Database Objects

12
Copyright © Oracle Corporation, 2001. All rights reserved.
Other Database Objects

12-2 Copyright © Oracle Corporation, 2001. All rights reserved.
Objectives
After completing this lesson, you should be able to
do the following:
• Create, maintain, and use sequences
• Create and maintain indexes
• Create private and public synonyms

Database Objects
Description
Basic unit of storage; composed of rows
and columns
Logically represents subsets of data from
one or more tables
Generates primary key values
Improves the performance of some queries
Alternative name for an object
Object
Table
View
Sequence
Index
Synonym

What Is a Sequence?
A sequence:
• Automatically generates unique numbers
• Is a sharable object
• Is typically used to create a primary key value
• Replaces application code
• Speeds up the efficiency of accessing sequence
values when cached in memory

The CREATE SEQUENCE Statement Syntax
Define a sequence to generate sequential numbers
automatically:
CREATE SEQUENCE sequence
[INCREMENT BY n]
[START WITH n]
[{MAXVALUE n | NOMAXVALUE}]
[{MINVALUE n | NOMINVALUE}]
[{CYCLE | NOCYCLE}]
[{CACHE n | NOCACHE}];
CREATE SEQUENCE sequence
[INCREMENT BY n]
[START WITH n]
[{MAXVALUE n | NOMAXVALUE}]
[{MINVALUE n | NOMINVALUE}]
[{CYCLE | NOCYCLE}]
[{CACHE n | NOCACHE}];

Creating a Sequence
• Create a sequence named DEPT_DEPTID_SEQ to be
used for the primary key of the DEPARTMENTS
table.
• Do not use the CYCLE option.
CREATE SEQUENCE dept_deptid_seq
INCREMENT BY 10
START WITH 120
MAXVALUE 9999
NOCACHE
NOCYCLE;
Sequence created.Sequence created.
CREATE SEQUENCE dept_deptid_seq
INCREMENT BY 10
START WITH 120
MAXVALUE 9999
NOCACHE
NOCYCLE;
Sequence created.Sequence created.

Confirming Sequences
• Verify your sequence values in the
USER_SEQUENCES data dictionary table.
• The LAST_NUMBER column displays the next
available sequence number if NOCACHE is
specified.
SELECT sequence_name, min_value, max_value,
increment_by, last_number
FROM user_sequences;
SELECT sequence_name, min_value, max_value,
increment_by, last_number
FROM user_sequences;

NEXTVAL and CURRVAL Pseudocolumns
• NEXTVAL returns the next available sequence
value. It returns a unique value every time it is
referenced, even for different users.
• CURRVAL obtains the current sequence value.
• NEXTVAL must be issued for that sequence before
CURRVAL contains a value.

Using a Sequence
• Insert a new department named “Support” in
location ID 2500.
• View the current value for the DEPT_DEPTID_SEQ
sequence.
INSERT INTO departments(department_id,
department_name, location_id)
VALUES (dept_deptid_seq.NEXTVAL,
'Support', 2500);
1 row created.1 row created.
INSERT INTO departments(department_id,
department_name, location_id)
VALUES (dept_deptid_seq.NEXTVAL,
'Support', 2500);
1 row created.1 row created.
SELECT dept_deptid_seq.CURRVAL
FROM dual;
SELECT dept_deptid_seq.CURRVAL
FROM dual;

Using a Sequence
• Caching sequence values in memory gives faster
access to those values.
• Gaps in sequence values can occur when:
– A rollback occurs
– The system crashes
– A sequence is used in another table
• If the sequence was created with NOCACHE, view
the next available value, by querying the
USER_SEQUENCES table.

Modifying a Sequence
Change the increment value, maximum value,
minimum value, cycle option, or cache option.
ALTER SEQUENCE dept_deptid_seq
INCREMENT BY 20
MAXVALUE 999999
NOCACHE
NOCYCLE;
Sequence altered.Sequence altered.
ALTER SEQUENCE dept_deptid_seq
INCREMENT BY 20
MAXVALUE 999999
NOCACHE
NOCYCLE;
Sequence altered.Sequence altered.

Guidelines for Modifying
a Sequence
• You must be the owner or have the ALTER privilege
for the sequence.
• Only future sequence numbers are affected.
• The sequence must be dropped and
re-created to restart the sequence at a different
number.
• Some validation is performed.

