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DDB Unit2

The document discusses techniques for translating global queries into fragment queries in a distributed database. It describes equivalence transformations that can be applied to simplify query expressions and produce equivalent fragment queries that will return the same results. Specifically, it discusses rules for pushing selections, projections, and joins down to fragments; using the algebra of qualified relations to evaluate qualifications; and pushing unions up to distribute grouping and aggregate functions across fragments.

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4K views2 pages

DDB Unit2

The document discusses techniques for translating global queries into fragment queries in a distributed database. It describes equivalence transformations that can be applied to simplify query expressions and produce equivalent fragment queries that will return the same results. Specifically, it discusses rules for pushing selections, projections, and joins down to fragments; using the algebra of qualified relations to evaluate qualifications; and pushing unions up to distribute grouping and aggregate functions across fragments.

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veerusite
Copyright
© Attribution Non-Commercial (BY-NC)
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[ III- IT- I semester DISTRIBUTED DATABASES -- Unit-2]

DISTRIBUTED DATABASES UNIT -2

 Translation of Global Queries to Fragment Queries  Equivalence Transformations for Queries  Transforming Global Queries into Fragment Queries  Distributed Grouping and Aggregate Function Evaluation  Parametric Queries

R.Veeranjaneyulu M.Tech PACE Institute of Technology & Sciences, Ongole

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[ III- IT- I semester DISTRIBUTED DATABASES -- Unit-2]

Translation of Global Queries to Fragment Queries:


In general, there are several different ways to transform a query over global relations into queries over fragments. These different transformations produce fragment queries which are equivalent i.e the produce the same result. The equivalence rules are used to for simplifying the query expressions: for instance, common sub expressions are identified, and operations are distributed to fragments. It has six specific rule to be precise..... 1) Use idempotence of selection and projection to generate appropriate selections and projections for each operand relation. 2) Push selections and projections down in the tree as far as possible. 3) Push selections down to the leaves of the tree, and then apply them using the algebra of qualified relations; substitute the selection result with the empty relation if the qualification of the result is contradictory. 4) Use the algebra of qualified relations to evaluate the qualification of operands of joins; substitute the sub tree, including the join and its operands, with the empty relation if the qualification of the result of join is contradictory. 5) In order to distribute joins which appear in the global query, unions must be pushed up, beyond the joins that we want to distribute. 6) In order to distribute grouping and aggregate function evaluations appearing in a global query, unions must be pushed up, beyond the corresponding group-by operation.

R.Veeranjaneyulu M.Tech PACE Institute of Technology & Sciences, Ongole

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