Release 2 (9.2)
Part Number A96521-01
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| Oracle9i Database Administrator's Guide Release 2 (9.2) Part Number A96521-01 |
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This chapter describes schema object management issues that are common across multiple types of schema objects. The following topics are presented:
| See Also:
Oracle9i SQL Reference for more information about syntax, authorizations, and restrictions for the SQL statements discussed in this chapter |
You can create several tables and views and grant privileges in one operation using the CREATE SCHEMA statement. The CREATE SCHEMA statement is useful if you want to guarantee the creation of several tables, views, and grants in one operation. If an individual table, view or grant fails, the entire statement is rolled back. None of the objects are created, nor are the privileges granted.
Specifically, the CREATE SCHEMA statement can include only CREATE TABLE, CREATE VIEW, and GRANT statements. You must have the privileges necessary to issue the included statements. You are not actually creating a schema, that is done when the user is created with a CREATE USER statement. Rather, you are populating the schema.
The following statement creates two tables and a view that joins data from the two tables:
CREATE SCHEMA AUTHORIZATION scott CREATE TABLE dept ( deptno NUMBER(3,0) PRIMARY KEY, dname VARCHAR2(15), loc VARCHAR2(25) CREATE TABLE emp ( empno NUMBER(5,0) PRIMARY KEY, ename VARCHAR2(15) NOT NULL, job VARCHAR2(10), mgr NUMBER(5,0), hiredate DATE DEFAULT (sysdate), sal NUMBER(7,2), comm NUMBER(7,2), deptno NUMBER(3,0) NOT NULL CONSTRAINT dept_fkey REFERENCES dept) CREATE VIEW sales_staff AS SELECT empno, ename, sal, comm FROM emp WHERE deptno = 30 WITH CHECK OPTION CONSTRAINT sales_staff_cnst GRANT SELECT ON sales_staff TO human_resources;
The CREATE SCHEMA statement does not support Oracle extensions to the ANSI CREATE TABLE and CREATE VIEW statements, including the STORAGE clause.
To rename an object, it must be in your schema. You can rename schema objects in either of the following ways:
If you drop and re-create an object, all privileges granted for that object are lost. Privileges must be regranted when the object is re-created.
Alternatively, a table, view, sequence, or a private synonym of a table, view, or sequence can be renamed using the RENAME statement. When using the RENAME statement, integrity constraints, indexes, and grants made for the object are carried forward for the new name. For example, the following statement renames the sales_staff view:
RENAME sales_staff TO dept_30;
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Note: You cannot use |
Before renaming a schema object, consider the following effects:
| See Also:
"Managing Object Dependencies" for more information about how Oracle manages object dependencies |
You analyze a schema object (table, index, or cluster) to:
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Note: Oracle Corporation strongly recommends that you use the However, you must use the |
The following topics are discussed in this section:
| See Also:
"Analyzing Index-Organized Tables" for information specific to analyzing index-organized tables |
You can use the DBMS_STATS package or the ANALYZE statement to gather statistics about the physical storage characteristics of a table, index, or cluster. These statistics are stored in the data dictionary and can be used by the optimizer to choose the most efficient execution plan for SQL statements accessing analyzed objects.
Oracle recommends using the more versatile DBMS_STATS package for gathering optimizer statistics, but you must use the ANALYZE statement to collect non-optimizer statistics, such as empty blocks, average space, and so forth.
The DBMS_STATS package allows both the gathering of statistics, including utilizing parallel execution, and the external manipulation of statistics. Statistics can be stored in tables outside of the data dictionary, where they can be manipulated without affecting the optimizer. Statistics can be copied between databases or backup copies can be made.
The following DBMS_STATS procedures enable the gathering of optimizer statistics:
GATHER_INDEX_STATSGATHER_TABLE_STATSGATHER_SCHEMA_STATSGATHER_DATABASE_STATS
See Also:
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The following statement computes statistics for the emp table:
ANALYZE TABLE emp COMPUTE STATISTICS;
The following query estimates statistics on the emp table, using the default statistical sample of 1064 rows:
ANALYZE TABLE emp ESTIMATE STATISTICS;
To specify the statistical sample that Oracle should use, include the SAMPLE option with the ESTIMATE STATISTICS option. You can specify an integer that indicates either a number of rows or index values, or a percentage of the rows or index values in the table. The following statements show examples of each option:
ANALYZE TABLE emp ESTIMATE STATISTICS SAMPLE 2000 ROWS; ANALYZE TABLE emp ESTIMATE STATISTICS SAMPLE 33 PERCENT;
In either case, if you specify a percentage greater than 50, or a number of rows or index values that is greater than 50% of those in the object, Oracle computes the exact statistics, rather than estimating.
If the data dictionary currently contains statistics for the specified object when an ANALYZE statement is issued, the new statistics replace the old statistics in the data dictionary.
There are some PL/SQL packaged procedures that effectively allow you to execute an ANALYZE statement. These are:
It is recommended that these packaged procedures only be used for collecting non-optimizer statistics.
