Oracle 8 Database Administration volume 2 instruction guide phần 3 doc

]trigger DISABLE Enabling Triggers Use the following command to enable a trigger: ALTER TRIGGER [ schema.. Oracle8: Database Administration 14-33 ......Getting Constraint and Trigger Inf

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Oracle8: Database Administration 14-27

Maintaining Constraints and Triggers

By default, a trigger is enabled when created

Disabling Triggers

Disabling a trigger may be necessary in the following conditions:

• An object that the trigger references is not available, but the DML

activity on the table cannot be stopped

• To speed up a large data load on the table that has the trigger

Use the following command to disable a trigger:

ALTER TRIGGER [ schema ]trigger DISABLE

Enabling Triggers

Use the following command to enable a trigger:

ALTER TRIGGER [ schema ]trigger ENABLE

What follows is the syntax for enabling or disabling all triggers in a table:

ALTER TABLE [ schema ]table

{ DISABLE | ENABLE } ALL TRIGGERS

Disabling and Enabling Triggers

• Use ALTER TRIGGER to disable or

enable one trigger.

• Use ALTER TABLE to disable or enable

all triggers.

ALTER TRIGGER scott.emp_conv_ln

DISABLE;

ALTER TABLE scott.employees

ENABLE ALL TRIGGERS;

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14-28 Oracle8: Database Administration

Constraints may be dropped when:

• They are no longer necessary

• They need to be modified but because they cannot be modified directly,

they must be dropped and added

Syntax

Use the following command to drop a constraint:

ALTER TABLE [ schema ] table

DROP {CONSTRAINT constraint

• When a primary key or a unique constraint is dropped, any associated

unique index is also dropped

• If a primary key or unique constraint is implemented using a nonunique

index, the associated index is not dropped, and must be dropped

manually if not required

Dropping Constraints

• Drop constraints using this command:

• Drop a table and any referencing

foreign key using this command:

ALTER TABLE scott.employees

DROP CONSTRAINT emp_ln_uk;

DROP TABLE departments

CASCADE CONSTRAINTS;

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Maintaining Constraints and Triggers

Dropping a Parent Table

Tables that are referenced by a foreign key can only be dropped if the

foreign key is dropped This can be achieved using the following command:

DROP [ schema ] table

CASCADE CONSTRAINTS

This may be necessary if the parent table needs to be reorganized

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Use the following command to drop a trigger that is no longer required:

DROP TRIGGER [ schema ] trigger

Dropping Triggers

DROP TRIGGER scott.audit_dept;

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Getting Constraint and Trigger Information Getting Constraint and Trigger Information Constraints and Their Status Use the following query to obtain the names, types, and status of all constraints on SCOTT’s EMP table: SVRMGR> SELECT constraint_name, constraint_type, deferrable, 2> deferred, validated 3> FROM dba_constraints 4> WHERE owner='SCOTT' 5> AND table_name='EMPLOYEES'; CONSTRAINT_NAME C DEFERRABLE DEFERRED VALIDATED - - - -

-EMP_DEPT_FK R DEFERRABLE DEFERRED VALIDATED EMP_PK P DEFERRABLE IMMEDIATE VALIDATED SYS_C00565 C NOT DEFERRABLE IMMEDIATE VALIDATED 3 rows selected 14-19 Copyright  Oracle Corporation, 1998 All rights reserved. Getting Constraint Information DBA_CONSTRAINTS OWNER

CONSTRAINT_NAME

CONSTRAINT_TYPE

TABLE_NAME

SEARCH_CONDITION

R_OWNER

R_CONSTRAINT_NAME DELETE_RULE

STATUS

DEFERRABLE

DEFERRED

VALIDATED

GENERATED

BAD

LAST_CHANGE DBA_CONS_COLUMNS OWNER

CONSTRAINT_NAME TABLE_NAME

COLUMN_NAME POSITION

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The following table shows the columns in DBA_CONSTRAINTS view that

are not self-evident

6> AND c.owner = cc.owner

7> AND c.constraint_name = cc.constraint_name

CONSTRAINT_TYPE The type of constraint is P if Primary key, U if Unique, R if

Foreign key, or C if Check constraint NOT NULL constraints are stored as check constraints.

SEARCH_CONDITION Shows the condition specified for a check constraint

R_OWNER

R_CONSTRAINT_NAME

Define the owner and name of the referenced constraint for foreign keys

GENERATED Indicates whether the constraint name is system generated

(Valid values are ‘USER NAME’ and ‘GENERATED NAME’.)

