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Oracle® Database SecureFiles and Large Objects Developer's Guide
11g Release 1 (11.1)

B28393-03
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2 Working with LOBs

This chapter describes the usage and semantics of LOBs that you need to be familiar with to use LOBs in your application. Various techniques for working with LOBs are covered.

Most of the discussions in this chapter regarding persistent LOBs assume that you are dealing with LOBs in tables that already exist. The task of creating tables with LOB columns is typically performed by your database administrator.

See Also:

This chapter contains these topics:

LOB Column States

The techniques you use when accessing a cell in a LOB column differ depending on the state of the given cell. A cell in a LOB Column can be in one of the following states:

Locking a Row Containing a LOB

You can lock a row containing a LOB to prevent other database users from writing to the LOB during a transaction. To lock a row containing a LOB, specify the FOR UPDATE clause when you select the row. While the row is locked, other users cannot lock or update the LOB, until you end your transaction.

Opening and Closing LOBs

The LOB APIs include operations that enable you to explicitly open and close a LOB instance. You can open and close a persistent LOB instance of any type: BLOB, CLOB, NCLOB, or BFILE. You open a LOB to achieve one or both of the following results:

If you open a LOB, then you must close the LOB at some point later in your session. This is the only requirement for an open LOB. While a LOB instance is open, you can perform as many operations as you want on the LOB—provided the operations are allowed in the given mode.

See Also:

"Opening Persistent LOBs with the OPEN and CLOSE Interfaces" for details on usage of these APIs.

LOB Locator and LOB Value

There are two techniques that you can use to access and modify LOB values:

Using the Data Interface for LOBs

You can perform bind and define operations on CLOB and BLOB columns in C applications using the data interface for LOBs in OCI. Doing so, enables you to insert or select out data in a LOB column without using a LOB locator as follows:

  • Using a bind variable associated with a LOB column to insert character data into a CLOB, or RAW data into a BLOB.

  • Using a define operation to define an output buffer in your application that holds character data selected from a CLOB, or RAW data selected from a BLOB.

    See Also:

    Chapter 14, "Data Interface for Persistent LOBs" for more information on implicit assignment of LOBs to other datatypes.

Using the LOB Locator to Access and Modify LOB Values

The value of a LOB instance stored in the database can be accessed through a LOB locator, a reference to the location of the LOB value. Database tables store only locators in CLOB, BLOB, NCLOB and BFILE columns. Note the following with respect to LOB locators and values:

  • To access or manipulate a LOB value, you pass the LOB locator to the various LOB APIs.

  • A LOB locator can be assigned to any LOB instance of the same type.

  • The characteristics of a LOB as being temporary or persistent have nothing to do with the locator. The characteristics of temporary or persistent apply only to the LOB instance.

LOB Locators and BFILE Locators

There are differences between the semantics of locators for LOB types BLOB, CLOB, and NCLOB on one hand, and the semantics of locators for the BFILE type on the other hand:

Regardless of where the value of a LOB is stored, a locator is stored in the table row of any initialized LOB column. Note that when the term locator is used without an identifying prefix term, it refers to both LOB locators and BFILE locators. Also, when you select a LOB from a table, the LOB returned is always a temporary LOB. For more information on working with locators for temporary LOBs, see "LOBs Returned from SQL Functions".

Table print_media

The table print_media of the Oracle Database Sample Schema PM, is used in many examples in this documentation and is defined as:

CREATE TABLE print_media
    ( product_id        NUMBER(6)
    , ad_id             NUMBER(6)
    , ad_composite      BLOB
    , ad_sourcetext     CLOB
    , ad_finaltext      CLOB
    , ad_fltextn        NCLOB
    , ad_textdocs_ntab  textdoc_tab
    , ad_photo          BLOB
    , ad_graphic        BFILE
    , ad_header         adheader_typ
    ) NESTED TABLE ad_textdocs_ntab STORE AS textdocs_nestedtab;

See Also:

"Creating a Table Containing One or More LOB Columns" for the details of how print_media and its associated tables and files are created.

Initializing a LOB Column to Contain a Locator

Any LOB instance that is NULL does not have a locator. Before you can pass a LOB instance to any LOB API routine, the instance must contain a locator. For example, you can select a NULL LOB from a row, but you cannot pass the instance to the PL/SQL DBMS_LOB.READ procedure. The following sub-sections describe how to initialize a persistent LOB column and how to initialize a BFILE column.