See Also:
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To verify the integrity of the structure of a table, index, cluster, or materialized view, use the ANALYZE statement with the VALIDATE STRUCTURE option. If the structure is valid, no error is returned. However, if the structure is corrupt, you receive an error message.
For example, in rare cases such as hardware or other system failures, an index can become corrupted and not perform correctly. When validating the index, you can confirm that every entry in the index points to the correct row of the associated table. If the index is corrupt, you can drop and re-create it.
If a table, index, or cluster is corrupt, you should drop it and re-create it. If a materialized view is corrupt, perform a complete refresh and ensure that you have remedied the problem. If the problem is not corrected, drop and re-create the materialized view.
The following statement analyzes the emp table:
ANALYZE TABLE emp VALIDATE STRUCTURE;
You can validate an object and all related objects (for example, indexes) by including the CASCADE option. The following statement validates the emp table and all associated indexes:
ANALYZE TABLE emp VALIDATE STRUCTURE CASCADE;
You can specify that you want to perform structure validation online while DML is occurring against the object being validated. There can be a slight performance impact when validating with ongoing DML affecting the object, but this is offset by the flexibility of being able to perform ANALYZE online. The following statement validates the emp table and all associated indexes online:
ANALYZE TABLE emp VALIDATE STRUCTURE CASCADE ONLINE;
You can look at the chained and migrated rows of a table or cluster using the ANALYZE statement with the LIST CHAINED ROWS clause. The results of this statement are stored in a specified table created explicitly to accept the information returned by the LIST CHAINED ROWS clause. These results are useful in determining whether you have enough room for updates to rows. For example, this information can show whether PCTFREE is set appropriately for the table or cluster.
To create the table to accept data returned by an ANALYZE ... LIST CHAINED ROWS statement, execute the UTLCHAIN.SQL or UTLCHN1.SQL script. These scripts are provided by Oracle. They create a table named CHAINED_ROWS in the schema of the user submitting the script.
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Note: Your choice of script to execute for creating the |
After a CHAINED_ROWS table is created, you specify it in the INTO clause of the ANALYZE statement. For example, the following statement inserts rows containing information about the chained rows in the emp_dept cluster into the CHAINED_ROWS table:
ANALYZE CLUSTER emp_dept LIST CHAINED ROWS INTO CHAINED_ROWS;
| See Also:
Oracle9i Database Reference for a description of the |
You can use the information in the CHAINED_ROWS table to reduce or eliminate migrated and chained rows in an existing table. Use the following procedure.
ANALYZE statement to collect information about migrated and chained rows.
ANALYZE TABLE order_hist LIST CHAINED ROWS;
SELECT * FROM CHAINED_ROWS WHERE TABLE_NAME = 'ORDER_HIST'; OWNER_NAME TABLE_NAME CLUST... HEAD_ROWID TIMESTAMP ---------- ---------- -----... ------------------ --------- SCOTT ORDER_HIST ... AAAAluAAHAAAAA1AAA 04-MAR-96 SCOTT ORDER_HIST ... AAAAluAAHAAAAA1AAB 04-MAR-96 SCOTT ORDER_HIST ... AAAAluAAHAAAAA1AAC 04-MAR-96
The output lists all rows that are either migrated or chained.
CREATE TABLE int_order_hist AS SELECT * FROM order_hist WHERE ROWID IN (SELECT HEAD_ROWID FROM CHAINED_ROWS WHERE TABLE_NAME = 'ORDER_HIST');
DELETE FROM order_hist WHERE ROWID IN (SELECT HEAD_ROWID FROM CHAINED_ROWS WHERE TABLE_NAME = 'ORDER_HIST');
INSERT INTO order_hist SELECT * FROM int_order_hist;
DROP TABLE int_order_history;
DELETE FROM CHAINED_ROWS WHERE TABLE_NAME = 'ORDER_HIST';
ANALYZE statement again, and query the output table.LONG column or long CHAR or VARCHAR2 columns.You can delete all rows of a table or all rows in a group of clustered tables so that the table (or cluster) still exists, but is completely empty. For example, consider a table that contains monthly data, and at the end of each month, you need to empty it (delete all rows) after archiving its data.
To delete all rows from a table, you have the following options:
Each of these options are discussed in the following sections
You can delete the rows of a table using the DELETE statement. For example, the following statement deletes all rows from the emp table:
DELETE FROM emp;
If there are many rows present in a table or cluster when using the DELETE statement, significant system resources are consumed as the rows are deleted. For example, CPU time, redo log space, and rollback segment space from the table and any associated indexes require resources. Also, as each row is deleted, triggers can be fired. The space previously allocated to the resulting empty table or cluster remains associated with that object. With DELETE you can choose which rows to delete, whereas TRUNCATE and DROP affect the entire object.
