Index & Query Optimization

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DBMS

Index and uer

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Optimization

Abel M. Ingaw

Ronald T. Bragas


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Index: a data structure that improves the speed of

data retrieval operations

Definition of Terms

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,

contains only the field/s a user is interested in

sorting on

Indexing: a data structure, which orders data of

one or more columns in database table


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defined based on its indexing attributes

can be one of the following types

Primary Index: If index is built on ordering

'key-field' of file

Indexing

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Secondary Index: If index is built on non-

ordering field of file

Clustering Index: If index is built on ordering

non-key field of file


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Dense Index: there is an index record for every

search key value in the database

Sparse Index: index records are not created

for every search key. An index record here

Ordered Indexing

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contains search key and actual pointer to the

data on the disk

Clustering Index: If index is built on orderingnon-key field of file


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Multilevel Index:

Index records are comprised of search-key

value and data pointers. This index itself is

Ordered Indexing

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s ore on e s a ong w e ac ua

database files.


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Part 2

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uery p m za on


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Query:

is a re uest or in ormation rom a database

Query Optimization

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Aims to choose a query which minimizes the usage

of resources

Query Optimization

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Helps to find the near optimum solution


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Scenario:

Table Name: employee

Columns:

Query Optimization

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employee_number, firstname, surname, address ,

tel_no varchar, salary,overtime_rate


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Example 1:

Find employee Fred Jone's salary(employee number

101832)

Query Optimization

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SQL:

SELECT salary FROM employee WHEREemployee_number = '101832';


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Problem:

No clue where to find this record

Solution:

Query Optimization

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SELECT employee_number, firstname, surname FROM

employee WHERE employee_number= '10875';


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Example 2:

What about if you want to select on more than one

criteria?

Query Optimization

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SQL 1:

SELECT firstname FROM employee


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Problem:

makes no use of an index at all. An index on firstname

is useless

Query Optimization

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o ut on:

SELECT firstname FROM employee WHERE

surname="Madida";


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Example 3:

find all the employees where half their overtime rate is

less than 20 pesos

Query Optimization

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SQL 1:

SELECT firstname FROM employee WHEREovertime_rate


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Problem:

Every single employee record is being read

School Algebra to the rescue:

Query Optimization

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x/2 = y' is the same as 'x = y*2


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Solution:

Add an index on overtime_rate

SELECT firstname FROM employee WHERE

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overtime_rate/2


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Example 4:

Having thousands of records, delete all records as fast

as possible.

Query Optimization

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SQL 1:

DELETE FROM employee


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Problem:

DELETE drops records one by one

Solution:

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TRUNCATE TABLE employee

TRUNCATE employee


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Conclusion

It's not only getting the data in that

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needs to be quick - sometimes you

need to get it out quickly too


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Index & Query Optimization

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