14 Managing Tables

Design Tables Before Creating Them

Usually, the application developer is responsible for designing the elements of an application, including the tables. Database administrators are responsible for establishing the attributes of the underlying tablespace that will hold the application tables. Either the DBA or the applications developer, or both working jointly, can be responsible for the actual creation of the tables, depending upon the practices for a site.

Working with the application developer, consider the following guidelines when designing tables:

• Use descriptive names for tables, columns, indexes, and clusters.

• Be consistent in abbreviations and in the use of singular and plural forms of table names and columns.

• Document the meaning of each table and its columns with the COMMENT command.

• Normalize each table.

• Select the appropriate datatype for each column.

• Define columns that allow nulls last, to conserve storage space.

• Cluster tables whenever appropriate, to conserve storage space and optimize performance of SQL statements.

Before creating a table, you should also determine whether to use integrity constraints. Integrity constraints can be defined on the columns of a table to enforce the business rules of your database automatically.

Consider Your Options for the Type of Table to Create

What types of tables can you create? Here are some choices:

Specify How Data Block Space Is to Be Used

By specifying the PCTFREE and PCTUSED parameters during the creation of each table, you can affect the efficiency of space utilization and amount of space reserved for updates to the current data in the data blocks of a table data segment. The PCTFREE and PCTUSED parameters are discussed in "Managing Space in Data Blocks".

Specify the Location of Each Table

It is advisable to specify the TABLESPACE clause in a CREATE TABLE statement to identify the tablespace that is to store the new table. Ensure that you have the appropriate privileges and quota on any tablespaces that you use. If you do not specify a tablespace in a CREATE TABLE statement, the table is created in your default tablespace.

When specifying the tablespace to contain a new table, ensure that you understand implications of your selection. By properly specifying a tablespace during the creation of each table, you can increase the performance of the database system and decrease the time needed for database administration.

The following situations illustrate how not specifying a tablespace, or specifying an inappropriate one, can affect performance:

• If users' objects are created in the SYSTEM tablespace, the performance of the database can suffer, since both data dictionary objects and user objects must contend for the same datafiles. Users' objects should not be stored in the SYSTEM tablespace. To avoid this, ensure that all users are assigned default tablespaces when they are created in the database.

• If application-associated tables are arbitrarily stored in various tablespaces, the time necessary to complete administrative operations (such as backup and recovery) for the data of that application can be increased.

Consider Parallelizing Table Creation

You can utilize parallel execution when creating tables using a subquery (AS SELECT) in the CREATE TABLE statement. Because multiple processes work together to create the table, performance of the table creation operation is improved.

Parallelizing table creation is discussed in the section "Parallelizing Table Creation".

Consider Using NOLOGGING When Creating Tables

To create a table most efficiently use the NOLOGGING clause in the CREATE TABLE ... AS SELECT statement. The NOLOGGING clause causes minimal redo information to be generated during the table creation. This has the following benefits:

• Space is saved in the redo log files.

• The time it takes to create the table is decreased.

• Performance improves for parallel creation of large tables.

The NOLOGGING clause also specifies that subsequent direct loads using SQL*Loader and direct load INSERT operations are not logged. Subsequent DML statements (UPDATE, DELETE, and conventional path insert) are unaffected by the NOLOGGING attribute of the table and generate redo.

If you cannot afford to lose the table after you have created it (for example, you will no longer have access to the data used to create the table) you should take a backup immediately after the table is created. In some situations, such as for tables that are created for temporary use, this precaution may not be necessary.

In general, the relative performance improvement of specifying NOLOGGING is greater for larger tables than for smaller tables. For small tables, NOLOGGING has little effect on the time it takes to create a table. However, for larger tables the performance improvement can be significant, especially when you are also parallelizing the table creation.

Consider Using Table Compression when Creating Tables

The Oracle Database table compression feature compresses data by eliminating duplicate values in a database block. Duplicate values in all the rows and columns in a block are stored once at the beginning of the block, in what is called a symbol table for that block. All occurrences of such values are replaced with a short reference to the symbol table.

Using table compression reduces disk use and memory use in the buffer cache, often resulting in better scale-up for read-only operations. Table compression can also speed up query execution. There is, however, a slight cost in CPU overhead.

Compression occurs when data is inserted with a bulk (direct-path) insert operation. A table can consist of compressed and uncompressed blocks transparently. Any DML operation can be applied to a table storing compressed blocks. However, conventional DML operations cause records to be stored uncompressed afterward the operations and the operations themselves are subject to a small performance overhead due to the nature of the table compression.

Consider using table compression when your data is mostly read only. Do not use table compression for tables that updated frequently.

Estimate Table Size and Plan Accordingly

Estimate the sizes of tables before creating them. Preferably, do this as part of database planning. Knowing the sizes, and uses, for database tables is an important part of database planning.

You can use the combined estimated size of tables, along with estimates for indexes, undo space, and redo log files, to determine the amount of disk space that is required to hold an intended database. From these estimates, you can make correct hardware purchases.

You can use the estimated size and growth rate of an individual table to better determine the attributes of a tablespace and its underlying datafiles that are best suited for the table. This can enable you to more easily manage the table disk space and improve I/O performance of applications that use the table.

Restrictions to Consider When Creating Tables

Here are some restrictions that may affect your table planning and usage:

• Tables containing object types cannot be imported into a pre-Oracle8 database.

• You cannot merge an exported table into a preexisting table having the same name in a different schema.

• You cannot move types and extent tables to a different schema when the original data still exists in the database.

• Oracle Database has a limit on the total number of columns that a table (or attributes that an object type) can have. See Oracle Database Reference for this limit.

  Further, when you create a table that contains user-defined type data, the database maps columns of user-defined type to relational columns for storing the user-defined type data. This causes additional relational columns to be created. This results in "hidden" relational columns that are not visible in a DESCRIBE table statement and are not returned by a SELECT * statement. Therefore, when you create an object table, or a relational table with columns of REF, varray, nested table, or object type, be aware that the total number of columns that the database actually creates for the table can be more than those you specify.

  
  

  See Also: Oracle Database Application Developer's Guide - Object-Relational Features for more information about user-defined types

  
  

Creating a Table

When you issue the following statement, you create a table named admin_emp in the hr schema and store it in the admin_tbs tablespace with an initial extent size of 50K:

CREATE TABLE        hr.admin_emp (
         empno      NUMBER(5) PRIMARY KEY,
         ename      VARCHAR2(15) NOT NULL,
         job        VARCHAR2(10),
         mgr        NUMBER(5),
         hiredate   DATE DEFAULT (sysdate),
         sal        NUMBER(7,2),
         comm       NUMBER(7,2),
         deptno     NUMBER(3) NOT NULL
                     CONSTRAINT admin_dept_fkey REFERENCES hr.departments
                     (department_id))
   TABLESPACE admin_tbs
   STORAGE ( INITIAL 50K);

In this CREATE TABLE statement, integrity constraints are defined on several columns of the table. Integrity constraints are discussed in "Managing Integrity Constraints".

