1_SQL_11_13_Notes_on_how_to_gain_knowledge.ppt

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

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
Database Objects

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}];

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.

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;

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.
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.

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.

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 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 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;

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.
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

Cntd..
 View index information in the
USER_INDEXES dictionary table
 Use synonyms to provide alternative
names for objects

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

1_SQL_11_13_Notes_on_how_to_gain_knowledge.ppt

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