You can drop a table and then re-create the table. For example, the following statements drop and then re-create the emp table:
DROP TABLE emp; CREATE TABLE emp ( ... );
When dropping and re-creating a table or cluster, all associated indexes, integrity constraints, and triggers are also dropped, and all objects that depend on the dropped table or clustered table are invalidated. Also, all grants for the dropped table or clustered table are dropped.
You can delete all rows of the table using the TRUNCATE statement. For example, the following statement truncates the emp table:
TRUNCATE TABLE emp;
Using the TRUNCATE statement provides a fast, efficient method for deleting all rows from a table or cluster. A TRUNCATE statement does not generate any rollback information and it commits immediately. It is a DDL statement and cannot be rolled back. A TRUNCATE statement does not affect any structures associated with the table being truncated (constraints and triggers) or authorizations. A TRUNCATE statement also specifies whether space currently allocated for the table is returned to the containing tablespace after truncation.
You can truncate any table or cluster in your own schema. Any user who has the DROP ANY TABLE system privilege can truncate a table or cluster in any schema.
Before truncating a table or clustered table containing a parent key, all referencing foreign keys in different tables must be disabled. A self-referential constraint does not have to be disabled.
As a TRUNCATE statement deletes rows from a table, triggers associated with the table are not fired. Also, a TRUNCATE statement does not generate any audit information corresponding to DELETE statements if auditing is enabled. Instead, a single audit record is generated for the TRUNCATE statement being issued. See Chapter 26, "Auditing Database Use" for information about auditing.
A hash cluster cannot be truncated, nor can tables within a hash or index cluster be individually truncated. Truncation of an index cluster deletes all rows from all tables in the cluster. If all the rows must be deleted from an individual clustered table, use the DELETE statement or drop and re-create the table.
The REUSE STORAGE or DROP STORAGE options of the TRUNCATE statement control whether space currently allocated for a table or cluster is returned to the containing tablespace after truncation. The default option, DROP STORAGE, reduces the number of extents allocated to the resulting table to the original setting for MINEXTENTS. Freed extents are then returned to the system and can be used by other objects.
Alternatively, the REUSE STORAGE option specifies that all space currently allocated for the table or cluster remains allocated to it. For example, the following statement truncates the emp_dept cluster, leaving all extents previously allocated for the cluster available for subsequent inserts and deletes:
TRUNCATE CLUSTER emp_dept REUSE STORAGE;
The REUSE or DROP STORAGE option also applies to any associated indexes. When a table or cluster is truncated, all associated indexes are also truncated. The storage parameters for a truncated table, cluster, or associated indexes are not changed as a result of the truncation.
Database triggers are procedures that are stored in the database and activated ("fired") when specific conditions occur, such as adding a row to a table. You can use triggers to supplement the standard capabilities of Oracle to provide a highly customized database management system. For example, you can create a trigger to restrict DML operations against a table, allowing only statements issued during regular business hours.
Database triggers can be associated with a table, schema, or database. They are implicitly fired when:
INSERT, UPDATE, DELETE) against an associated tableALTER, CREATE, DROP) on objects within a database or schemaSTARTUP, SHUTDOWN, SERVERERROR)This is not a complete list. See the Oracle9i SQL Reference for a full list of statements and database events that cause triggers to fire
Create triggers with the CREATE TRIGGER statement. They can be defined as firing BEFORE or AFTER the triggering event, or INSTEAD OF it. The following statement creates a trigger scott.emp_permit_changes on table scott.emp. The trigger fires before any of the specified statements are executed.
CREATE TRIGGER scott.emp_permit_changes BEFORE DELETE OR INSERT OR UPDATE ON scott.emp . pl/sql block .
You can later remove a trigger from the database by issuing the DROP TRIGGER statement.
A trigger can be in either of two distinct modes:
An enabled trigger executes its trigger body if a triggering statement is issued and the trigger restriction, if any, evaluates to true. By default, triggers are enabled when first created.
A disabled trigger does not execute its trigger body, even if a triggering statement is issued and the trigger restriction (if any) evaluates to true.
To enable or disable triggers using the ALTER TABLE statement, you must own the table, have the ALTER object privilege for the table, or have the ALTER ANY TABLE system privilege. To enable or disable an individual trigger using the ALTER TRIGGER statement, you must own the trigger or have the ALTER ANY TRIGGER system privilege.
See Also:
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You enable a disabled trigger using the ALTER TRIGGER statement with the ENABLE option. To enable the disabled trigger named reorder on the inventory table, enter the following statement:
ALTER TRIGGER reorder ENABLE;
To enable all triggers defined for a specific table, use the ALTER TABLE statement with the ENABLE ALL TRIGGERS option. To enable all triggers defined for the INVENTORY table, enter the following statement:
ALTER TABLE inventory ENABLE ALL TRIGGERS;
Consider temporarily disabling a trigger if one of the following conditions is true:
You disable a trigger using the ALTER TRIGGER statement with the DISABLE option. To disable the trigger reorder on the inventory table, enter the following statement:
ALTER TRIGGER reorder DISABLE;
You can disable all triggers associated with a table at the same time using the ALTER TABLE statement with the DISABLE ALL TRIGGERS option. For example, to disable all triggers defined for the inventory table, enter the following statement:
ALTER TABLE inventory DISABLE ALL TRIGGERS;
Integrity constraints are rules that restrict the values for one or more columns in a table. Constraint clauses can appear in either CREATE TABLE or ALTER TABLE statements, and identify the column or columns affected by the constraint and identify the conditions of the constraint.