Creating a Temporary Table

It is also possible to create a temporary table. The definition of a temporary table is visible to all sessions, but the data in a temporary table is visible only to the session that inserts the data into the table. Use the CREATE GLOBAL TEMPORARY TABLE statement to create a temporary table. The ON COMMIT clause indicate if the data in the table is transaction-specific (the default) or session-specific, the implications of which are as follows:

Temporary tables are useful in applications where a result set is to be buffered, perhaps because it is constructed by running multiple DML operations. For example, consider the following:

A Web-based airlines reservations application allows a customer to create several optional itineraries. Each itinerary is represented by a row in a temporary table. The application updates the rows to reflect changes in the itineraries. When the customer decides which itinerary she wants to use, the application moves the row for that itinerary to a persistent table.

During the session, the itinerary data is private. At the end of the session, the optional itineraries are dropped.

This statement creates a temporary table that is transaction specific:

CREATE GLOBAL TEMPORARY TABLE admin_work_area
        (startdate DATE,
         enddate DATE,
         class CHAR(20))
      ON COMMIT DELETE ROWS;

Indexes can be created on temporary tables. They are also temporary and the data in the index has the same session or transaction scope as the data in the underlying table.

Unlike permanent tables, temporary tables and their indexes do not automatically allocate a segment when they are created. Instead, segments are allocated when the first INSERT (or CREATE TABLE AS SELECT) is performed. This means that if a SELECT, UPDATE, or DELETE is performed before the first INSERT, the table appears to be empty.

DDL operations (except TRUNCATE) are allowed on an existing temporary table only if no session is currently bound to that temporary table.

If you rollback a transaction, the data you entered is lost, although the table definition persists.

A transaction-specific temporary table allows only one transaction at a time. If there are several autonomous transactions in a single transaction scope, each autonomous transaction can use the table only as soon as the previous one commits.

Because the data in a temporary table is, by definition, temporary, backup and recovery of temporary table data is not available in the event of a system failure. To prepare for such a failure, you should develop alternative methods for preserving temporary table data.

Parallelizing Table Creation

When you specify the AS SELECT clause to create a table and populate it with data from another table, you can utilize parallel execution. The CREATE TABLE ... AS SELECT statement contains two parts: a CREATE part (DDL) and a SELECT part (query). Oracle Database can parallelize both parts of the statement. The CREATE part is parallelized if one of the following is true:

• A PARALLEL clause is included in the CREATE TABLE ... AS SELECT statement

• An ALTER SESSION FORCE PARALLEL DDL statement is specified

The query part is parallelized if all of the following are true:

• The query includes a parallel hint specification (PARALLEL or PARALLEL_INDEX) or the CREATE part includes the PARALLEL clause or the schema objects referred to in the query have a PARALLEL declaration associated with them.

• At least one of the tables specified in the query requires either a full table scan or an index range scan spanning multiple partitions.

If you parallelize the creation of a table, that table then has a parallel declaration (the PARALLEL clause) associated with it. Any subsequent DML or queries on the table, for which parallelization is possible, will attempt to use parallel execution.

The following simple statement parallelizes the creation of a table and stores the result in a compressed format, using table compression:

CREATE TABLE hr.admin_emp_dept
     PARALLEL COMPRESS
     AS SELECT * FROM hr.employees
     WHERE department_id = 10;

In this case, the PARALLEL clause tells the database to select an optimum number of parallel execution servers when creating the table.

Advantages of Using Direct-Path INSERT

The following are performance benefits of direct-path INSERT:

• During direct-path INSERT, you can disable the logging of redo and undo entries. Conventional insert operations, in contrast, must always log such entries, because those operations reuse free space and maintain referential integrity.

• To create a new table with data from an existing table, you have the choice of creating the new table and then inserting into it, or executing a CREATE TABLE ... AS SELECT statement. By creating the table and then using direct-path INSERT operations, you update any indexes defined on the target table during the insert operation. The table resulting from a CREATE TABLE ... AS SELECT statement, in contrast, does not have any indexes defined on it; you must define them later.

• Direct-path INSERT operations ensure atomicity of the transaction, even when run in parallel mode. Atomicity cannot be guaranteed during parallel direct-path loads (using SQL*Loader).

• If errors occur during parallel direct-path loads, some indexes could be marked UNUSABLE at the end of the load. Parallel direct-path INSERT, in contrast, rolls back the statement if errors occur during index update.

• Direct-path INSERT must be used if you want to store the data in compressed form using table compression.

Enabling Direct-Path INSERT

You can implement direct-path INSERT operations by using direct-path INSERT statements, inserting data in parallel mode, or by using the Oracle SQL*Loader utility in direct-path mode. Direct-path inserts can be done in either serial or parallel mode.

To activate direct-path INSERT in serial mode, you must specify the APPEND hint in each INSERT statement, either immediately after the INSERT keyword, or immediately after the SELECT keyword in the subquery of the INSERT statement.

When you are inserting in parallel DML mode, direct-path INSERT is the default. In order to run in parallel DML mode, the following requirements must be met:

• You must have Oracle Enterprise Edition installed.

• You must enable parallel DML in your session. To do this, run the following statement:

  ALTER SESSION { ENABLE | FORCE } PARALLEL DML;
  
  

• You must specify the parallel attribute for the target table, either at create time or subsequently, or you must specify the PARALLEL hint for each insert operation.

To disable direct-path INSERT, specify the NOAPPEND hint in each INSERT statement. Doing so overrides parallel DML mode.

How Direct-Path INSERT Works

You can use direct-path INSERT on both partitioned and nonpartitioned tables.

Specifying the Logging Mode for Direct-Path INSERT

Direct-path INSERT lets you choose whether to log redo and undo information during the insert operation.

• You can specify logging mode for a table, partition, index, or LOB storage at create time (in a CREATE statement) or subsequently (in an ALTER statement).

• If you do not specify either LOGGING or NOLOGGING at these times:

  • The logging attribute of a partition defaults to the logging attribute of its table.

  • The logging attribute of a table or index defaults to the logging attribute of the tablespace in which it resides.

  • The logging attribute of LOB storage defaults to LOGGING if you specify CACHE for LOB storage. If you do not specify CACHE, then the logging attributes defaults to that of the tablespace in which the LOB values resides.

• You set the logging attribute of a tablespace in a CREATE TABLESPACE or ALTER TABLESPACE statements.

  
  

  Note: If the database or tablespace is in FORCE LOGGING mode, then direct path INSERT always logs, regardless of the logging setting.

  
  

Additional Considerations for Direct-Path INSERT

The following are some additional considerations when using direct-path INSERT.

