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Questions and Answers
What is the primary difference between Exhaustive Search Optimization and Heuristic Based Optimization?
What is the primary difference between Exhaustive Search Optimization and Heuristic Based Optimization?
In Heuristic Based Optimization, why are cross-product operations avoided?
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?
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?
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?
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?
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?
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?
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?
In heuristic optimization of query trees, what is the objective?
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What is the purpose of cost-based query optimization?
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.