This section discusses the concepts of constraints and identifies the SQL statements used to define and manage integrity constraints. The following topics are contained in this section:
See Also:
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You can specify that a constraint is enabled (ENABLE) or disabled (DISABLE). If a constraint is enabled, data is checked as it is entered or updated in the database, and data that does not conform to the constraint's rule is prevented from being entered. If a constraint is disabled, then data that does not conform can be allowed to enter the database.
Additionally, you can specify that existing data in the table must conform to the constraint (VALIDATE). Conversely, if you specify NOVALIDATE, you are not ensured that existing data conforms.
An integrity constraint defined on a table can be in one of the following states:
For details about the meaning of these states and an understanding of their consequences, see the Oracle9i SQL Reference. Some of these consequences are discussed here.
To enforce the rules defined by integrity constraints, the constraints should always be enabled. However, consider temporarily disabling the integrity constraints of a table for the following performance reasons:
In all three cases, temporarily disabling integrity constraints can improve the performance of the operation, especially in data warehouse configurations.
It is possible to enter data that violates a constraint while that constraint is disabled. Thus, you should always enable the constraint after completing any of the operations listed in the bullets above.
While a constraint is enabled, no row violating the constraint can be inserted into the table. However, while the constraint is disabled such a row can be inserted. This row is known as an exception to the constraint. If the constraint is in the enable novalidated state, violations resulting from data entered while the constraint was disabled remain. The rows that violate the constraint must be either updated or deleted in order for the constraint to be put in the validated state.
You can identify exceptions to a specific integrity constraint while attempting to enable the constraint. See "Reporting Constraint Exceptions". All rows violating constraints are noted in an EXCEPTIONS table, which you can examine.
When a constraint is in the enable novalidate state, all subsequent statements are checked for conformity to the constraint. However, any existing data in the table is not checked. A table with enable novalidated constraints can contain invalid data, but it is not possible to add new invalid data to it. Enabling constraints in the novalidated state is most useful in data warehouse configurations that are uploading valid OLTP data.
Enabling a constraint does not require validation. Enabling a constraint novalidate is much faster than enabling and validating a constraint. Also, validating a constraint that is already enabled does not require any DML locks during validation (unlike validating a previously disabled constraint). Enforcement guarantees that no violations are introduced during the validation. Hence, enabling without validating enables you to reduce the downtime typically associated with enabling a constraint.
Using integrity constraint states in the following order can ensure the best benefits:
Some benefits of using constraints in this order are:
When an integrity constraint is defined in a CREATE TABLE or ALTER TABLE statement, it can be enabled, disabled, or validated or not validated as determined by your specification of the ENABLE/DISABLE clause. If the ENABLE/DISABLE clause is not specified in a constraint's definition, Oracle automatically enables and validates the constraint.
The following CREATE TABLE and ALTER TABLE statements both define and disable integrity constraints:
CREATE TABLE emp ( empno NUMBER(5) PRIMARY KEY DISABLE, . . . ; ALTER TABLE emp ADD PRIMARY KEY (empno) DISABLE;
An ALTER TABLE statement that defines and disables an integrity constraint never fails because of rows in the table that violate the integrity constraint. The definition of the constraint is allowed because its rule is not enforced.
The following CREATE TABLE and ALTER TABLE statements both define and enable integrity constraints:
CREATE TABLE emp ( empno NUMBER(5) CONSTRAINT emp.pk PRIMARY KEY, . . . ; ALTER TABLE emp ADD CONSTRAINT emp.pk PRIMARY KEY (empno);
An ALTER TABLE statement that defines and attempts to enable an integrity constraint can fail because rows of the table violate the integrity constraint. If this case, the statement is rolled back and the constraint definition is not stored and not enabled.
When you enable a UNIQUE or PRIMARY KEY constraint an associated index is created.
You can use the ALTER TABLE statement to enable, disable, modify, or drop a constraint. When Oracle is using a UNIQUE or PRIMARY KEY index to enforce a constraint, and constraints associated with that index are dropped or disabled, the index is dropped, unless you specify otherwise.
While enabled foreign keys reference a PRIMARY or UNIQUE key, you cannot disable or drop the PRIMARY or UNIQUE key constraint or the index.
The following statements disable integrity constraints. The second statement specifies that the associated indexes are to be kept.
