e computer notes - Other database objects

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

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]
[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
START WITH 120
MAXVALUE 9999
NOCACHE
NOCYCLE;
Sequence created.

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

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 ³Support´ in
location ID 2500.
INSERT INTO departments(department_id,
department_name, location_id)
VALUES (dept_deptid_seq.NEXTVAL,
'Support', 2500);
1 row created.
" View the current value for the DEPT_DEPTID_SEQ
sequence.
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.

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.

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.
CREATE INDEXindex
ON table (column[, column]...);
"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.

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

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.
SELECT ic.index_name, ic.column_name,
ic.column_position col_pos,ix.uniqueness
FRO M user_indexes ix, user_ind_columns ic
WHERE ic.index_name = ix.index_name
AND ic.table_name = 'EMPLOYEES';

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

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] SYNONY M synonym
FOR object;

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

e computer notes - Other database objects

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e computer notes - Other database objects