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Managing LOBs: Database Administration

This chapter describes administrative tasks that must be performed to setup, maintain, and use a database that contains LOBs.

This chapter contains these topics:

• Database Utilities for Loading Data into LOBs
• Managing Temporary LOBs
• Managing BFILEs
• Changing Tablespace Storage for a LOB

Database Utilities for Loading Data into LOBs

The following utilities are recommended for bulk loading data into LOB columns as part of database setup or maintenance tasks:

• SQL*Loader
• Oracle DataPump

  Note: Application Developers: If you are loading data into a LOB in your application, then using the LOB APIs is recommended. See Chapter 14, "LOB APIs for Basic Operations" for details on APIs that allow you to load LOBs from files.

Using SQL*Loader to Load LOBs

There are two general techniques for using SQL*Loader to load data into LOBs:

• Loading data from a primary data file
• Loading from a secondary data file using LOBFILEs

Consider the following issues when loading LOBs with SQL*Loader:

• For SQL*Loader conventional path loads, failure to load a particular LOB does not result in the rejection of the record containing that LOB; instead, the record ends up containing an empty LOB.

  For SQL*Loader direct-path loads, the LOB could be empty or truncated. LOBs are sent in pieces to the server for loading. If there is an error, then the LOB piece with the error is discarded and the rest of that LOB is not loaded. In other words, if the entire LOB with the error is contained in the first piece, then that LOB column will either be empty or truncated.

• When loading from LOBFILEs specify the maximum length of the field corresponding to a LOB-type column. If the maximum length is specified, then it is taken as a hint to help optimize memory usage. It is important that the maximum length specification does not underestimate the true maximum length.
• When using SQL*Loader direct-path load, loading LOBs can take up substantial memory. If the message "SQL*Loader 700 (out of memory)" appears when loading LOBs, then internal code is probably batching up more rows in each load call than can be supported by your operating system and process memory. A workaround is to use the ROWS option to read a smaller number of rows in each data save.
• You can also use the Direct Path API to load LOBs.
• Using LOBFILEs is recommended when loading columns containing XML data in CLOBs or XMLType columns. Whether you perform a direct-path load or a conventional path load with SQL*Loader depends on whether you need to validate XML documents upon loading.
  • If the XML document must be validated upon loading, then use conventional path load.
  • If it is not necessary to ensure that the XML document is valid or you can safely assume that the XML document is valid, then you can perform a direct-path load. Performance is higher when you use direct-path load because the overhead of XML validation is incurred.

  A conventional path load executes SQL INSERT statements to populate tables in an Oracle database. A direct path load eliminates much of the Oracle database overhead by formatting Oracle data blocks and writing the data blocks directly to the database files.

  A direct-path load does not compete with other users for database resources, so it can usually load data at near disk speed. Considerations inherent to direct path loads, such as restrictions, security, and backup implications, are discussed in Oracle Database Utilities.

• Tables to be loaded must already exist in the database. SQL*Loader never creates tables. It loads existing tables that either already contain data or are empty.
• The following privileges are required for a load:
  • You must have INSERT privileges on the table to be loaded.
  • You must have DELETE privilege on the table to be loaded, when using the REPLACE or TRUNCATE option to empty out the old data before loading the new data in its place.

    See Also: For details on using SQL*Loader to load LOBs and other details on SQL*Loader usage, refer to the Oracle Database Utilities guide.

Using SQL*Loader to Populate a BFILE Column

This section describes how to load data from files in the file system into a BFILE column.

Note that the BFILE datatype stores unstructured binary data in operating system files outside the database. A BFILE column or attribute stores a file locator that points to a server-side external file containing the data.

The SQL*Loader assumes that the necessary DIRECTORY objects have already been created. See "Directory Object" for more information on creating directory objects.

A control file field corresponding to a BFILE column consists of column name followed by the BFILE directive.

The BFILE directive takes as arguments a DIRECTORY object name followed by a BFILE name. Both of these can be provided as string constants, or they can be dynamically sourced through some other field.

The following two examples illustrate the loading of BFILES.