ALTER TABLE dept DISABLE CONSTRAINT dname_ukey; ALTER TABLE dept DISABLE PRIMARY KEY KEEP INDEX, DISABLE UNIQUE (dname, loc) KEEP INDEX;
The following statements enable novalidate disabled integrity constraints:
ALTER TABLE dept ENABLE NOVALIDATE CONSTRAINT dname_ukey; ALTER TABLE dept ENABLE NOVALIDATE PRIMARY KEY, ENABLE NOVALIDATE UNIQUE (dname, loc);
The following statements enable or validate disabled integrity constraints:
ALTER TABLE dept MODIFY CONSTRAINT dname_key VALIDATE; ALTER TABLE dept MODIFY PRIMARY KEY ENABLE NOVALIDATE;
The following statements enable disabled integrity constraints:
ALTER TABLE dept ENABLE CONSTRAINT dname_ukey; ALTER TABLE dept ENABLE PRIMARY KEY, ENABLE UNIQUE (dname, loc);
To disable or drop a UNIQUE key or PRIMARY KEY constraint and all dependent FOREIGN KEY constraints in a single step, use the CASCADE option of the DISABLE or DROP clauses. For example, the following statement disables a PRIMARY KEY constraint and any FOREIGN KEY constraints that depend on it:
ALTER TABLE dept DISABLE PRIMARY KEY CASCADE;
The ALTER TABLE ... RENAME CONSTRAINT statement enables you to rename any currently existing constraint for a table. The new constraint name must not conflict with any existing constraint names for a user.
The following statement renames the dname_ukey constraint for table dept:
ALTER TABLE dept RENAME CONSTRAINT dname_ukey TO dname_unikey;
When you rename a constraint, all dependencies on the base table remain valid.
The RENAME CONSTRAINT clause provides a means of renaming system generated constraint names.
You can drop an integrity constraint if the rule that it enforces is no longer true, or if the constraint is no longer needed. You can drop the constraint using the ALTER TABLE statement with one of the following clauses:
The following two statements drop integrity constraints. The second statement keeps the index associated with the PRIMARY KEY constraint:
ALTER TABLE dept DROP UNIQUE (dname, loc); ALTER TABLE emp DROP PRIMARY KEY KEEP INDEX, DROP CONSTRAINT dept_fkey;
If FOREIGN KEYs reference a UNIQUE or PRIMARY KEY, you must include the CASCADE CONSTRAINTS clause in the DROP statement, or you cannot drop the constraint.
When Oracle checks a constraint, it signals an error if the constraint is not satisfied. You can defer checking the validity of constraints until the end of a transaction.
When you issue the SET CONSTRAINTS statement, the SET CONSTRAINTS mode lasts for the duration of the transaction, or until another SET CONSTRAINTS statement resets the mode.
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Notes:
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Within the application being used to manipulate the data, you must set all constraints deferred before you actually begin processing any data. Use the following DML statement to set all deferrable constraints deferred:
SET CONSTRAINTS ALL DEFERRED;
You can check for constraint violations before committing by issuing the SET CONSTRAINTS ALL IMMEDIATE statement just before issuing the COMMIT. If there are any problems with a constraint, this statement fails and the constraint causing the error is identified. If you commit while constraints are violated, the transaction is rolled back and you receive an error message.
If exceptions exist when a constraint is validated, an error is returned and the integrity constraint remains novalidated. When a statement is not successfully executed because integrity constraint exceptions exist, the statement is rolled back. If exceptions exist, you cannot validate the constraint until all exceptions to the constraint are either updated or deleted.
You cannot use the CREATE TABLE statement to determine which rows are in violation. To determine which rows violate the integrity constraint, issue the ALTER TABLE statement with the EXCEPTIONS option in the ENABLE clause. The EXCEPTIONS option places the ROWID, table owner, table name, and constraint name of all exception rows into a specified table.
You must create an appropriate exceptions report table to accept information from the EXCEPTIONS option of the ENABLE clause before enabling the constraint. You can create an exception table by executing the UTLEXCPT.SQL script or the UTLEXPT1.SQL script.
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Note: Your choice of script to execute for creating the |
Both of these scripts create a table named EXCEPTIONS. You can create additional exceptions tables with different names by modifying and resubmitting the script.
The following statement attempts to validate the PRIMARY KEY of the dept table, and if exceptions exist, information is inserted into a table named EXCEPTIONS:
ALTER TABLE dept ENABLE PRIMARY KEY EXCEPTIONS INTO EXCEPTIONS;
If duplicate primary key values exist in the dept table and the name of the PRIMARY KEY constraint on dept is sys_c00610, the following rows might be placed in the table EXCEPTIONS by the previous statement:
SELECT * FROM EXCEPTIONS; ROWID OWNER TABLE_NAME CONSTRAINT ------------------ --------- -------------- ----------- AAAAZ9AABAAABvqAAB SCOTT DEPT SYS_C00610 AAAAZ9AABAAABvqAAG SCOTT DEPT SYS_C00610
A more informative query would be to join the rows in an exception report table and the master table to list the actual rows that violate a specific constraint, as shown in the following example:
SELECT deptno, dname, loc FROM dept, EXCEPTIONS WHERE EXCEPTIONS.constraint = 'SYS_C00610' AND dept.rowid = EXCEPTIONS.row_id; DEPTNO DNAME LOC ---------- -------------- ----------- 10 ACCOUNTING NEW YORK 10 RESEARCH DALLAS
All rows that violate a constraint must be either updated or deleted from the table containing the constraint. When updating exceptions, you must change the value violating the constraint to a value consistent with the constraint or to a null. After the row in the master table is updated or deleted, the corresponding rows for the exception in the exception report table should be deleted to avoid confusion with later exception reports. The statements that update the master table and the exception report table should be in the same transaction to ensure transaction consistency.
