Oracle8i Application Developer's Guide - Large Objects (LOBs)
Release 2 (8.1.6)

Part Number A76940-01





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Basic Components, 4 of 5

The LOB Locator

LOB Value and Locators

Inline storage of the LOB value

Data stored in a LOB is termed the LOB's value. The value of an internal LOB may or may not be stored inline with the other row data. If you do not set DISABLE STORAGE IN ROW and the internal LOB value is less than approximately 4,000 bytes, then the value is stored inline; otherwise it is stored outside the row. Since LOBs are intended to be large objects, inline storage will only be relevant if your application mixes small and large LOBs.

As mentioned in Chapter 7, "Modeling and Design", "ENABLE | DISABLE STORAGE IN ROW" , the LOB value is automatically moved out of the row once it extends beyond approximately 4,000 bytes.

LOB Locators

Regardless of where the value of the internal LOB is stored, a locator is stored in the row. You can think of a LOB locator as a pointer to the actual location of the LOB value. A LOB locator is a locator to an internal LOB while a BFILE locator is a locator to an external LOB. When the term locator is used without an identifying prefix term, it refers to both LOB locators and BFILE locators.

LOB Locator Operations

Setting the LOB Column/Attribute to Contain a Locator

Invoking 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 routine will raise 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.

The following INSERT statement:

See Chapter 8, "Sample Application", for the definition of table Multimedia_tab.

INSERT INTO Multimedia_tab VALUES (101, 'JFK interview', EMPTY_CLOB(), NULL,
    BFILENAME('AUDIO_DIR', 'JFK_interview'), NULL);

Similarly, the LOB attributes for the Map_typ column in Multimedia_tab can be initialized to NULL or set to empty as shown below. Note that you cannot initialize a LOB object attribute with a literal.

INSERT INTO Multimedia_tab 
          EMPTY_BLOB(), NULL, NULL, NULL, 
          Map_typ('Moon Mountain', 23, 34, 45, 56, EMPTY_BLOB(), NULL);

Accessing a LOB Through a Locator


Performing a SELECT on a LOB returns the locator instead of the LOB value. In the following PL/SQL fragment you select the LOB locator for story and place it in the PL/SQL locator variable Image1 defined in the program block. When you use PL/SQL DBMS_LOB functions to manipulate the LOB value, you refer to the LOB using the locator.

    Image1       BLOB;
    ImageNum     INTEGER := 101;
    SELECT story INTO Image1 FROM Multimedia_tab
        WHERE clip_id = ImageNum;
    DBMS_OUTPUT.PUT_LINE('Size of the Image is: ' ||
    /* more LOB routines */

In the case of OCI, locators are mapped to locator pointers which are used to manipulate the LOB value. The OCI LOB interface is described Chapter 3, "LOB Programmatic Environments" and in the Oracle Call Interface Programmer's Guide.

Using LOB locators and transaction boundaries, and read consistent locators are described in Chapter 5, "Advanced Topics".

1 Note:You could use SQL to populate a LOB column with data even if it contained NULL, i.e., unless its a LOB attribute. However, you cannot use one of the six programmatic environment interfaces on a NULL LOB!

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