Reasons for Using the ALTER TABLE Statement

You can use the ALTER TABLE statement to perform any of the following actions that affect a table:

• Modify physical characteristics (PCTFREE, PCTUSED, INITRANS, or storage parameters)

• Move the table to a new segment or tablespace

• Explicitly allocate an extent or deallocate unused space

• Add, drop, or rename columns, or modify an existing column definition (datatype, length, default value, and NOT NULL integrity constraint)

• Modify the logging attributes of the table

• Modify the CACHE/NOCACHE attributes

• Add, modify or drop integrity constraints associated with the table

• Enable or disable integrity constraints or triggers associated with the table

• Modify the degree of parallelism for the table

• Rename a table

• Add or modify index-organized table characteristics

• Alter the characteristics of an external table

• Add or modify LOB columns

• Add or modify object type, nested table, or varray columns

Many of these operations are discussed in succeeding sections.

Altering Physical Attributes of a Table

When altering the data block space usage parameters (PCTFREE and PCTUSED) of a table, note that new settings apply to all data blocks used by the table, including blocks already allocated and subsequently allocated for the table. However, the blocks already allocated for the table are not immediately reorganized when space usage parameters are altered, but as necessary after the change. The data block storage parameters are described in "Managing Space in Data Blocks".

When altering the transaction entry setting INITRANS of a table, note that a new setting for INITRANS applies only to data blocks subsequently allocated for the table. To better understand this transaction entry setting parameter, see "Specifying the INITRANS Parameter".

The storage parameters INITIAL and MINEXTENTS cannot be altered. All new settings for the other storage parameters (for example, NEXT, PCTINCREASE) affect only extents subsequently allocated for the table. The size of the next extent allocated is determined by the current values of NEXT and PCTINCREASE, and is not based on previous values of these parameters. Storage parameters are discussed in "Managing Storage Parameters".

Moving a Table to a New Segment or Tablespace

The ALTER TABLE ... MOVE statement enables you to relocate data of a nonpartitioned table or of a partition of a partitioned table into a new segment, and optionally into a different tablespace for which you have quota. This statement also lets you modify any of the storage attributes of the table or partition, including those which cannot be modified using ALTER TABLE. You can also use the ALTER TABLE ... MOVE statement with the COMPRESS keyword to store the new segment using table compression.

The following statement moves the hr.admin_emp table to a new segment, specifying new storage parameters:

ALTER TABLE hr.admin_emp MOVE
      STORAGE ( INITIAL 20K
                NEXT 40K
                MINEXTENTS 2
                MAXEXTENTS 20
                PCTINCREASE 0 );

Moving a table changes the rowids of the rows in the table. This causes indexes on the table to be marked UNUSABLE, and DML accessing the table using these indexes will receive an ORA-01502 error. The indexes on the table must be dropped or rebuilt. Likewise, any statistics for the table become invalid and new statistics should be collected after moving the table.

If the table includes LOB column(s), this statement can be used to move the table along with LOB data and LOB index segments (associated with this table) which the user explicitly specifies. If not specified, the default is to not move the LOB data and LOB index segments.

Manually Allocating Storage for a Table

Oracle Database dynamically allocates additional extents for the data segment of a table, as required. However, perhaps you want to allocate an additional extent for a table explicitly. For example, in an Oracle Real Application Clusters environment, an extent of a table can be allocated explicitly for a specific instance.

A new extent can be allocated for a table using the ALTER TABLE ... ALLOCATE EXTENT clause.

You can also explicitly deallocate unused space using the DEALLOCATE UNUSED clause of ALTER TABLE. This is described in "Reclaiming Unused Space".

Modifying an Existing Column Definition

Use the ALTER TABLE ... MODIFY statement to modify an existing column definition. You can modify column datatype, default value, or column constraint.

You can increase the length of an existing column, or decrease it, if all existing data satisfies the new length. You can change a column from byte semantics to CHAR semantics or vice versa. You must set the initialization parameter BLANK_TRIMMING=TRUE to decrease the length of a nonempty CHAR column.

If you are modifying a table to increase the length of a column of datatype CHAR, realize that this can be a time consuming operation and can require substantial additional storage, especially if the table contains many rows. This is because the CHAR value in each row must be blank-padded to satisfy the new column length.

Adding Table Columns

To add a column to an existing table, use the ALTER TABLE ... ADD statement.

The following statement alters the hr.admin_emp table to add a new column named bonus:

ALTER TABLE hr.admin_emp
      ADD (bonus NUMBER (7,2));

If a new column is added to a table, the column is initially NULL unless you specify the DEFAULT clause. When you specify a default value, the database updates each row in the new column with the values specified. Specifying a DEFAULT value is not supported for tables using table compression.

You can add a column with a NOT NULL constraint to a table only if the table does not contain any rows, or you specify a default value.

Renaming Table Columns

Oracle Database lets you rename existing columns in a table. Use the RENAME COLUMN clause of the ALTER TABLE statement to rename a column. The new name must not conflict with the name of any existing column in the table. No other clauses are allowed in conjunction with the RENAME COLUMN clause.

The following statement renames the comm column of the hr.admin_emp table.

ALTER TABLE hr.admin_emp
      RENAME COLUMN comm TO commission;

As noted earlier, altering a table column can invalidate dependent objects. However, when you rename a column, the database updates associated data dictionary tables to ensure that function-based indexes and check constraints remain valid.

Oracle Database also lets you rename column constraints. This is discussed in "Renaming Constraints".

Dropping Table Columns

You can drop columns that are no longer needed from a table, including an index-organized table. This provides a convenient means to free space in a database, and avoids your having to export/import data then re-create indexes and constraints.

You cannot drop all columns from a table, nor can you drop columns from a table owned by SYS. Any attempt to do so results in an error.

ALTER TABLE hr.admin_emp DROP COLUMN sal;

ALTER TABLE hr.admin_emp DROP (bonus, commission);

ALTER TABLE hr.admin_emp SET UNUSED (hiredate, mgr);

SELECT * FROM DBA_UNUSED_COL_TABS;

OWNER                       TABLE_NAME                  COUNT
--------------------------- --------------------------- -----
HR                          ADMIN_EMP                       2

ALTER TABLE hr.admin_emp DROP UNUSED COLUMNS CHECKPOINT 250;

Features of Online Table Redefinition

Online table redefinition enables you to:

• Modify the storage parameters of the table

• Move the table to a different tablespace in the same schema

• Add support for parallel queries

• Add or drop partitioning support

• Re-create the table to reduce fragmentation

• Change the organization of a normal table (heap organized) to an index-organized table and vice versa

• Add or drop a column

The DBMS_REDEFINITION Package

The mechanism for performing online redefinition is the PL/SQL package DBMS_REDEFINITION. Execute privileges on this package is granted to EXECUTE_CATALOG_ROLE. In addition to having execute privileges on this package, you must be granted the following privileges:

• CREATE ANY TABLE

• ALTER ANY TABLE

• DROP ANY TABLE

• LOCK ANY TABLE

• SELECT ANY TABLE

• CREATE ANY TRIGGER

• CREATE ANY INDEX

  
  

  See Also: PL/SQL Packages and Types Reference for a description of the DBMS_REDEFINITION package

  
  

Steps for Online Redefinition of Tables

In order to perform an online redefinition of a table the user must perform the following steps.