CONNECT system/manager
GRANT CREATE ANY DIRECTORY to samp; 
CONNECT samp/samp
CREATE OR REPLACE DIRECTORY adgraphic_photo as '/tmp';
CREATE OR REPLACE DIRECTORY adgraphic_dir as '/tmp';

In the following example only the file name is specified dynamically.

Control file:

LOAD DATA
INFILE sample9.dat
INTO TABLE Print_media
FIELDS TERMINATED BY ','
(product_id  INTEGER EXTERNAL(6),
 FileName    FILLER CHAR(30),
 ad_graphic  BFILE(CONSTANT "modem_graphic_2268_21001", FileName))

Data file:

007, modem_2268.jpg,
008, monitor_3060.jpg,
009, keyboard_2056.jpg,

In the following example, the BFILE and the DIRECTORY object are specified dynamically.

Control file:

LOAD DATA
INFILE sample10.dat
INTO TABLE Print_media
FIELDS TERMINATED BY ','
(
 product_id INTEGER EXTERNAL(6),
 ad_graphic BFILE (DirName, FileName),
 FileName  FILLER CHAR(30),
 DirName   FILLER CHAR(30)
)

Data file:

007,monitor_3060.jpg,ADGRAPHIC_PHOTO,
008,modem_2268.jpg,ADGRAPHIC_PHOTO,
009,keyboard_2056.jpg,ADGRAPHIC_DIR,

Using Oracle DataPump to Transfer LOB Data

You can use Oracle DataPump to transfer LOB data from one database to another.

Managing Temporary LOBs

The database keeps track of temporary LOBs in each session, and provides a v$ view called v$temporary_lobs. From the session, the application can determine which user owns the temporary LOB. As a database administrator, you can use this view to monitor and guide any emergency cleanup of temporary space used by temporary LOBs.

Managing Temporary Tablespace for Temporary LOBs

Temporary tablespace is used to store temporary LOB data. As a database administrator you control data storage resources for temporary LOB data by controlling user access to temporary tablespaces and by the creation of different temporary tablespaces.

Managing BFILEs

This section describes administrative tasks for managing databases that contain BFILEs.

Rules for Using Directory Objects and BFILEs

When creating a directory object or BFILEs, ensure that the following conditions are met:

• The operating system file must not be a symbolic or hard link.
• The operating system directory path named in the Oracle DIRECTORY object must be an existing operating system directory path.
• The operating system directory path named in the Oracle DIRECTORY object should not contain any symbolic links in its components.

Setting Maximum Number of Open BFILEs

A limited number of BFILEs can be open simultaneously in each session. The initialization parameter, SESSION_MAX_OPEN_FILES defines an upper limit on the number of simultaneously open files in a session.

The default value for this parameter is 10. That is, you can open a maximum of 10 files at the same time in each session if the default value is used. If you want to alter this limit, then the database administrator can change the value of this parameter in the init.ora file. For example:

SESSION_MAX_OPEN_FILES=20

If the number of unclosed files reaches the SESSION_MAX_OPEN_FILES value, then you will not be able to open any more files in the session. To close all open files, use the DBMS_LOB.FILECLOSEALL call.

Changing Tablespace Storage for a LOB

As the database administrator, you can use the following techniques to change the default storage for a LOB after the table has been created:

• Using ALTER TABLE... MODIFY: You can change LOB tablespace storage as follows:

  Note: The ALTER TABLE syntax for modifying an existing LOB column uses the MODIFY LOB clause, not the LOB...STORE AS clause. The LOB...STORE AS clause is only for newly added LOB columns. There are two kinds of LOB storage clauses: LOB_storage_clause and modify_LOB_storage_clause. In the ALTER TABLE MODIFY LOB statement, you can only specify modify_LOB_storage_clause.

  ALTER TABLE test MODIFY 
    LOB (lob1)
      STORAGE  (
      NEXT         4M
      MAXEXTENTS   100
      PCTINCREASE  50
  

              )
  
  

• Using ALTER TABLE... MOVE: You can also use the MOVE clause of the ALTER TABLE statement to change LOB tablespace storage. For example:

  ALTER TABLE test MOVE
      TABLESPACE tbs1
      LOB (lob1, lob2)
      STORE AS (
            TABLESPACE tbs2
            DISABLE STORAGE IN ROW);