To correct the exceptions in the previous examples, you might issue the following transaction:
UPDATE dept SET deptno = 20 WHERE dname = 'RESEARCH'; DELETE FROM EXCEPTIONS WHERE constraint = 'SYS_C00610'; COMMIT;
When managing exceptions, the goal is to eliminate all exceptions in your exception report table.
| See Also:
Oracle9i Database Reference for a description of the |
Oracle provides the following views that enable you to see constraint definitions on tables and to identify columns that are specified in constraints:
| See Also:
Oracle9i Database Reference contains descriptions of the columns in these views |
This section describes the various object dependencies, and contains the following topics:
First, review Table 21-1, which shows how objects are affected by changes to other objects on which they depend.
Oracle automatically recompiles an invalid view or PL/SQL program unit the next time it is used. In addition, a user can force Oracle to recompile a view or program unit using the appropriate SQL statement with the COMPILE clause. Forced compilations are most often used to test for errors when a dependent view or program unit is invalid, but is not currently being used. In these cases, automatic recompilation would not otherwise occur until the view or program unit was executed. To identify invalid dependent objects, query the views USER/ALL/DBA_OBJECTS.
To recompile a view manually, you must have the ALTER ANY TABLE system privilege or the view must be contained in your schema. Use the ALTER VIEW statement with the COMPILE clause to recompile a view. The following statement recompiles the view emp_dept contained in your schema:
ALTER VIEW emp_dept COMPILE;
To recompile a standalone procedure manually, you must have the ALTER ANY PROCEDURE system privilege or the procedure must be contained in your schema. Use the ALTER PROCEDURE/FUNCTION statement with the COMPILE clause to recompile a standalone procedure or function. The following statement recompiles the stored procedure update_salary contained in your schema:
ALTER PROCEDURE update_salary COMPILE;
To recompile a package manually, you must have the ALTER ANY PROCEDURE system privilege or the package must be contained in your schema. Use the ALTER PACKAGE statement with the COMPILE clause to recompile either a package body or both a package specification and body. The following statement recompiles just the body of the package acct_mgmt:
ALTER PACKAGE acct_mgmt COMPILE BODY;
The next example compiles both the body and specification of the package acct_mgmt:
ALTER PACKAGE acct_mgmt COMPILE PACKAGE;
Object names referenced in SQL statements can consist of several pieces, separated by periods. The following describes how Oracle resolves an object name.
scott.emp, scott is the first piece. If there is only one piece, the one piece is considered the first piece.
If no schema is found in Step c, the object cannot be qualified and Oracle returns an error.
scott.emp.deptno is the name, scott is qualified as a schema, emp is qualified as a table, and deptno must correspond to a column (because emp is a table). If emp is qualified as a package, deptno must correspond to a public constant, variable, procedure, or function of that package.When global object names are used in a distributed database, either explicitly or indirectly within a synonym, the local Oracle resolves the reference locally. For example, it resolves a synonym to a remote table's global object name. The partially resolved statement is shipped to the remote database, and the remote Oracle completes the resolution of the object as described here.
Because of how Oracle resolves references, it is possible for an object to depend on the nonexistence of other objects. This situation occurs when the dependent object uses a reference that would be interpreted differently were another object present. For example, assume the following:
company schema contains a table named emp.PUBLIC synonym named emp is created for company.emp and the SELECT privilege for company.emp is granted to the PUBLIC role.jward schema does not contain a table or private synonym named emp.jward creates a view in his schema with the following statement:
CREATE VIEW dept_salaries AS SELECT deptno, MIN(sal), AVG(sal), MAX(sal) FROM emp GROUP BY deptno ORDER BY deptno;
When jward creates the dept_salaries view, the reference to emp is resolved by first looking for jward.emp as a table, view, or private synonym, none of which is found, and then as a public synonym named emp, which is found. As a result, Oracle notes that jward.dept_salaries depends on the nonexistence of jward.emp and on the existence of public.emp.
Now assume that jward decides to create a new view named emp in his schema using the following statement:
CREATE VIEW emp AS SELECT empno, ename, mgr, deptno FROM company.emp;
Notice that jward.emp does not have the same structure as company.emp.