1. Choose one of the following two methods of redefinition:

   • The first method of redefinition is to use the primary keys or pseudo-primary keys to perform the redefinition. Pseudo-primary keys are unique keys with all component columns having NOT NULL constraints. For this method, the versions of the tables before and after redefinition should have the same primary key columns. This is the preferred and default method of redefinition.

   • The second method of redefinition is to use rowids. For this method, the table to be redefined should not be an index organized table. Also, in this method of redefinition, a hidden column named M_ROW$$ is added to the post-redefined version of the table and it is recommended that this column be marked as unused or dropped after the redefinition is completed.

2. Verify that the table can be online redefined by invoking the DBMS_REDEFINITION.CAN_REDEF_TABLE() procedure and use the OPTIONS_FLAG parameter to specify the method of redefinition to be used. If the table is not a candidate for online redefinition, then this procedure raises an error indicating why the table cannot be online redefined.

3. Create an empty interim table (in the same schema as the table to be redefined) with all of the desired attributes. If columns are to be dropped, do not include them in the definition of the interim table. If a column is to be added, then add the column definition to the interim table.

   It is possible to perform table redefinition in parallel. If you specify a degree of parallelism on both of the tables and you ensure that parallel execution is enabled for the session, the database will use parallel execution whenever possible to perform the redefinition. You can use the PARALLEL clause of the ALTER SESSION statement to enable parallel execution.

4. Start the redefinition process by calling DBMS_REDEFINITION.START_REDEF_TABLE(), providing the following:

   • The table to be redefined

   • The interim table name

   • The column mapping

   • The method of redefinition

   • Optionally, the columns to be used in ordering rows

   • Optionally, specify the ORDER BY columns

   If the column mapping information is not supplied, then it is assumed that all the columns (with their names unchanged) are to be included in the interim table. If the column mapping is supplied, then only those columns specified explicitly in the column mapping are considered. If the method of redefinition is not specified, then the default method of redefinition using primary keys is assumed.

   You can optionally specify the ORDERBY_COLS parameter to specify how rows should be ordered during the initial instantiation of the interim table.

5. You have two methods for creating (cloning) dependent objects such as triggers, indexes, grants, and constraints on the interim table. Method 1 is the most automatic and preferred method, but there may be times that you would choose to use method 2.

   • Method 1: Automatically Creating Dependent Objects

     Use the COPY_TABLE_DEPENDENTS procedure to automatically create dependent objects such as triggers, indexes, grants, and constraints on the interim table. This procedure also registers the dependent objects. Registering the dependent objects enables the identities of these objects and their cloned counterparts to be automatically swapped later as part of the redefinition completion process. The result is that when the redefinition is completed, the names of the dependent objects will be the same as the names of the original dependent objects.

     You can discover if errors occurred while copying dependent objects by checking the NUM_ERRORS output variable. If the IGNORE_ERRORS parameter is set to TRUE, the COPY_TABLE_DEPENDENTS procedure continues cloning dependent objects even if an error is encounter when creating an object. The errors can later be viewed by querying the DBA_REDIFINITION_ERRORS view. Reasons for errors include a lack of system resources or a change in the logical structure of the table.

     If IGNORE_ERRORS is set to FALSE, the COPY_TABLE_DEPENDENTS procedure stops cloning objects as soon as any error is encountered.

     After you correct any errors you can attempt again to clone the failing object or objects by reexecuting the COPY_TABLE_DEPENDENTS procedure. Optionally you can create the objects manually and then register them as explained in method 2.

     The COPY_TABLE_DEPENDENTS procedure can be used multiple times as necessary. If an object has already been successfully cloned, it will ignore the operation.

   • Method 2: Manually Creating Dependent Objects

     You can manually create dependent objects on the interim table.

     
     

     Note: In previous releases you were required to manually create the triggers, indexes, grants, and constraints on the interim table, and there may still be situations where to want to or must do so. In such cases, any referential constraints involving the interim table (that is, the interim table is either a parent or a child table of the referential constraint) must be created disabled. Until the redefinition process is either completed or aborted, any trigger defined on the interim table will not execute.

     
     

     Use the REGISTER_DEPENDENT_OBJECT procedure after you create dependent objects manually. You can also use the COPY_TABLE_DEPENDENTS procedure to do the registration. Note that the COPY_TABLE_DEPENDENTS procedure does not clone objects that are registered manually.

     You would also use the REGISTER_DEPENDENT_OBJECT procedure if the COPY_TABLE_DEPENDENTS procedure failed to copy a dependent object and manual intervention is required.

     You can query the DBA_REDEFINITION_OBJECTS view to determine which dependent objects are registered. This view shows dependent objects that were registered explicitly with the REGISTER_DEPENDENT_OBJECT procedure or implicitly with the COPY_TABLE_DEPENDENTS procedure. Only current information is shown in the view.

     The UNREGISTER_DEPENDENT_OBJECT procedure can be used to unregister a dependent object on the table being redefined and on the interim table.

6. Execute the DBMS_REDEFINITION.FINISH_REDEF_TABLE procedure to complete the redefinition of the table. During this procedure, the original table is locked in the exclusive mode for a very short time, independent of the amount of data in the original table. However, FINISH_REDEF_TABLE will wait for all pending DML that was initiated before it was invoked to commit before completing the redefinition.

   As a result of this procedure, the following occur:

   1. The original table is redefined such that it has all the attributes, indexes, constraints, grants and triggers of the interim table

   2. The referential constraints involving the interim table now involve the post redefined table and are enabled.

   3. Dependent objects that were registered, either explicitly using REGISTER_DEPENDENT_OBJECT or implicitly using COPY_TABLE_DEPENDENTS, are renamed automatically.

      
      

      Note: If no registration is done or no automatic cloning is done, then you must manually rename the dependent objects.

      
      

7. If the redefinition was done using rowids, the post-redefined table will have a hidden column (M_ROW$$) and it is recommended that the user set this hidden column to unused as follows:

   ALTER TABLE table_name SET UNUSED (M_ROW$$)
   
   

The following is the end result of the redefinition process:

• The original table is redefined with the attributes and features of the interim table.

• The triggers, grants, indexes and constraints defined on the interim table after START_REDEF_TABLE() and before FINISH_REDEF_TABLE() are now defined on the post-redefined table. Any referential constraints involving the interim table before the redefinition process was finished now involve the post-redefinition table and are enabled.

• Any indexes, triggers, grants and constraints defined on the original table (prior to redefinition) are transferred to the interim table and are dropped when the user drops the interim table. Any referential constraints involving the original table before the redefinition now involve the interim table and are disabled.

• Any PL/SQL procedures and cursors defined on the original table (prior to redefinition) are invalidated. They are automatically revalidated (this revalidation can fail if the shape of the table was changed as a result of the redefinition process) whenever they are used next.

