Oracle 8 Database Administration volume 2 instruction guide phần 2 pot

Oracle8: Database Administration 13-25 ......Reorganizing Indexes Analyze the index to perform the following: does not verify whether index entries correspond to data in the table.. Orac

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Oracle8: Database Administration 13-25

Reorganizing Indexes

Analyze the index to perform the following:

does not verify whether index entries correspond to data in the table

Syntax

ANALYZE INDEX [ schema.]index VALIDATE STRUCTURE

After running this command, query INDEX_STATS to obtain information

about the index as shown in the following example:

SVRMGR> SELECT blocks, pct_used, distinct_keys

Reorganize the index if it has a high proportion of deleted rows—for

example, when the ratio of DEL_LF_ROWS to LF_ROWS exceeds 30%

Checking Index Validity

ANALYZE INDEX scott.ord_region_id_idx

VALIDATE STRUCTURE;

INDEX_STATS

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Lesson 13: Managing Indexes

Dropping Indexes

Indexes may need to be dropped in the following scenarios:

index prior to large data loads and recreating them after the load:

unnecessarily, especially if they are based on volatile tables This is

generally the case in an OLTP system, where ad hoc queries are

generated at year-end or quarter-end to gather information for review

meetings

during certain types of operations such as loading In this case the index

Dropping Indexes

• Drop and re-create an index before bulk

loads.

• Drop indexes that are infrequently

needed and build them when necessary.

• Drop and recreate invalid indexes.

DROP INDEX scott.dept_dname_idx;

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Dropping Indexes

Syntax

Use the following command to drop an index:

DROP INDEX [schema.] index

OEM

Note

An index cannot be dropped if it is used to implement an integrity constraint

that is enabled Constraints are discussed in the lesson “Maintaining Data

Integrity.”

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Obtaining Index Information

The data dictionary views DBA_INDEXES and DBA_IND_COLUMNS

give the information on the indexes and the columns indexed

Checking Indexes and Their Validity

Use the following command to verify the name, type, and status of the

indexes owned by the user SCOTT:

SVRMGR> SELECT index_name, tablespace_name, index_type,

Obtaining Index Information

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Obtaining Index Information

The column INDEX_TYPE indicates whether the index is bitmap or normal

Use the following query to list the names of all reverse key indexes:

Finding Columns in an Index

The following query lists all the indexes owned by the user SCOTT and

shows the tables and columns on which the indexes are built:

SVRMGR> SELECT index_name, table_owner, table_name, column_name 2> FROM dba_ind_columns

3> WHERE index_owner = 'SCOTT'

4> ORDER BY index_name, column_position;

INDEX_NAME TABLE_OWNER TABLE_NAME COLUMN_NAME - - - - EMP_LNAME_IDX SCOTT EMP LAST_NAME ORD_ORD_NO_IDX SCOTT ORD ORD_NO

ORD_REGION_ID_IDX SCOTT ORD REGION_ID

3 rows selected.

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Summary

Quick Reference

Initialization parameters CREATE_BITMAP_AREA_SIZE

Dynamic performance views None

Data dictionary tables/views DBA_INDEXES

DBA_IND_COLUMNS DBA_OBJECTS IND$

INDEX_STATS

CREATE UNIQUE INDEX CREATE BITMAP INDEX CREATE INDEX REVERSE ALTER INDEX STORAGE ALTER INDEX INITRANS MAXTRANS ALTER INDEX ALLOCATE EXTENT ALTER INDEX DEALLOCATE UNUSED ALTER INDEX REBUILD

ALTER INDEX REBUILD REVERSE ALTER INDEX REBUILD NOREVERSE ANALYZE INDEX VALIDATE STRUCTURE DROP INDEX

Packaged procedures and

functions

None

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14

Maintaining Data Integrity

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Lesson 14: Maintaining Data Integrity

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• Obtaining constraint and trigger

information from the data dictionary

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Overview

Data integrity guarantees that data in a database adheres to business rules

There are three primary ways in which data integrity can be maintained:

Mapping the business rules using one of the three methods is a design

decision made by the designer The database administrator is primarily

concerned with implementing the methods chosen by the designer and

balancing the performance needs against integrity requirements

Application code may be implemented either as stored procedures within the

database or as applications running on the client This lesson focuses on the

use of database triggers and integrity constraints

Data Integrity

Application

code

Integrity constraint

Database trigger

Table Data

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Overview

Database Triggers

Database triggers are PL/SQL programs that are executed when a certain

event such as an insert or an update of a column occurs on a table Triggers

can be enabled or disabled—that is, they can be set to execute when the

event occurs, or they can be set not to execute even though they are defined

Database triggers are usually created only to enforce a complex business

rule that cannot be defined as an integrity constraint

Integrity Constraints

Integrity constraints are the preferred mechanism for enforcing business

rules because they:

coding

The following sections explain the behavior of integrity constraints and

discuss how they are implemented by the Oracle server

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Integrity Constraints and Triggers

As shown in the slide, there are five types of declarative integrity constraints

Although the NOT NULL and CHECK constraints do not directly require

DBA attention, the primary key, unique, and foreign key constraints must be

managed to ensure high availability and acceptable performance levels

Instructor Note

A foreign key constraint can be defined to disable the deletion of a parent

row when child rows exist or the deletion of all the child rows when the

corresponding parent row is deleted

Designates a column or combination of columns as unique

Designates a column or combination of columns as the table’s primary key Designates a column or combination of columns as the foreign key in a

referential integrity constraint Specifies a condition that each row of the table must satisfy

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Integrity Constraints and Triggers

An integrity constraint can be in one of the following states:

Disabled

A constraint that is disabled is not checked, even though the constraint

definition is still stored in the data dictionary Data in the table as well as

new data that is entered or updated may not conform to the rules defined by

the constraint

Enabled Novalidate (Enforced)

If a constraint is enabled novalidate, new data that violates the constraint

cannot be entered However, the table may contain data that is invalid—that

is, data that violates the constraint This is usually an intermediate stage that

ensures that all new data is checked before being accepted into the table

Constraint States

Disabled Enabled

novalidate

Enabled validate

Existing data New data

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Enabled Validate

If a constraint is in this state, then all data in the table is guaranteed to adhere

to the constraint In addition, this state prevents any invalid data from being

entered This is the normal state of operation of a constraint for online

transaction processing

When a constraint is enabled validate from a disabled state, the table is

locked and all data in the table is checked for conformance This may cause

DML operations such as a data load to wait, so it may be advisable to move

from a disabled state to enable novalidate and then to move to enable

validate

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The point in a transaction at which a constraint is checked can be controlled

by defining the constraint appropriately

Nondeferred or Immediate Constraints

Nondeferred constraints, also known as immediate constraints, are enforced

at the end of every DML statement A constraint violation causes the

statement to be rolled back If a constraint causes an action such as delete

cascade, the action is taken as part of the statement that caused it

A constraint that is defined as nondeferrable cannot be modified to be

enforced at the end of a transaction

Deferred Constraints

Deferred constraints are constraints that are checked only when a transaction

commits If any constraint violations are detected at commit time, the entire

transaction is rolled back These constraints are most useful when both the

parent and child rows in a foreign key relationship are entered at the same

time, as in the case of an order entry system, where the order and the items

in the order are entered at the same time

Deferred Constraints

DML statement

Check nondeferred constraints

COMMIT

Check deferred constraints

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For a constraint to be deferred, it must be defined as a deferrable constraint

at the time of creation A constraint that is defined as deferrable can be

specified as one of the following:

• Initially immediate specifies that by default it should function as an

immediate constraint, unless explicitly set otherwise

• Initially deferred specifies that by default the constraint should only be

enforced at the end of the transaction

Although the default mode of enforcement of a deferrable constraint is

specified and stored in the data dictionary, applications can modify the

constraint to work as either deferred or immediate This is accomplished by

using either an ALTER SESSION command or a SET CONSTRAINT

command as shown below:

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Primary and unique keys are enforced using indexes You can control the

location and type of index that is used for enforcing these constraints

Oracle Server follows this procedure to implement unique and primary key

constraints:

leading part of an index, the index will be used to enforce the constraint

3 If the constraint is enabled and there is no index that uses the constraint

columns as a leading part of the index, an index with the same name as

the constraint is created using the following rules:

created

Use existing index

Is an index available for use?