As it attempts to resolve references in object definitions, Oracle internally makes note of dependencies that the new dependent object has on "nonexistent" objects--schema objects that, if they existed, would change the interpretation of the object's definition. Such dependencies must be noted in case a nonexistent object is later created. If a nonexistent object is created, all dependent objects must be invalidated so that dependent objects can be recompiled and verified and all dependent function-based indexes must be marked unusable.
Therefore, in the previous example, as jward.emp is created, jward.dept_salaries is invalidated because it depends on jward.emp. Then when jward.dept_salaries is used, Oracle attempts to recompile the view. As Oracle resolves the reference to emp, it finds jward.emp (public.emp is no longer the referenced object). Because jward.emp does not have a sal column, Oracle finds errors when replacing the view, leaving it invalid.
In summary, you must manage dependencies on nonexistent objects checked during object resolution in case the nonexistent object is later created.
If your database is very large or contains an unusually large number of objects, columns in tables, constraint definitions, users, or other definitions, the tables that make up the data dictionary might at some point be unable to acquire additional extents. For example, a data dictionary table could require an additional extent, but there is not enough contiguous space in the SYSTEM tablespace. If this happens, you cannot create new objects, even though the tablespace intended to hold the objects has sufficient space. To remedy this situation, you can change the storage parameters of the underlying data dictionary tables, just as you can change the storage settings for user-created segments. This allows the data dictionary tables to be allocated more extents. For example, you can adjust the values of NEXT or PCTINCREASE for a data dictionary table.
This section describes aspects of changing data dictionary storage parameters, and contains the following topics:
The following tables and clusters contain the definitions of all the user-created objects in the database:
Of all of the data dictionary segments, the following are the most likely to require change:
For the clustered tables, you must change the storage settings for the cluster, not for the table.
Oracle returns an error if a user tries to create a new object that requires Oracle to allocate an additional extent to the data dictionary when it is unable to allocate an extent. The following error message indicates this kind of problem.
ORA-1653 unable to extend tablenamebynumin tablespacename
If you receive this error message and the segment you were trying to change (such as a table or rollback segment) has not reached the limits specified for it in its definition, check the storage settings for the object that contains its definition.
For example, if you received an ORA-1653 while trying to define a new PRIMARY KEY constraint on a table and there is sufficient space for the index that Oracle must create for the key, check if CON$ or C_COBJ# cannot be allocated another extent. To do this, query DBA_SEGMENTS. If another extent cannot be allocated, consider changing the storage parameters for CON$ or C_COBJ#. See "Example 7: Displaying Segments that Cannot Allocate Additional Extents".
Oracle provides data dictionary views and PL/SQL packages that allow you to display information about schema objects. Views and packages that are unique to a particular schema object are described in the chapter of this book associated with that object. This section describes views and packages that are generic in nature and apply to multiple schema objects.
These Oracle supplied PL/SQL packages provide information about schema objects:
The following sections contain examples of using some of these packages.
| See Also:
Oracle9i Supplied PL/SQL Packages and Types Reference for a description of PL/SQL packages |
The DBMS_METADATA package is a powerful tool for obtaining the complete definition of a schema object. It enables you to obtain all of the attributes of an object in one pass. The object is described as DDL that can be used to (re)create it.
In this example the GET_DDL function is used to fetch the DDL for all tables in the current schema, filtering out nested tables and overflow segments. The SET_TRANSFORM_PARAM (with the handle value equal to DBMS_METADATA.SESSION_TRANSFORM meaning "for the current session") is used to specify that storage clauses are not to be returned in the SQL DDL. Afterwards, the session-level transform parameters are reset to their defaults. Once set, transform parameter values remain in effect until specifically reset to their defaults.
EXECUTE DBMS_METADATA.SET_TRANSFORM_PARAM( DBMS_METADATA.SESSION_TRANSFORM,'STORAGE',false); SELECT DBMS_METADATA.GET_DDL('TABLE',u.table_name) FROM USER_ALL_TABLES u WHERE u.nested='NO' AND (u.iot_type is null or u.iot_type='IOT'); EXECUTE DBMS_METADATA.SET_TRANSFORM_PARAM( DBMS_METADATA.SESSION_TRANSFORM,'DEFAULT');
| See Also:
Oracle9i XML Developer's Kits Guide - XDK for detailed information and further examples relating to the use of the |
The following SQL*Plus example uses the DBMS_SPACE package to obtain unused space information.