BAD Indicates that the constraint is to be rewritten to avoid such

situations as Year 2000 problems (This might happen because earlier releases of Oracle allowed 2-digit years to

be specified in check constraints.) LAST_CHANGE The date when the constraint was last enabled or disabled

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Getting Constraint and Trigger Information

Finding Primary Key–Foreign Key Relationships

To find foreign keys on SCOTT’s EMP table and the parent constraints, use

the following query:

SVRMGR> SELECT c.constraint_name AS "Foreign Key",

2> p.constraint_name AS "Referenced Key",

11> AND c.r_constraint_name = p.constraint_name;

Foreign Key Referenced Key C OWNER TABLE_NAME

- - - - EMP_DEPT_FK DEPT_PK P SCOTT DEPARTMENTS

-1 row selected.

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TRIGGER_NAME

TRIGGER_TYPE

TRIGGERING_EVENT

TABLE_OWNER

TABLE_NAME

STATUS

DESCRIPTION

TRIGGER_BODY DBA_TRIGGER_COLS TRIGGER_OWNER TRIGGER_NAME TABLE_OWNER

TABLE_NAME

COLUMN_NAME

DBA_OBJECTS OWNER OBJECT_NAME OBJECT_TYPE STATUS

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Getting Constraint and Trigger Information

Triggers and Trigger Columns

To find the names of all triggers on SCOTT’s EMP table, and the columns if

any, in the UPDATE OF clause, use the following query:

SVRMGR> SELECT t.owner, t.trigger_name, t.trigger_type,

2 t.triggering_event, t.status,

3 c.column_name, o.status as "VALIDITY"

4 FROM dba_triggers t, dba_trigger_cols c,

5 dba_objects o

6 WHERE t.owner = c.trigger_owner (+)

7 AND t.trigger_name = c.trigger_name (+)

8 AND t.owner = o.owner

9 AND t.trigger_name = o.object_name

10 AND o.object_type = 'TRIGGER'

11 AND t.table_owner = 'SCOTT'

12 AND t.table_name = 'EMPLOYEES';

OWNER RIGGER_NAME TRIGGER_TYPE TRIGGERING_EVENT STATUS

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Summary

• Implementing constraints and triggers

• Using appropriate strategy for creating

and maintaining constraints

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Summary

Quick Reference

Initialization parameters None

Dynamic performance views None

Data dictionary views DBA_CONSTRAINTS

DBA_CONS_COLUMNS DBA_TRIGGERS

DBA_TRIGGER_COLS Commands CREATE TABLE CONSTRAINT

ALTER TABLE ADD CONSTRAINT

EXCEPTIONS INTO CREATE OR REPLACE TRIGGER ALTER TABLE DISABLE CONSTRAINT ALTER TABLE ENABLE NOVALIDATE CONSTRAINT

ALTER TABLE ENABLE VALIDATE CONSTRAINT EXCEPTIONS INTO ALTER TRIGGER DISABLE

ALTER TABLE DISABLE ALL TRIGGERS ALTER TRIGGER ENABLE

ALTER TABLE ENABLE ALL TRIGGERS ALTER TRIGGER COMPILE

ALTER TABLE DROP CONSTRAINT DROP TABLE CASCADE CONSTRAINTS DROP TRIGGER

Packaged procedures and

functions

None

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15

Using Clusters and Index-Organized Tables

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Objectives

Objectives

• Creating and maintaining clusters

• Using index-organized tables

• Retrieving information about clusters

and tables from the data dictionary

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Overview

In a regular table, the user has very limited control over how row data is

distributed When the table is first created, the rows are generally inserted

into a segment starting with the first block of the first extent But once other

DML operations are carried out, several factors, such as the order of blocks

in the free list and row migration, make it difficult to influence the ordering

of row data within the table

Clusters offer some degree of control over how rows are stored When a

cluster is used, the Oracle server stores all rows that have the same key value

in the same block, if possible

When index-organized tables are used, the user exercises greater control

over data ordering Data in an index-organized table is in the order of the key

specified

This lesson focuses on the use of these two means of storing data

Distribution of Rows Within a

Table

Cluster Index-organized

table Table

Random

Ordering of Rows

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Clusters

Clusters

A cluster can be used to store related sets of rows within the same Oracle

Server block The slide shows an example from an order processing system,

where the order table, ORD, and the items in the order, ITEM, are stored in a

cluster

Difference Between Clustered and Unclustered Tables

If they were stored as regular tables, ORD and ITEM are placed in different

segments This means that the tables use their own set of blocks—a block

that is used to store a row from the ORD table does not contain data from the

ITEM table, and vice versa

If the tables ORD and ITEM are stored in a cluster, they share the same

cluster segment A block in this segment can store rows from both tables If

a table is stored in a cluster, the cluster becomes the physical unit of storage

and the table is a logical entity—that is, part of the cluster

Clusters

Clustered ORD and ITEM tables

Cluster Key (ORD_NO)

101 ORD_DT CUST_CD 05-JAN-97 R01

PROD QTY A4102 20 A5675 19 W0824 10

102 ORD_DT CUST_CD 07-JAN-97 N45

PROD QTY A2091 11 G7830 20 N9587 26

Unclustered ORD

and ITEM tables

ORD_NO PROD QTY

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Cluster Characteristics

Clusters have the following characteristics:

• Clusters have a cluster key, which is used to identify the rows that need

to be stored together

• The cluster key may consist of one or more columns

• Tables in a cluster have columns that correspond to the cluster key

• Clustering is a mechanism that is transparent to the applications using

the tables Data in a clustered table can be manipulated as though it were

stored in a regular table

• Updating one of the columns in the cluster key may entail physically

relocating the row

• The cluster key is independent of the primary key The tables in a cluster

can have a primary key, which may be the cluster key or a different set of

columns

• Clusters are usually created to improve performance Random access to

clustered data may be faster, but full table scans on clustered tables are

generally slower

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An index cluster uses an index, known as a cluster index, to maintain the

data within the cluster

• The cluster index must be available to store, access, or maintain data in

an index cluster

• The cluster index is used to point to the block that contains the rows with

a given key value

• The structure of a cluster index is similar to that of a normal index

Although a normal index does not store a NULL key value, cluster

indexes store NULL keys There is only one entry for each key value in

the cluster index Therefore, they are likely to be smaller than a normal

index on the same set of key values

Cluster Types

Hash function

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• To store or retrieve rows from a cluster, the Oracle server uses the cluster

index to locate the first row that corresponds to the given key value and

then retrieves the rows for the given key

• If several rows in an index cluster have the same cluster key, the cluster

key is not repeated for each row In a table with a large number of rows

per key value, use of an index cluster may reduce the amount of space

needed to store data

Hash Cluster

A hash cluster uses a function to calculate the location of a row The hash

function uses the cluster key and can either be user defined or system

generated

When a row is inserted into a table in a hash cluster:

• The hash key columns are used to compute a hash value

• The row is stored based on the hash value

The hash function is used to locate the row while retrieving the data from a

hashed table Where applicable, a hash cluster can provide greater

performance gains than an index cluster

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Creating Clusters

Creating Clusters

1 Create a cluster.

2 Create a cluster index.

Creating Index Clusters

CREATE CLUSTER scott.ord_clu

(ord_no NUMBER(3))

SIZE 200 TABLESPACE DATA01

STORAGE(INITIAL 5M NEXT 5M PCTINCREASE 0);

CREATE INDEX scott.ord_clu_idx

ON CLUSTER scott.ord_clu

TABLESPACE INDX01

STORAGE(INITIAL 1M NEXT 1M PCTINCREASE 0);

Creating Index Clusters

CREATE TABLE scott.ord

(ord_no NUMBER(3)

CONSTRAINT ord_pk PRIMARY KEY,

ord_dt DATE, cust_cd VARCHAR2(3))

CLUSTER scott.ord_clu(ord_no);

3 Create tables in the cluster.

CREATE TABLE scott.item

(ord_no NUMBER(3) CONSTRAINT item_ord_fk

REFERENCES scott.ord,

prod VARCHAR2(5), qty NUMBER(3),

CONSTRAINT item_pk PRIMARY KEY(ord_no,prod))

CLUSTER scott.ord_clu(ord_no);

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To create tables in an index cluster, follow these steps:

1 Create the cluster

2 Create the cluster index

3 Create tables in the cluster

You can create tables in the cluster before creating the index All three steps

must be completed before inserting data into the table

Creating the Cluster

Use the following command to create a cluster:

CREATE CLUSTER [ schema ] cluster (column dattype [, column datatype ] )

where: schema is the owner of the cluster

cluster is the name of the cluster

column is the name of a key column

data type is the data type and size of the key column

SIZE specifies the space required by all rows

corresponding to a key value in bytes,kilobytes, or megabytes

INDEX specifies that it is an index cluster

Note

If SIZE is not defined, it defaults to the size of one block The Oracle server

uses this value to estimate the maximum number of key values that can be

accommodated in each block of the cluster In the example, SIZE is set to

200 For a block size of 2048, after allowing for the header, about 1900 bytes

are available in the block for storing data Therefore, a block can

accommodate a maximum of 9 key values

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Creating Clusters

Creating the Cluster Index

Use the following command to create a cluster index:

CREATE INDEX [ schema ] index

ON CLUSTER [ schema ] cluster

The key columns do not need to be specified for a cluster index because they

have already been defined while creating the cluster Place the cluster index

in a tablespace different from that used for creating the cluster

Creating Tables in the Cluster

Specify the cluster name to create tables in the cluster:

CREATE TABLE [schema.]table

( column_definition

[, column_definition ]

[, [CONSTRAINT constraint] out_of_line_constraint ]

)

CLUSTER [schema.]cluster (column [, column ] )

The CLUSTER clause specifies that the table must be placed in a cluster

Note

A table that is placed in a cluster cannot have any physical attributes of its

own because it is not a segment by itself and is a part of the cluster

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