Intermediate Synchronization

After the redefinition process has been started by calling START_REDEF_TABLE() and before FINISH_REDEF_TABLE() has been called, it is possible that a large number of DML statements have been executed on the original table. If you know this is the case, it is recommended that you periodically synchronize the interim table with the original table. This is done by calling the DBMS_REDEFINITION.SYNC_INTERIM_TABLE() procedure. Calling this procedure reduces the time taken by FINISH_REDEF_TABLE() to complete the redefinition process.

The small amount of time that the original table is locked during FINISH_REDEF_TABLE() is independent of whether SYNC_INTERIM_TABLE() has been called.

Terminate and Clean Up After Errors

In the event that an error is raised during the redefinition process, or if you choose to terminate the redefinition process, call DBMS_REDEFINITION.ABORT_REDEF_TABLE(). This procedure drops temporary logs and tables associated with the redefinition process. After this procedure is called, you can drop the interim table and its associated objects.

Example of Online Table Redefinition

This example illustrates online redefinition of the previously created table hr.admin_emp, which at this point only contains columns: empno, ename, job, deptno. The table is redefined as follows:

• New columns mgr, hiredate, sal, and bonus (these existed in the original table but were dropped in previous examples) are added.

• The new column bonus is initialized to 0

• The column deptno has its value increased by 10.

• The redefined table is partitioned by range on empno.

The steps in this redefinition are illustrated below.

1. Verify that the table is a candidate for online redefinition. In this case you specify that the redefinition is to be done using primary keys or pseudo-primary keys.

   BEGIN
   DBMS_REDEFINITION.CAN_REDEF_TABLE('hr','admin_emp',
           dbms_redefinition.cons_use_pk);
   END;
   /
   
   

2. Create an interim table hr.int_admin_emp.

   CREATE TABLE hr.int_admin_emp
          (empno      NUMBER(5) PRIMARY KEY,
            ename      VARCHAR2(15) NOT NULL,
            job        VARCHAR2(10),
            mgr        NUMBER(5),
            hiredate   DATE DEFAULT (sysdate),
            sal        NUMBER(7,2),
            deptno     NUMBER(3) NOT NULL,
            bonus      NUMBER (7,2) DEFAULT(1000))
        PARTITION BY RANGE(empno)
          (PARTITION emp1000 VALUES LESS THAN (1000) TABLESPACE admin_tbs,
           PARTITION emp2000 VALUES LESS THAN (2000) TABLESPACE admin_tbs2);
   
   

3. Start the redefinition process.

   BEGIN
   DBMS_REDEFINITION.START_REDEF_TABLE('hr', 'admin_emp','int_admin_emp',
          'empno empno, ename ename, job job, deptno+10 deptno, 0 bonus',
           dbms_redefinition.cons_use_pk);
   END;
   /
   
   

4. Automatically create any triggers, indexes and constraints on hr.int_admin_emp.

   BEGIN
   DBMS_REDEFINITION.COPY_TABLE_DEPENDENTS('hr', 'admin_emp','int_admin_emp',
          TRUE, TRUE, TRUE, FALSE);
   END;
   
   

5. Optionally, synchronize the interim table hr.int_admin_emp.

   BEGIN 
   DBMS_REDEFINITION.SYNC_INTERIM_TABLE('hr', 'admin_emp', 'int_admin_emp');
   END;
   /
   
   

6. Complete the redefinition.

   BEGIN
   DBMS_REDEFINITION.FINISH_REDEF_TABLE('hr', 'admin_emp', 'int_admin_emp');
   END;
   /
   
   

   The table hr.admin_emp is locked in the exclusive mode only for a small window toward the end of this step. After this call the table hr.admin_emp is redefined such that it has all the attributes of the hr.int_admin_emp table.

7. Drop the interim table.

Restrictions for Online Redefinition of Tables

The following restrictions apply to the online redefinition of tables:

• If the table is to be redefined using primary key or pseudo-primary keys (unique keys or constraints with all component columns having not null constraints), then the table to be redefined must have the same primary key or pseudo-primary key columns. If the table is to be redefined using rowids, then the table must not be an index-organized table.

• Tables that are replicated in an n-way master configuration can be redefined, but horizontal subsetting (subset of rows in the table), vertical subsetting (subset of columns in the table), and column transformations are not allowed.

• The overflow table of an index-organized table cannot be online redefined.

• Tables with user-defined types (objects, REFs, collections, typed tables) cannot be online redefined.

• Tables with BFILE columns cannot be online redefined.

• Tables with LONG columns can be online redefined, but those columns must be converted to CLOBS. Tables with LONG RAW columns must be converted to BLOBS. Tables with LOB columns are acceptable.

• The table to be redefined cannot be part of a cluster.

• Tables in the SYS and SYSTEM schema cannot be online redefined.

• Temporary tables cannot be redefined.

• A subset of rows in the table cannot be redefined.

• Only simple deterministic expressions, sequences, and SYSDATE can be used when mapping the columns in the interim table to those of the original table. For example, subqueries are not allowed.

• If new columns (which are not instantiated with existing data for the original table) are being added as part of the redefinition, then they must not be declared NOT NULL until the redefinition is complete.

• There cannot be any referential constraints between the table being redefined and the interim table.

• Table redefinition cannot be done NOLOGGING.

• Tables with materialized view logs defined on them cannot be online redefined.

What Is the Recycle Bin?

The recycle bin is actually a data dictionary table containing information about dropped objects. Dropped tables and any associated objects such as indexes, constraints, nested tables, and the likes are not removed and still occupy space. They continue to count against user space quotas, until specifically purged from the recycle bin or the unlikely situation where they must be purged by the database because of tablespace space constraints.

Each user can be thought of as having his own recycle bin, since unless a user has the SYSDBA privilege, the only objects that the user has access to in the recycle bin are those that the user owns. A user can view his objects in the recycle bin using the following statement:

SELECT * FROM RECYCLEBIN;

When you drop a tablespace including its contents, the objects in the tablespace are not placed in the recycle bin and the database purges any entries in the recycle bin for objects located in the tablespace. The database also purges any recycle bin entries for objects in a tablespace when you drop the tablespace, not including contents, and the tablespace is otherwise empty. Likewise:

• When you drop a user, any objects belonging to the user are not placed in the recycle bin and any objects in the recycle bin are purged.

• When you drop a cluster, its member tables are not placed in the recycle bin and any former member tables in the recycle bin are purged.

• When you drop a type, any dependent objects such as subtypes are not placed in the recycle bin and any former dependent objects in the recycle bin are purged.

Renaming Objects in the Recycle Bin

When a dropped table is moved to the recycle bin, the table and its associated objects are given system-generated names. This is necessary to avoid name conflicts that may arise if multiple tables have the same name. This could occur under the following circumstances:

• A user drops a table, re-creates it with the same name, then drops it again.

• Two users have tables with the same name, and both users drop their tables.