Yes

Constraint deferrable?

Create nonunique index

Yes

No Key enabled?

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You need to consider several factors in maintaining tables that are in a

foreign key relationship

DDL Involving Parent Table

the following command to perform both actions using a single

statement:

DROP TABLE table CASCADE CONSTRAINTS

foreign key

parent is dropped The following command can be used to achieve this:

DROP TABLESPACE tablespace INCLUDING CONTENTS

Foreign Key Considerations

To

Drop parent table

Truncate parent table

Drop tablespace

containing parent

table

Avoid locks on child

table while performing

DML on parent table

Perform DML on child

table

Perform Cascade constraints Disable/drop foreign key Use CASCADE CONSTRAINTS clause

Create index on foreign key

Ensure tablespace containing parent key index online

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DML on Tables in a Foreign Key Relationship

If the DELETE CASCADE option is not used when rows are deleted from

the parent table, Oracle Server needs to ensure that there are no rows in the

child table with the corresponding foreign key Similarly, an update to the

parent key is permitted only when there are no child rows with the old key

value If there is no index on the foreign key on the child table, the Oracle

server locks the child table and prevents changes to ensure referential

integrity If there is an index on the table, the referential integrity is

maintained by locking the index entries and avoiding more restrictive locks

on the child table If both tables need to be updated concurrently from

different transactions, create an index on the foreign key columns

When data is inserted into or the foreign key column is updated in the child

table, the Oracle server checks the index on the parent table that is used for

enforcing the referenced key Therefore, the operation succeeds only if the

tablespace containing the index is online Note that the tablespace

containing the parent table does not need to be online to perform DML

operations on the child table

Note

Besides the advantage mentioned above, creating indexes on foreign key

columns is recommended because it has other performance advantages

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Database triggers are PL/SQL programs that are implicitly executed when a

DML statement is executed on the associated table

Trigger Types

There are twelve possible trigger combinations that can be defined on a table

based on the following three properties that can be specified for a trigger:

DML Statement

A trigger can be set to execute when INSERT, UPDATE, or DELETE, or

any combination of these statements, is executed on a table When defining

an update trigger, if column names are specified, the trigger fires only when

the columns mentioned are updated

Trigger types

• INSERT or UPDATE or DELETE

• BEFORE or AFTER

• ROW or STATEMENT

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Trigger States

Triggers can be enabled or disabled A disabled trigger does not execute

when the corresponding DML operation is carried out on the table A trigger

that is enabled fires on execution of the corresponding DML command

Triggers are event driven, so when a trigger is enabled, no action is carried

out on an existing data

Creating a Trigger: Example

The following trigger uses the INITCAP function to convert an employee’s

last name before it is stored

CREATE TRIGGER scott.emp_conv_ln

BEFORE INSERT OR UPDATE OF last_name ON scott.employees

FOR EACH ROW

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Implementing Constraints and Triggers

A constraint can be defined either when a table is created or when a table is

altered to add the constraint

Syntax: In-Line Constraint

At the time the table is created, the constraint can be created in line using the

following syntax to define the column:

column datatype [CONSTRAINT constraint]

in_line_constraint

[defer_spec]

in_line_constraint :==

{[NOT] NULL

|PRIMARY KEY [USING INDEX index_clause]

Defining Constraints While

Creating a TableCREATE TABLE scott.employees(

empno NUMBER(4)

CONSTRAINT emp_pk PRIMARY KEY DEFERRABLE

USING INDEX STORAGE(INITIAL 100K NEXT 100K) TABLESPACE indx01,

last_name VARCHAR2(30)

CONSTRAINT emp_ln_nn NOT NULL, deptno NUMBER(2))

TABLESPACE data01;

Tiêu đề	Managing Indexes
Trường học	Oracle Corporation
Chuyên ngành	Database Administration
Thể loại	tài liệu hướng dẫn
Năm xuất bản	1998

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