37 lines
1.7 KiB
Markdown
37 lines
1.7 KiB
Markdown
### [Relational Algebra - Recap](obsidian://open?vault=Computer%20Science%20Year%201&file=Semester%201%2FDatabase%20Systems%2FWeek%2010%2FWeek%2010%20Database%20Systems)
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- High level & procedural
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- Defines how to build relations
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- SELECT represented by pi ( projection )
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- FROM represented by sigma ( selection )
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- WHERE represented by x ( cartesian product )
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#### Data Manipulation Language
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- Provides set of operations
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- Data retrieval - query language
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- ex. SQL
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#### Closure
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- Operations work on one or more relations to define another relation without changing the original
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- Both operands and results are relations, output from one operation can become the input for another operation.
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- This allows nested expressions.
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#### Theta Join
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R theta(f) S defines a relation that contains tuples satisfying F from the Cartesian product of two relations R and S.
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Takes the Cartesian product of two relations r and s than selects the rows which satisfy the condition F.
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R theta(f) S is the same as sigma(f) (R x S ).
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F may contain comparison operators to form a condition.
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#### Query Processing
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The activities involved in parsing, validating, optimising and executing a query.
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Purpose is to transform a query written in a high level language into correct and efficient strategy in low level language. Also to execute strategy to retrieve data.
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#### Common Optimisation Rules
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- Begin with initial query tree for SQL
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- Move SELECT operations down the tree
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- Apply more restrictive SELECT operations first ( eg. equalities before range queries )
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- Replace Cartesian products followed by selection with theta joins ( eg. *sigma(f) ( RxS )* -> *R theta(f) S* )
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- Move PROJECT operations down the query tree ( add project operations as inputs to theta joins ). |