Removing a Sequence
• Remove a sequence from the data dictionary by
using the DROP SEQUENCE statement.
• Once removed, the sequence can no longer be
referenced.
DROP SEQUENCE dept_deptid_seq;
Sequence dropped.Sequence dropped.
DROP SEQUENCE dept_deptid_seq;
Sequence dropped.Sequence dropped.

What Is an Index?
An index:
• Is a schema object
• Is used by the Oracle server to speed up the
retrieval of rows by using a pointer
• Can reduce disk I/O by using a rapid path access
method to locate data quickly
• Is independent of the table it indexes
• Is used and maintained automatically by the Oracle
server

How Are Indexes Created?
• Automatically: A unique index is created
automatically when you define a PRIMARY KEY or
UNIQUE constraint in a table definition.
• Manually: Users can create nonunique indexes on
columns to speed up access to the rows.

Creating an Index
• Create an index on one or more columns.
• Improve the speed of query access to the
LAST_NAME column in the EMPLOYEES table.
CREATE INDEX emp_last_name_idx
ON employees(last_name);
Index created.Index created.
CREATE INDEX emp_last_name_idx
ON employees(last_name);
Index created.Index created.
CREATE INDEX index
ON table (column[, column]...);
CREATE INDEX index
ON table (column[, column]...);

When to Create an Index
You should create an index if:
• A column contains a wide range of values
• A column contains a large number of null values
• One or more columns are frequently used together
in a WHERE clause or a join condition
• The table is large and most queries are expected
to retrieve less than 2 to 4 percent of the rows

When Not to Create an Index
It is usually not worth creating an index if:
• The table is small
• The columns are not often used as a condition in
the query
• Most queries are expected to retrieve more than 2
to 4 percent of the rows in the table
• The table is updated frequently
• The indexed columns are referenced as part of an
expression

SELECT ic.index_name, ic.column_name,
ic.column_position col_pos,ix.uniqueness
FROM user_indexes ix, user_ind_columns ic
WHERE ic.index_name = ix.index_name
AND ic.table_name = 'EMPLOYEES';
Confirming Indexes
• The USER_INDEXES data dictionary view contains
the name of the index and its uniqueness.
• The USER_IND_COLUMNS view contains the index
name, the table name, and the column name.

Function-Based Indexes
• A function-based index is an index based on
expressions.
• The index expression is built from table columns,
constants, SQL functions, and user-defined
functions.
CREATE INDEX upper_dept_name_idx
ON departments(UPPER(department_name));
Index created.
SELECT *
FROM departments
WHERE UPPER(department_name) = 'SALES';

Removing an Index
• Remove an index from the data dictionary by using
the DROP INDEX command.
• Remove the UPPER_LAST_NAME_IDX index from
the data dictionary.
• To drop an index, you must be the owner of the
index or have the DROP ANY INDEX privilege.
DROP INDEX upper_last_name_idx;
Index dropped.Index dropped.
DROP INDEX upper_last_name_idx;
Index dropped.Index dropped.
DROP INDEX index;DROP INDEX index;

Synonyms
Simplify access to objects by creating a synonym
(another name for an object). With synonyms, you can:
• Ease referring to a table owned by another user
• Shorten lengthy object names
CREATE [PUBLIC] SYNONYM synonym
FOR object;
CREATE [PUBLIC] SYNONYM synonym
FOR object;

Creating and Removing Synonyms
• Create a shortened name for the
DEPT_SUM_VU view.
• Drop a synonym.
CREATE SYNONYM d_sum
FOR dept_sum_vu;
Synonym Created.Synonym Created.
CREATE SYNONYM d_sum
FOR dept_sum_vu;
Synonym Created.Synonym Created.
DROP SYNONYM d_sum;
Synonym dropped.Synonym dropped.
DROP SYNONYM d_sum;
Synonym dropped.Synonym dropped.

Summary
In this lesson, you should have learned how to:
• Automatically generate sequence numbers by
using a sequence generator
• View sequence information in the
USER_SEQUENCES data dictionary table
• Create indexes to improve query retrieval speed
• View index information in the USER_INDEXES
dictionary table
• Use synonyms to provide alternative names for
objects

Practice 12 Overview
This practice covers the following topics:
• Creating sequences
• Using sequences
• Creating nonunique indexes
• Displaying data dictionary information about
sequences and indexes
• Dropping indexes

Database Objects

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