How does query tree represent relational algebra expression, Database Management System

Assignment Help:

How does a query tree represent a relational algebra expression?  Discuss any three rules for query optimisation, giving example as to when should each rule be applied.   

 Ans: A query tree is a tree structure which refers to a relational algebra expression. It denotes the input relations as leaf nodes of the tree, and denotes the relational algebra operations as internal nodes. An execution of the query tree contains executing an internal node operation while its operands are available and after that replacing that internal node by the relation which results from executing the operation. The execution terminates while the root node is executed and generates the result relation for the query.  For instance, the query tree for the relational algebra expression

  ∏E_No =(σp_No = 'DB2003' (Assigned_To) :

The three rules for query optimization are like this:

  • In a cascade (sequence) of ∏ operations, each one but the last one can be ignored :

   ∏1(∏2(......∏n ( R))) ≡ ∏1(R )

  • If a selection condition c includes just only those attributes A1, A2, .... An in the projection list, the two operations can be commuted :

   ∏A1, A2,......An (σC (R ) ≡ σC  (∏A1, A2, ......An (R ))..... ))

  • A conjunctive selection condition can be split up into a cascade of individual σ operations:

  σC1 AND C2 AND ....AND  Cn  (R ) ≡ σC1C2(....(σCn (R ))....))


Related Discussions:- How does query tree represent relational algebra expression

How is the database organised in oracle, How is the database organised in O...

How is the database organised in Oracle? Database organized within Oracle in terms of table and table holds attributes and values. A database consists of one or more logical

Update operations, The Update Operations : Update operations are used fo...

The Update Operations : Update operations are used for altering database values. The constraint violations faced by this operation are logically the similar as the problem faced

What is called an evaluation primitive, What is called an evaluation primit...

What is called an evaluation primitive? A relational algebra operation annotated with instructions on how to evaluate is known as an evaluation primitive.

What is a records management consultation, What is a records management con...

What is a records management consultation? Our organization is an authority for disseminating your specific requirements and giving you with a solution to effectively manage yo

Synonyms and indexes, Synonyms and Indexes Some of the essential proper...

Synonyms and Indexes Some of the essential properties of indexes are: An Index is a schema Object Indexes can be formed explicitly or automatically Indexes are m

What are the constituents of file, What are the Constituents of File? Also ...

What are the Constituents of File? Also elaborates all the possible file operations? A file is organised logically as sequence of record. These records are mapped onto disk blo

What is boyce-code normal form, Boyce-Code Normal Form (BCNF) A relatio...

Boyce-Code Normal Form (BCNF) A relationship is said to be in BCNF if it is already in 3NF and the left hand side of each dependency is a candidate key. A relation who is in 3N

What is a block and a block number, What is a block and a block number? ...

What is a block and a block number? A block is a contiguous sequence of sectors from a single track of one platter. Every request specifies the address on the disk to be refere

Write Your Message!

Captcha
Free Assignment Quote

Assured A++ Grade

Get guaranteed satisfaction & time on delivery in every assignment order you paid with us! We ensure premium quality solution document along with free turntin report!

All rights reserved! Copyrights ©2019-2020 ExpertsMind IT Educational Pvt Ltd