The renaming convention is as follows:

BIN$unique_id$version

where:

• unique_id is a 26-character globally unique identifier for this object, which makes the recycle bin name unique across all databases

• version is a version number assigned by the database

Viewing and Querying Objects in the Recycle Bin

Oracle Database provides two views for obtaining information about objects in the recycle bin:

One use for these views is to identify the name that the database has assigned to a dropped object, as shown in the following example:

SELECT object_name, original_name FROM dba_recyclebin
   WHERE owner = 'HR';

OBJECT_NAME                    ORIGINAL_NAME
------------------------------ --------------------------------
BIN$yrMKlZaLMhfgNAgAIMenRA==$0 EMPLOYEES

You can also view the contents of the recycle bin using the SQL*Plus command SHOW RECYCLEBIN.

SQL> show recyclebin

ORIGINAL NAME    RECYCLEBIN NAME                OBJECT TYPE  DROP TIME
---------------- ------------------------------ ------------ -------------------
EMPLOYEES        BIN$yrMKlZaVMhfgNAgAIMenRA==$0 TABLE        2003-10-27:14:00:19

You can query objects that are in the recycle bin, just as you can query other objects. However, you must specify the name of the object as it is identified in the recycle bin. For example:

SELECT * FROM "BIN$yrMKlZaVMhfgNAgAIMenRA==$0";

Purging Objects in the Recycle Bin

If you decide that you are never going to restore an item from the recycle bin, you can use the PURGE statement to remove the items and their associated objects from the recycle bin and release their storage space. You need the same privileges as if you were dropping the item.

When you use the PURGE statement to purge a table, you can use the name that the table is known by in the recycle bin or the original name of the table. The recycle bin name can be obtained from either the DBA_ or USER_RECYCLEBIN view as shown in "Viewing and Querying Objects in the Recycle Bin". The following hypothetical example purges the table hr.int_admin_emp, which was renamed to BIN$jsleilx392mk2=293$0 when it was placed in the recycle bin:

PURGE TABLE BIN$jsleilx392mk2=293$0;

You can achieve the same result with the following statement:

PURGE TABLE int_admin_emp;

You can use the PURGE statement to purge all the objects in the recycle bin that are from a specified tablespace or only the tablespace objects belonging to a specified user, as shown in the following examples:

PURGE TABLESPACE example;
PURGE TABLESPACE example USER oe;

Users can purge the recycle bin or their own objects, and release space for objects, by using the following statement:

PURGE RECYCLEBIN;

If you have the SYSDBA privilege, then you can purge the entire recycle bin by specifying DBA_RECYCLEBIN, instead of RECYCLEBIN in the previous statement.

You can also use the PURGE statement to purge an index from the recycle bin or to purge from the recycle bin all objects in a specified tablespace.

Restoring Tables from the Recycle Bin

Use the FLASHBACK TABLE ... TO BEFORE DROP statement to recover objects from the recycle bin. You can specify either the name of the table in the recycle bin or the original table name. An optional RENAME TO clause lets you rename the table as you recover it. The recycle bin name can be obtained from either the DBA_ or USER_RECYCLEBIN view as shown in "Viewing and Querying Objects in the Recycle Bin". To use the FLASHBACK TABLE ... TO BEFORE DROP statement, you need the same privileges you need to drop the table.

The following example restores int_admin_emp table and assigns to it a new name:

FLASHBACK TABLE int_admin_emp TO BEFORE DROP 
   RENAME TO int2_admin_emp;

The system-generated recycle bin name is very useful if you have dropped a table multiple times. For example, suppose you have three versions of the int2_admin_emp table in the recycle bin and you want to recover the second version. You can do this by issuing two FLASHBACK TABLE statements, or you can query the recycle bin and then flashback to the appropriate system-generated name, as shown in the following example:

SELECT object_name, original_name FROM recyclebin;    

OBJECT_NAME                    ORIGINAL_NAME
------------------------------ --------------------------------
BIN$yrMKlZaLMhfgNAgAIMenRA==$0 INT2_ADMIN_EMP
BIN$yrMKlZaVMhfgNAgAIMenRA==$0 INT2_ADMIN_EMP
BIN$yrMKlZaQMhfgNAgAIMenRA==$0 INT2_ADMIN_EMP

FLASHBACK TABLE BIN$yrMKlZaVMhfgNAgAIMenRA==$0 TO BEFORE DROP 
   RENAME TO int2_admin_emp;

What Are Index-Organized Tables?

An index-organized table has a storage organization that is a variant of a primary B-tree. Unlike an ordinary (heap-organized) table whose data is stored as an unordered collection (heap), data for an index-organized table is stored in a B-tree index structure in a primary key sorted manner. Each leaf block in the index structure stores both the key and nonkey columns.

The structure of an index-organized table provides the following benefits:

• Fast random access on the primary key because an index-only scan is sufficient. And, because there is no separate table storage area, changes to the table data (such as adding new rows, updating rows, or deleting rows) result only in updating the index structure.

• Fast range access on the primary key because the rows are clustered in primary key order.

• Lower storage requirements because duplication of primary keys is avoided. They are not stored both in the index and underlying table, as is true with heap-organized tables.

Index-organized tables have full table functionality. They support features such as constraints, triggers, LOB and object columns, partitioning, parallel operations, online reorganization, and replication. And, they offer these additional features:

• Key compression

• Overflow storage area and specific column placement

• Secondary indexes, including bitmap indexes.

Index-organized tables are ideal for OLTP applications, which require fast primary key access and high availability. Queries and DML on an orders table used in electronic order processing are predominantly primary-key based and heavy volume causes fragmentation resulting in a frequent need to reorganize. Because an index-organized table can be reorganized online and without invalidating its secondary indexes, the window of unavailability is greatly reduced or eliminated.

Index-organized tables are suitable for modeling application-specific index structures. For example, content-based information retrieval applications containing text, image and audio data require inverted indexes that can be effectively modeled using index-organized tables. A fundamental component of an internet search engine is an inverted index that can be modeled using index-organized tables.

These are but a few of the applications for index-organized tables.

Creating Index-Organized Tables

You use the CREATE TABLE statement to create index-organized tables, but you must provide additional information:

• An ORGANIZATION INDEX qualifier, which indicates that this is an index-organized table

• A primary key, specified through a column constraint clause (for a single column primary key) or a table constraint clause (for a multiple-column primary key).

Optionally, you can specify the following:

• An OVERFLOW clause, which preserves dense clustering of the B-tree index by storing the row column values exceeding a specified threshold in a separate overflow data segment.

• A PCTTHRESHOLD value, which defines the percentage of space reserved in the index block for an index-organized table. Any portion of the row that exceeds the specified threshold is stored in the overflow segment. In other words, the row is broken at a column boundary into two pieces, a head piece and tail piece. The head piece fits in the specified threshold and is stored along with the key in the index leaf block. The tail piece is stored in the overflow area as one or more row pieces. Thus, the index entry contains the key value, the nonkey column values that fit the specified threshold, and a pointer to the rest of the row.

• An INCLUDING clause, which can be used to specify nonkey columns that are to be stored in the overflow data segment.

