Chapter FourteenINDEX and SYNONYM

Dr. Chitsaz

Objectives:• Create and maintain Indexes•Types and applications of Indexes• Create Synonym• Application of Index and Synonym

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INDEX

• We are interested in finding the location of an object

• For large tables INDEX speed up the time to locate an object.

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INDEX

• Object used to speed up the retrieval of rows

• (can) Reduce the disk I/O• Is independent of the table indexes• Used and maintained by the system

automatically (Oracle Server)

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INDEX CREATION:

• UNIQUE INDEXES:When using primary key or unique, index is created automatically

• NONUNIQUE INDEXES: Can be created by user on column(s)

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UNIQUE INDEXE

CREATE TABLE student(Name VARCHAR2(80),ID NUMBER(9) PRIMARY KEY,

GPA NUMBER(3,2),B_Date DATE,Major CHAR(4));

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CREATE INDEX:

• CREATE [BITMAP | UNIQUE ] INDEX indexName ON tableName

(col, col, __ ) [REVERSE];• REVERSE: to reverse the order of index• UNIQUE: use B_tree

• CREATE INDEX stud_indexON student (name);

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CREATE FUNCTION BASED INDEX:-Use of index with functions- SELECT *

FROM STUDENTWHERE LOWER (name)=‘ john’;

-CREATE INDEX stud_lower_indexON student (LOWER (name));

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Index Unique Scan

SELECT * FROM STUDENTWHERE ID=11111;

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Index Range Scan

SELECT * FROM STUDENTWHERE name LIKE ‘JO%’;

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When Indexes Are Used

• Use of Equal (or IN): SELECT *

FROM STUDENTWHERE name =‘JOHN’;

• Use of Range Values on Indexed ColumnSELECT * FROM STUDENTWHERE name <‘J’;

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When Indexes Are Used

• Use of MIN or Max

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When Indexes Are Not Used

• Use of Function: SELECT *

FROM STUDENTWHERE LOWER(name) =‘john’;

• Use of NULL or NOT NULLSELECT * FROM STUDENTWHERE name is NULL;

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When Indexes Are Not Used

• Use of Not Equal: SELECT *

FROM STUDENTWHERE name != ‘john’;

• OrSELECT * FROM STUDENTWHERE name NOT IN (SELECT name

FROM studentWHERE GPA=3);

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When to use an index:

• Columns that are used frequently in WHERE clause

• Columns containing a wide range of values• Columns containing a large number of

NULLS• In large tables, where most retrieves are

less than 5% of the rows• Tables with JOIN, or ORDER BY, MAX/MIN

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When to use an index:

Examples:Phone numberY/NZip codeFlagCity, State

Online transaction

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When to use an index:

Examples:Phone number indexY/N Bit mapZip code Depend on local or national zip code

Flag Bit mapCity, State

Online transaction (live access) do not use bit map

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When not to use an index:

• Small tables• Tables that are updated frequently• Retrieve is more than 5% of rows

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When not to use a bitmap index:

• Small tables• Frequently updated tables

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INDEXES

SELECT INDEX_NAMEFROM USER_INDEXES //INDWHERE TABLE_NAME=‘STUDENT’;

• SELECT *FROM USER_IND_COLUMNS;

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Confirming indexes:

• SELECT A.index_name, A.column_name,A.column_position, B.uniqueness

FROM user_indexes B,user_ind_columns A

WHERE A.index_name = B.index_nameAND A.table_name = ‘STUDENT';

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INDEXESUSER_INDEXES: (are grouped in 4 categories)

1-IDENTIFICATION: INDEX_NAMETABLE_NAMEINDEX_TYPE

2-SPACE RELATED:TABLESPACE_NAMEINITIAL_EXTENT

3-STATISTICS RELATED:USER_STATSLAST_ANALYZED

4-OTHERS:DROPPEDJOIN_INDEX

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Removing index:

• DROP INDEX stud_index;

• When a table is dropped, the index on that table is automatically drop.

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SYNONYMS

• Access an object by creating synonyms• Refer to a table owned by another user• Shorter length object names

CREATE[PUBLIC] SYNONYM synonymFOR object;

CREATE SYNONYM ssFOR student_view;

• DROP SYNONYM ss;

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SYNONYM

SELECT SYNONYM_NAME, TABLE_OWNER, TABLE_NAME, DB_LINK

FROM USER_SYNONYMS; //SYN

USER_SYNONYMS: Very useful for debugging

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SYNONYM

SELECT DISTINCT DB_LINKFROM USER_SYNONYMSWHERE DB_LINK IS NOT NULL;

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QUESTIONS:

How many indexes to use on a table?Advantages?Disadvantages?

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QUESTIONS:

Do we need all three indexes?CREATE INDEX indx_1 on student (name, id, GPA)CREATE INDEX indx_2 on student (name, id)CREATE INDEX indx_3 on student (name)