation
- Query tree: a tree data structure that corresponds to a relational algebra expression, representing input relations as leaf nodes and relational algebra operations as internal nodes.
- Query graph: a graph data structure that corresponds to a relational calculus expression, without indicating an order of operations.
Query Tree Optimization
- The task of heuristic optimization of query trees is to find a final query tree that is efficient to execute.
Cost-Based Query Optimization
- Estimate and compare the costs of executing a query using different execution strategies and choose the strategy with the lowest cost estimate.
- Key issues: cost function and number of execution strategies to be considered.
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Description
Learn about Exhaustive Search Optimization and Heuristic Based Optimization techniques used in query optimization. Understand how these methods work and their impact on improving query performance.
