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

Differentiate between a unique key and a primary key, Differentiate between...

Differentiate between a unique key and a primary key. - By default, clustered index on column are created by primary key while nonclustered index are created by unique key.

Information infrasture, what benefit can a company gain by managing its own...

what benefit can a company gain by managing its own infrasture and services

Advantages and disadvantages of using a database system, What are the advan...

What are the advantages and disadvantages of using a database system. Sol. Advantages of Database System (DBMS) :  (1) Data Redundancy and Inconsistency: Since differen

Explain trigger, Explain trigger? Trigger: A database trigger is proc...

Explain trigger? Trigger: A database trigger is procedural code which is automatically executed within response to certain events on a particular table in a database. Trigger

Stored and derived attributes, Attributes that are directly kept in the dat...

Attributes that are directly kept in the data base. For example, 'Birth date' attribute of a person.   Derived  Attributes that are not stored directly but can be derived

What is the difference between link and reference, What is the difference b...

What is the difference between link and reference? The fundamental difference between links and references is that links are symmetrical on the other hand references refer only

Define blocks, Define blocks? The database system resides eternally on ...

Define blocks? The database system resides eternally on non-volatile storage, and is into fixed-length storage units known as blocks.

#title using the invoice, using the invoice table shown below,write the rel...

using the invoice table shown below,write the relational schema,draw a dependencies,inclunding all partial and transitive dependencies.assume that the table does not contain repeat

Log based recovery, Log based recovery: Let us first describe the term tran...

Log based recovery: Let us first describe the term transaction log in the context of DBMS. A transaction log is a record in DBMS that remains track of all the transactions of a dat

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