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 to control as concurrent tasks, Control as Concurrent Tasks The ex...

Control as Concurrent Tasks The expression control as simultaneous task means applying control for those events of object which can happen simultaneously. An object can be imp

DW Job Scheduling, Define a job scheduling strategy that will meet business...

Define a job scheduling strategy that will meet business requirement of reporting availability by 6am CST for the following cubes? Show the job scheduling dependencies in a pictori

Datawarehouse, 1) Define a job scheduling strategy that will meet business ...

1) Define a job scheduling strategy that will meet business requirement of reporting availability by 6am CST for the following cubes? Show the job scheduling dependencies in a pict

Data envelopment analysis , I need a help with my assignment of data envelo...

I need a help with my assignment of data envelopment analysis which is due for submission next week on monday the 10th Dec. Will you be able to help so that I can submit it on tim

Designing the database schema , The domain that you will model in this assi...

The domain that you will model in this assignment is that of courses and students at a university. So as not to make the task too large and unspecified, you will here get a descrip

Goals, what are the purposes of dbms in details

what are the purposes of dbms in details

Enforce referential integrity, Use Access 2007 to create a database calle...

Use Access 2007 to create a database called UniLib.mdb. - this file should be created on your personal drive at the university (or C: if you're using Access 2007 at home) and t

Data dictionary, The database is to record information about artists for a ...

The database is to record information about artists for a (music) production company. An artist is the name of a singer or band; the name of the artist, the year the artist started

What is a foreign key, What is a foreign key, and what is it used for? ...

What is a foreign key, and what is it used for? A foreign key is used to establish relationships between relations in the relational model. Technically, a foreign key is a colu

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