Initializing a Persistent LOB Column

Before you can start writing data to a persistent LOB using the supported programmatic environment interfaces (PL/SQL, OCI, OCCI, Pro*C/C++, Pro*COBOL, Visual Basic, Java, or OLEDB), the LOB column/attribute must be made non-NULL, that is, it must contain a locator.

You can accomplish this by initializing the persistent LOB to empty in an INSERT/UPDATE statement using the functions EMPTY_BLOB for BLOBs or EMPTY_CLOB for CLOBs and NCLOBs.

Note:

You can use SQL to populate a LOB column with data even if it contains a NULL value.

See Also:

Chapter 5, "LOB Storage" for more information on initializing LOB columns.

Running the EMPTY_BLOB() or EMPTY_CLOB() function in and of itself does not raise an exception. However, using a LOB locator that was set to empty to access or manipulate the LOB value in any PL/SQL DBMS_LOB or OCI function raises an exception.

Valid places where empty LOB locators may be used include the VALUES clause of an INSERT statement and the SET clause of an UPDATE statement.

See Also:

Note:

Character strings are inserted using the default character set for the instance.

The following INSERT statement in the PM, table print_media:

  • Populates ad_sourcetext with the character string 'my Oracle',

  • Sets ad_composite, ad_finaltext, and ad_fltextn to an empty value,

  • Sets ad_photo to NULL, and

  • Initializes ad_graphic to point to the file my_picture located under the logical directory my_directory_object.

CREATE OR REPLACE DIRECTORY my_directory_object AS 'oracle/work/tklocal';
INSERT INTO print_media VALUES (1726, 1, EMPTY_BLOB(), 
    'my Oracle', EMPTY_CLOB(), EMPTY_CLOB(),
    NULL, NULL, BFILENAME('my_directory_object', 'my_picture'), NULL);

Similarly, the LOB attributes for the ad_header column in print_media can be initialized to NULL, empty, or a character/raw literal, which is shown in the following statement:

INSERT INTO print_media (product_id, ad_id, ad_header) 
    VALUES (1726, 1, adheader_typ('AD FOR ORACLE', sysdate, 
    'Have Grid', EMPTY_BLOB()));

Initializing BFILEs

Before you can access BFILE values using LOB APIs, the BFILE column or attribute must be made non-NULL. You can initialize the BFILE column to point to an external operating system file by using the BFILENAME function.

See Also:

"Accessing BFILEs" for more information on initializing BFILE columns.

Accessing LOBs

You can access a LOB instance using the following techniques:

Accessing a LOB Using SQL

Support for columns that use LOB datatypes is built into many SQL functions. This support enables you to use SQL semantics to access LOB columns in SQL. In most cases, you can use the same SQL semantics on a LOB column that you would use on a VARCHAR2 column.

See Also:

For details on SQL semantics support for LOBs, see Chapter 10, "SQL Semantics and LOBs".

Accessing a LOB Using the Data Interface

You can select a LOB directly into CHAR or RAW buffers using the LONG-to-LOB API in OCI and PL/SQL. In the following PL/SQL example, ad_finaltext is selected into a VARCHAR2 buffer final_ad.

DECLARE
    final_ad VARCHAR(32767);
BEGIN
    SELECT ad_finaltext INTO final_ad FROM print_media
        WHERE product_id = 2056 and ad_id = 12001 ; 
    /* PUT_LINE can only output up to 255 characters at a time  */
    ...
    DBMS_OUTPUT.PUT_LINE(final_ad);
    /* more calls to read final_ad */
    ...
END;

See Also:

For more details on accessing LOBs using the data interface, see Chapter 14, "Data Interface for Persistent LOBs".

Accessing a LOB Using the Locator Interface

You can access and manipulate a LOB instance by passing the LOB locator to the LOB APIs supplied with the database. An extensive set of LOB APIs is provided with each supported programmatic environment. In OCI, a LOB locator is mapped to a locator pointer which is used to access the LOB value.

Note:

In all environments, including OCI, the LOB APIs operate on the LOB value implicitly—there is no need to "dereference" the LOB locator.

See Also:

LOB Rules and Restrictions

This section provides details on LOB rules and restrictions.

Rules for LOB Columns

LOB columns are subject to the following rules and restrictions:

  • You cannot specify a LOB as a primary key column.

  • Oracle Database has limited support for remote LOBs. Remote LOBs are supported in three ways.