CREATE TABLE admin_docindex(
        token char(20), 
        doc_id NUMBER,
        token_frequency NUMBER,
        token_offsets VARCHAR2(512),
        CONSTRAINT pk_admin_docindex PRIMARY KEY (token, doc_id))
    ORGANIZATION INDEX 
    TABLESPACE admin_tbs
    PCTTHRESHOLD 20
    OVERFLOW TABLESPACE admin_tbs2;

CREATE OR REPLACE TYPE admin_typ AS OBJECT
    (col1 NUMBER, col2 VARCHAR2(6));
CREATE TABLE admin_iot (c1 NUMBER primary key, c2 admin_typ)
    ORGANIZATION INDEX;

CREATE TABLE admin_iot2 OF admin_typ (col1 PRIMARY KEY)
    ORGANIZATION INDEX;

CREATE TYPE project_t AS OBJECT(pno NUMBER, pname VARCHAR2(80));
/
CREATE TYPE project_set AS TABLE OF project_t;
/
CREATE TABLE proj_tab (eno NUMBER, projects PROJECT_SET)
    NESTED TABLE projects STORE AS emp_project_tab
                ((PRIMARY KEY(nested_table_id, pno)) 
    ORGANIZATION INDEX)
    RETURN AS LOCATOR;

CREATE TABLE admin_iot4(a INT, b INT, c INT, d INT, 
                primary key(c,b))
    ORGANIZATION INDEX;

CREATE TABLE admin_docindex2(
        token CHAR(20), 
        doc_id NUMBER,
        token_frequency NUMBER,
        token_offsets VARCHAR2(512),
        CONSTRAINT pk_admin_docindex2 PRIMARY KEY (token, doc_id))
    ORGANIZATION INDEX 
    TABLESPACE admin_tbs
    PCTTHRESHOLD 20
    INCLUDING token_frequency
    OVERFLOW TABLESPACE admin_tbs2;

CREATE TABLE admin_iot3(i PRIMARY KEY, j, k, l) 
     ORGANIZATION INDEX 
     PARALLEL
     AS SELECT * FROM hr.jobs;

CREATE TABLE admin_iot5(i INT, j INT, k INT, l INT, PRIMARY KEY (i, j, k)) 
    ORGANIZATION INDEX COMPRESS;

CREATE TABLE admin_iot6(i INT, j INT, k INT, l INT, PRIMARY KEY(i, j, k)) 
    ORGANIZATION INDEX COMPRESS 2;

CREATE TABLE admin_iot7(i INT, j INT, k INT, l INT, PRIMARY KEY (i, j, k)) 
    ORGANIZATION INDEX COMPRESS 1;

ALTER TABLE admin_iot5 MOVE NOCOMPRESS;

Maintaining Index-Organized Tables

Index-organized tables differ from ordinary tables only in physical organization. Logically, they are manipulated in the same manner as ordinary tables. You can specify an index-organized table just as you would specify a regular table in INSERT, SELECT, DELETE, and UPDATE statements.

ALTER TABLE admin_docindex INITRANS 4 OVERFLOW INITRANS 6;

ALTER TABLE admin_docindex PCTTHRESHOLD 15 INCLUDING doc_id;

ALTER TABLE admin_iot3 ADD OVERFLOW TABLESPACE admin_tbs2;

ALTER TABLE admin_docindex MOVE;

ALTER TABLE admin_docindex MOVE ONLINE;

ALTER TABLE admin_docindex MOVE TABLESPACE admin_tbs2 
    OVERFLOW TABLESPACE admin_tbs3;

CREATE TABLE admin_iot_lob
   (c1 number (6) primary key,
    admin_lob CLOB)
   ORGANIZATION INDEX
   LOB (admin_lob) STORE AS (TABLESPACE admin_tbs2);
.
.
.
ALTER TABLE admin_iot_lob MOVE LOB (admin_lob) STORE AS (TABLESPACE admin_tbs3); 

Creating Secondary Indexes on Index-Organized Tables

You can create secondary indexes on an index organized tables to provide multiple access paths. Secondary indexes on index-organized tables differ from indexes on ordinary tables in two ways:

• They store logical rowids instead of physical rowids. This is necessary because the inherent movability of rows in a B-tree index results in the rows having no permanent physical addresses. If the physical location of a row changes, its logical rowid remains valid. One effect of this is that a table maintenance operation, such as ALTER TABLE ... MOVE, does not make the secondary index unusable.

• The logical rowid also includes a physical guess which identifies the database block address at which the row is likely to be found. If the physical guess is correct, a secondary index scan would incur a single additional I/O once the secondary key is found. The performance would be similar to that of a secondary index-scan on an ordinary table.

Unique and nonunique secondary indexes, function-based secondary indexes, and bitmap indexes are supported as secondary indexes on index-organized tables.

CREATE INDEX Doc_id_index on Docindex(Doc_id, Token);

SELECT Token FROM Docindex WHERE Doc_id = 1;

Analyzing Index-Organized Tables

Just like ordinary tables, index-organized tables are analyzed using the DBMS_STATS package, or the ANALYZE statement.

EXECUTE DBMS_STATS.GATHER_TABLE_STATS ('HR','COUNTRIES');

Using the ORDER BY Clause with Index-Organized Tables

If an ORDER BY clause only references the primary key column or a prefix of it, then the optimizer avoids the sorting overhead, as the rows are returned sorted on the primary key columns.

The following queries avoid sorting overhead because the data is already sorted on the primary key:

SELECT * FROM admin_docindex2 ORDER BY token, doc_id;
SELECT * FROM admin_docindex2 ORDER BY token;

If, however, you have an ORDER BY clause on a suffix of the primary key column or non-primary-key columns, additional sorting is required (assuming no other secondary indexes are defined).

SELECT * FROM admin_docindex2 ORDER BY doc_id;
SELECT * FROM admin_docindex2 ORDER BY token_frequency;

Converting Index-Organized Tables to Regular Tables

You can convert index-organized tables to regular tables using the Oracle import or export utilities, or the CREATE TABLE ... AS SELECT statement.

To convert an index-organized table to a regular table:

• Export the index-organized table data using conventional path.

• Create a regular table definition with the same definition.

• Import the index-organized table data, making sure IGNORE=y (ensures that object exists error is ignored).

  
  

  Note: Before converting an index-organized table to a regular table, be aware that index-organized tables cannot be exported using pre-Oracle8 versions of the Export utility.

  
  
  
  

  See Also: Oracle Database Utilities for more details about using the IMPORT and EXPORT utilities

  
  

Creating External Tables

You create external tables using the ORGANIZATION EXTERNAL clause of the CREATE TABLE statement. You are not in fact creating a table; that is, an external table does not have any extents associated with it. Rather, you are creating metadata in the data dictionary that enables you to access external data.

The following example creates an external table and then uploads the data to a database table. Alternatively, you can unload data through the external table framework by specifying the AS subquery clause of the CREATE TABLE statement. External table data pump unload can use only the ORACLE_DATAPUMP access driver.

