Chapter Fourteen
INDEX and SYNONYM
Dr. Chitsaz
Objectives:
• Create and maintain Indexes
•Types and applications of Indexes
• Create Synonym
• Application of Index and Synonym
INDEX
• We are interested in finding the location of
an object
• For large tables INDEX speed up the time
to locate an object.
2
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_index
ON student (name);
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CREATE FUNCTION BASED
INDEX:
-Use of index with functions
SELECT
*
FROM
WHERE
STUDENT
LOWER (name)=‘ john’;
-CREATE INDEX stud_lower_index
ON student (LOWER (name));
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Index Unique Scan
SELECT
FROM
WHERE
*
STUDENT
ID=11111;
8
Index Range Scan
SELECT
FROM
WHERE
*
STUDENT
name LIKE ‘JO%’;
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When Indexes Are Used
• Use of Equal (or IN):
SELECT
*
FROM
WHERE
STUDENT
name =‘JOHN’;
• Use of Range Values on Indexed Column
SELECT
FROM
WHERE
*
STUDENT
name <‘J’;
10
When Indexes Are Used
• Use of MIN or Max
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When Indexes Are Not Used
• Use of Function:
SELECT
*
FROM
WHERE
STUDENT
LOWER(name) =‘john’;
• Use of NULL or NOT NULL
SELECT
FROM
WHERE
*
STUDENT
name is NULL;
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When Indexes Are Not Used
• Use of Not Equal:
SELECT
FROM
WHERE
*
STUDENT
name != ‘john’;
SELECT
FROM
WHERE
*
STUDENT
name NOT IN (SELECT name
FROM student
WHERE GPA=3);
• Or
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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
14
When to use an index:
Examples:
Phone number
Y/N
Zip code
Flag
City, State
Online transaction
15
When to use an index:
Examples:
Phone number
Y/N
Zip code
Flag
City, State
Online transaction
index
Bit map
Depend on local or national zip code
Bit map
(live access) do not use bit map
16
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_NAME
FROM
USER_INDEXES //IND
WHERE 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_name
AND
A.table_name = ‘STUDENT';
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INDEXES
USER_INDEXES: (are grouped in 4 categories)
1-IDENTIFICATION:
INDEX_NAME
TABLE_NAME
INDEX_TYPE
2-SPACE RELATED:
TABLESPACE_NAME
INITIAL_EXTENT
3-STATISTICS RELATED:
USER_STATS
LAST_ANALYZED
4-OTHERS:
DROPPED
JOIN_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 synonym
FOR
object;
CREATE SYNONYM ss
FOR student_view;
• DROP SYNONYM ss;
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SYNONYM
SELECT
FROM
SYNONYM_NAME, TABLE_OWNER,
TABLE_NAME, DB_LINK
USER_SYNONYMS; //SYN
USER_SYNONYMS: Very useful for debugging
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SYNONYM
SELECT
FROM
WHERE
DISTINCT DB_LINK
USER_SYNONYMS
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)
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