    1. Create table as select or insert as select.

    CREATE TABLE t AS SELECT * FROM table1@remote_site;
    INSERT INTO t SELECT * FROM table1@remote_site;
    UPDATE t SET lobcol = (SELECT lobcol FROM table1@remote_site);
    INSERT INTO table1@remote_site SELECT * FROM local_table;
    UPDATE table1@remote_site SET lobcol = (SELECT lobcol FROM local_table);
    DELETE FROM table1@remote_site <WHERE clause involving non_lob_columns>
    

    In statements structured like the preceding examples, only standalone LOB columns are allowed in the select list.

    2. Functions on remote LOBs returning scalars. SQL and PL/SQL functions having a LOB parameter and returning a scalar datatype are supported. Other SQL functions and DBMS_LOB APIs are not supported for use with remote LOB columns. For example, the following statement is supported:

    CREATE TABLE tab AS SELECT DBMS_LOB.GETLENGTH@dbs2(clob_col) len FROM tab@dbs2;
    CREATE TABLE tab AS SELECT LENGTH(clob_col) len FROM tab@dbs2;
    

    However, the following statement is not supported because DBMS_LOB.SUBSTR returns a LOB:

    CREATE TABLE tab AS SELECT DBMS_LOB.SUBSTR(clob_col) from tab@dbs2; 
    

    3. Data Interface for remote LOBs. You can insert a character or binary buffer into a remote CLOB or BLOB, and select a remote CLOB or BLOB into a character or binary buffer. For example (in PL/SQL):

    SELECT clobcol1, type1.blobattr INTO varchar_buf1, raw_buf2 FROM
       table1@remote_site;
    INSERT INTO table1@remotesite (clobcol1, type1.blobattr) VALUES varchar_buf1,
       raw_buf2;
    INSERT INTO table1@remotesite (lobcol) VALUES ('test');
    UPDATE table1 SET lobcol = 'xxx';
    

    These are the only supported syntax involving LOBs in remote tables. No other usage is supported.

  • Clusters cannot contain LOBs, either as key or nonkey columns.

  • The following data structures are supported only as temporary instances. You cannot store these instances in database tables:

    • VARRAY of any LOB type

    • VARRAY of any type containing a LOB type, such as an object type with a LOB attribute

    • ANYDATA of any LOB type

    • ANYDATA of any type containing a LOB

  • You cannot specify LOB columns in the ORDER BY clause of a query, or in the GROUP BY clause of a query or in an aggregate function.

  • You cannot specify a LOB column in a SELECT... DISTINCT or SELECT... UNIQUE statement or in a join. However, you can specify a LOB attribute of an object type column in a SELECT... DISTINCT statement or in a query that uses the UNION or MINUS set operator if the column's object type has a MAP or ORDER function defined on it.

  • The first (INITIAL) extent of a LOB segment must contain at least three database blocks.

  • When creating an AFTER UPDATE DML trigger, you cannot specify a LOB column in the UPDATE OF clause.

  • You cannot specify a LOB column as part of an index key. However, you can specify a LOB column in the indextype specification of a domain index. In addition, Oracle Text lets you define an index on a CLOB column.

  • In an INSERT... AS SELECT operation, you can bind up to 4000 bytes of data to LOB columns and attributes.

  • If a table has both LONG and LOB columns, you cannot bind more than 4000 bytes of data to both the LONG and LOB columns in the same SQL statement. However, you can bind more than 4000 bytes of data to either the LONG or the LOB column.

Note:

For a table on which you have defined an AFTER UPDATE DML trigger, if you use OCI functions or DBMS_LOB package to change the value of a LOB column or the LOB attribute of an object type column, the database does not fire the DML trigger.

See Also:

Restrictions for LOB Operations

Other general LOB restrictions include the following:

  • In SQL Loader, A field read from a LOB cannot be used as an argument to a clause. See "Database Utilities for Loading Data into LOBs".

  • Session migration is not supported for BFILEs in shared server (multithreaded server) mode. This implies that operations on open BFILEs can persist beyond the end of a call to a shared server. In shared server sessions, BFILE operations are bound to one shared server, they cannot migrate from one server to another.

  • Case-insensitive searches on CLOB colums often do not succeed. For example, to do a case-insensitive search on a CLOB colum:

    ALTER SESSION SET NLS_COMP=LINGUISTIC;
    ALTER SESSION SET NLS_SORT=BINARY_CI;
    SELECT * FROM ci_test WHERE LOWER(clob_col) LIKE 'aa%';
    

    The select fails without the LOWER function. Oracle Text does do case-insensitive searches. Use DBMS_LOB.INSTR as another alternative. See Chapter 10, "SQL Semantics and LOBs".