The file empxt1.dat contains the following sample data:

360,Jane,Janus,ST_CLERK,121,17-MAY-2001,3000,0,50,jjanus
361,Mark,Jasper,SA_REP,145,17-MAY-2001,8000,.1,80,mjasper
362,Brenda,Starr,AD_ASST,200,17-MAY-2001,5500,0,10,bstarr
363,Alex,Alda,AC_MGR,145,17-MAY-2001,9000,.15,80,aalda

The file empxt2.dat contains the following sample data:

401,Jesse,Cromwell,HR_REP,203,17-MAY-2001,7000,0,40,jcromwel
402,Abby,Applegate,IT_PROG,103,17-MAY-2001,9000,.2,60,aapplega
403,Carol,Cousins,AD_VP,100,17-MAY-2001,27000,.3,90,ccousins
404,John,Richardson,AC_ACCOUNT,205,17-MAY-2001,5000,0,110,jrichard

The following hypothetical SQL statements create an external table in the hr schema named admin_ext_employees and load its data into the hr.employees table.

CONNECT  /  AS SYSDBA;
-- Set up directories and grant access to hr 
CREATE OR REPLACE DIRECTORY admin_dat_dir
    AS '/flatfiles/data'; 
CREATE OR REPLACE DIRECTORY admin_log_dir 
    AS '/flatfiles/log'; 
CREATE OR REPLACE DIRECTORY admin_bad_dir 
    AS '/flatfiles/bad'; 
GRANT READ ON DIRECTORY admin_dat_dir TO hr; 
GRANT WRITE ON DIRECTORY admin_log_dir TO hr; 
GRANT WRITE ON DIRECTORY admin_bad_dir TO hr;
-- hr connects 
CONNECT hr/hr
-- create the external table
CREATE TABLE admin_ext_employees
                   (employee_id       NUMBER(4), 
                    first_name        VARCHAR2(20),
                    last_name         VARCHAR2(25), 
                    job_id            VARCHAR2(10),
                    manager_id        NUMBER(4),
                    hire_date         DATE,
                    salary            NUMBER(8,2),
                    commission_pct    NUMBER(2,2),
                    department_id     NUMBER(4),
                    email             VARCHAR2(25) 
                   ) 
     ORGANIZATION EXTERNAL 
     ( 
       TYPE ORACLE_LOADER 
       DEFAULT DIRECTORY admin_dat_dir 
       ACCESS PARAMETERS 
       ( 
         records delimited by newline 
         badfile admin_bad_dir:'empxt%a_%p.bad' 
         logfile admin_log_dir:'empxt%a_%p.log' 
         fields terminated by ',' 
         missing field values are null 
         ( employee_id, first_name, last_name, job_id, manager_id, 
           hire_date char date_format date mask "dd-mon-yyyy", 
           salary, commission_pct, department_id, email 
         ) 
       ) 
       LOCATION ('empxt1.dat', 'empxt2.dat') 
     ) 
     PARALLEL 
     REJECT LIMIT UNLIMITED; 
-- enable parallel for loading (good if lots of data to load)
ALTER SESSION ENABLE PARALLEL DML;
-- load the data in hr employees table
INSERT INTO employees (employee_id, first_name, last_name, job_id, manager_id,
                       hire_date, salary, commission_pct, department_id, email) 
            SELECT * FROM admin_ext_employees;

The following paragraphs contain descriptive information about this example.

The first few statements in this example create the directory objects for the operating system directories that contain the data sources, and for the bad record and log files specified in the access parameters. You must also grant READ or WRITE directory object privileges, as appropriate.

The TYPE specification indicates the access driver of the external table. The access driver is the API that interprets the external data for 5the database. Oracle Database provides two access drivers: ORACLE_LOADER and ORACLE_DATAPUMP. If you omit the TYPE specification, ORACLE_LOADER is the default access driver. You must specify the ORACLE_DATAPUMP access driver if you specify the AS subquery clause to unload data from one Oracle Database and reload it into the same or a different Oracle Database.

The access parameters, specified in the ACCESS PARAMETERS clause, are opaque to the database. These access parameters are defined by the access driver, and are provided to the access driver by the database when the external table is accessed. See Oracle Database Utilities for a description of the ORACLE_LOADER access parameters.

The PARALLEL clause enables parallel query on the data sources. The granule of parallelism is by default a data source, but parallel access within a data source is implemented whenever possible. For example, if PARALLEL=3 were specified, then more than one parallel execution server could be working on a data source. But, parallel access within a data source is provided by the access driver only if all of the following conditions are met:

• The media allows random positioning within a data source

• It is possible to find a record boundary from a random position

• The data files are large enough to make it worthwhile to break up into multiple chunks

  
  

  Note: Specifying a PARALLEL clause is of value only when dealing with large amounts of data. Otherwise, it is not advisable to specify a PARALLEL clause, and doing so can be detrimental.

  
  

The REJECT LIMIT clause specifies that there is no limit on the number of errors that can occur during a query of the external data. For parallel access, this limit applies to each parallel execution server independently. For example, if REJECT LIMIT is specified, each parallel query process is allowed 10 rejections. Hence, the only precisely enforced values for REJECT LIMIT on parallel query are 0 and UNLIMITED.

In this example, the INSERT INTO TABLE statement generates a dataflow from the external data source to the Oracle Database SQL engine where data is processed. As data is parsed by the access driver from the external table sources and provided to the external table interface, the external data is converted from its external representation to its Oracle Database internal datatype.

Altering External Tables

You can use any of the ALTER TABLE clauses shown in Table 14-1to change the characteristics of an external table. No other clauses are permitted.

ALTER TABLE admin_ext_employees
   REJECT LIMIT 100;

ALTER TABLE admin_ext_employees
   PROJECT COLUMN REFERNCED;

ALTER TABLE admin_ext_employees
   PROJECT COLUMN ALL;

ALTER TABLE admin_ext_employees 
    DEFAULT DIRECTORY admin_dat2_dir;

ALTER TABLE admin_ext_employees
    ACCESS PARAMETERS
       (FIELDS TERMINATED BY ';');

ALTER TABLE admin_ext_employees
   LOCATION ('empxt3.txt',
             'empxt4.txt');

Dropping External Tables

For an external table, the DROP TABLE statement removes only the table metadata in the database. It has no affect on the actual data, which resides outside of the database.

System and Object Privileges for External Tables

System and object privileges for external tables are a subset of those for regular table. Only the following system privileges are applicable to external tables:

• CREATE ANY TABLE

• ALTER ANY TABLE

• DROP ANY TABLE

• SELECT ANY TABLE

Only the following object privileges are applicable to external tables:

• ALTER

• SELECT

However, object privileges associated with a directory are:

• READ

• WRITE

For external tables, READ privileges are required on directory objects that contain data sources, while WRITE privileges are required for directory objects containing bad, log, or discard files.