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Database Management Systems/Managing Database
Notes Figure 12.1: Query Processing
12.1.1 Optimisation
A relational algebra expression may have many equivalent expressions.
Example: ( (EMP)) is equivalent to ( (EMP)).
(salary < 5000) salary salary salary <5000
Each relational algebraic operation can be evaluated using one of the several different algorithms.
Correspondingly, a relational-algebraic expression can be evaluated in many ways.
An expression that specifies detailed evaluation strategy is known as evaluation-plan, for example,
you can use an index on salary to find employees with salary < 5000 or we can perform complete
relation scan and discard employees with salary 5000. The basis of selection of any of the
scheme will be the cost.
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