SQL> VARIABLE total_blocks NUMBER SQL> VARIABLE total_bytes NUMBER SQL> VARIABLE unused_blocks NUMBER SQL> VARIABLE unused_bytes NUMBER SQL> VARIABLE lastextf NUMBER SQL> VARIABLE last_extb NUMBER SQL> exec DBMS_SPACE.UNUSED_SPACE('SCOTT', 'EMP', 'TABLE', :total_blocks, - > :total_bytes,:unused_blocks, :unused_bytes, :lastextf, - > :last_extb, :lastusedblock); PL/SQL procedure successfully completed. SQL> PRINT TOTAL_BLOCKS ------------ 5 TOTAL_BYTES ----------- 10240 ... LASTUSEDBLOCK ------------- 3
These views display information about schema objects:
The following sections contain examples of using some of these views.
| See Also:
Oracle9i Database Reference for a complete description of data dictionary views |
The following query lists all of the objects owned by the user issuing the query:
SELECT OBJECT_NAME, OBJECT_TYPE FROM USER_OBJECTS; OBJECT_NAME OBJECT_TYPE ------------------------- ------------------- EMP_DEPT CLUSTER EMP TABLE DEPT TABLE EMP_DEPT_INDEX INDEX PUBLIC_EMP SYNONYM EMP_MGR VIEW
Column information, such as name, datatype, length, precision, scale, and default data values can be listed using one of the views ending with the _COLUMNS suffix. For example, the following query lists all of the default column values for the emp and dept tables:
SELECT TABLE_NAME, COLUMN_NAME, DATA_DEFAULT FROM USER_TAB_COLUMNS WHERE TABLE_NAME = 'DEPT' OR TABLE_NAME = 'EMP'; TABLE_NAME COLUMN_NAME DATA_DEFAULT ---------- ------------- -------------------- DEPT DEPTNO DEPT DNAME DEPT LOC 'NEW YORK' EMP EMPNO EMP ENAME EMP JOB EMP MGR EMP HIREDATE SYSDATE EMP SAL EMP COMM EMP DEPTNO
Notice that not all columns have user-specified defaults. These columns automatically have NULL as the default.
When you create a view or a synonym, the view or synonym is based on its underlying base object. The ALL/USER/DBA_DEPENDENCIES data dictionary views can be used to reveal the dependencies for a view. The ALL/USER/DBA_SYNONYMS data dictionary views can be used to list the base object of a synonym. For example, the following query lists the base objects for the synonyms created by the user jward:
SELECT TABLE_OWNER, TABLE_NAME, SYNONYM_NAME FROM DBA_SYNONYMS WHERE OWNER = 'JWARD'; TABLE_OWNER TABLE_NAME SYNONYM_NAME ---------------------- ----------- ----------------- SCOTT DEPT DEPT SCOTT EMP EMP
The following query returns the name of each rollback segment, the tablespace that contains each, and the size of each rollback segment:
SELECT SEGMENT_NAME, TABLESPACE_NAME, BYTES, BLOCKS, EXTENTS FROM DBA_SEGMENTS WHERE SEGMENT_TYPE = 'ROLLBACK'; SEGMENT_NAME TABLESPACE_NAME BYTES BLOCKS EXTENTS ------------ --------------- --------- ------- --------- RS1 SYSTEM 20480 10 2 RS2 TS1 40960 20 3 SYSTEM SYSTEM 184320 90 3
General information about the currently allocated extents in a database is stored in the DBA_EXTENTS data dictionary view. For example, the following query identifies the extents associated with rollback segments and the size of each of those extents:
SELECT SEGMENT_NAME, BYTES, BLOCKS FROM DBA_EXTENTS WHERE SEGMENT_TYPE = 'ROLLBACK'; SEGMENT_NAME BYTES BLOCKS --------------- --------- -------- RS1 10240 5 RS1 10240 5 SYSTEM 51200 25 SYSTEM 51200 25 SYSTEM 51200 25
Notice that the rs1 rollback segment is comprised of two extents, both 10K, while the SYSTEM rollback segment is comprised of three equally sized extents of 50K.
Information about the free extents (extents not allocated to any segment) in a database is stored in the DBA_FREE_SPACE data dictionary view. For example, the following query reveals the amount of free space available as free extents in each tablespace:
SELECT TABLESPACE_NAME, FILE_ID, BYTES, BLOCKS FROM DBA_FREE_SPACE; TABLESPACE_NAME FILE_ID BYTES BLOCKS ------------------- --------- -------- ---------- SYSTEM 1 8120320 3965 SYSTEM 1 10240 5 TS1 2 10432512 5094
You can also use DBA_FREE_SPACE, in combination with the views DBA_SEGMENTS, DBA_TABLES, DBA_CLUSTERS, DBA_INDEXES, and DBA_ROLLBACK_SEGS, to determine if any other segment is unable to allocate additional extents for data dictionary objects only.
A segment may not be allocated to an extent for any of the following reasons:
SEG.MAX_EXTENTS).The following query returns the names, owners, and tablespaces of all segments that satisfy any of the above criteria:
SELECT a.SEGMENT_NAME, a.SEGMENT_TYPE, a.TABLESPACE_NAME, a.OWNER FROM DBA_SEGMENTS a WHERE a.NEXT_EXTENT >= (SELECT MAX(b.BYTES) FROM DBA_FREE_SPACE b WHERE b.TABLESPACE_NAME = a.TABLESPACE_NAME) OR a.EXTENTS = a.MAX_EXTENTS OR a.EXTENTS = 'data_block_size' ;
Once you have identified a segment that cannot allocate additional extents, you can solve the problem in either of two ways, depending on its cause:
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