1. Database SecureFiles and Large Objects Developer's Guide
  2. Value Semantics LOBs
  3. PL/SQL Semantics for LOBs

8 PL/SQL Semantics for LOBs

Topics:

8.1 PL/SQL Statements and Variables

In PL/SQL, semantic changes have been made.

Note:

Most discussions concerning PL/SQL semantics, and CLOBs and VARCHAR2s, also apply to BLOBs and RAWs, unless otherwise noted. In the text, BLOB and RAW are not explicitly mentioned.

PL/SQL semantics support is described in the following sections:

8.2 Implicit Conversions Between CLOB and VARCHAR2

Implicit conversions from CLOB to VARCHAR2 and from VARCHAR2 to CLOB data types are allowed in PL/SQL.

These conversions enable you to perform the following operations in your application:

  • CLOB columns can be selected into VARCHAR2 PL/SQL variables

  • VARCHAR2 columns can be selected into CLOB variables

  • Assignment and parameter passing between CLOBs and VARCHAR2s

Accessing a CLOB as a VARCHAR2 in PL/SQL

The following example illustrates the way CLOB data is accessed when the CLOBs are treated as VARCHAR2s: 

declare
   myStoryBuf VARCHAR2(4001);
BEGIN
   SELECT ad_sourcetext INTO myStoryBuf FROM print_media WHERE ad_id = 12001;
   -- Display Story by printing myStoryBuf directly
END;
/

Assigning a CLOB to a VARCHAR2 in PL/SQL

declare
myLOB CLOB;
BEGIN
SELECT 'ABCDE' INTO myLOB FROM print_media WHERE ad_id = 11001;
-- myLOB is a temporary LOB.
-- Use myLOB as a lob locator
  DBMS_OUTPUT.PUT_LINE('Is temp? '||DBMS_LOB.ISTEMPORARY(myLOB));
END;
/

8.3 Explicit Conversion Functions

In SQL and PL/SQL, the certain explicit conversion functions convert other data types to and from CLOB, NCLOB, and BLOB as part of the LONG-to-LOB migration:

  • TO_CLOB(): Converting from VARCHAR2, NVARCHAR2, or NCLOB to a CLOB

  • TO_NCLOB(): Converting from VARCHAR2, NVARCHAR2, or CLOB to an NCLOB

  • TO_BLOB(): Converting from RAW to a BLOB

  • TO_CHAR() converts a CLOB to a CHAR type. When you use this function to convert a character LOB into the database character set, if the LOB value to be converted is larger than the target type, then the database returns an error. Implicit conversions also raise an error if the LOB data does not fit.

  • TO_NCHAR() converts an NCLOB to an NCHAR type. When you use this function to convert a character LOB into the national character set, if the LOB value to be converted is larger than the target type, then the database returns an error. Implicit conversions also raise an error if the LOB data does not fit.

  • CAST does not directly support any of the LOB data types. When you use CAST to convert a CLOB value into a character data type, an NCLOB value into a national character data type, or a BLOB value into a RAW data type, the database implicitly converts the LOB value to character or raw data and then explicitly casts the resulting value into the target data type. If the resulting value is larger than the target type, then the database returns an error.

Other explicit conversion functions are not supported, such as, TO_NUMBER(), see Table 7-1.

Note:

LOBs do not support duplicate LONG binds.

See Also:

Migrating Columns from LONGs to LOBs for more information about conversion functions

8.3.1 VARCHAR2 and CLOB in PL/SQL Built-In Functions

CLOB and VARCHAR2 are two distinct types.

However, depending on the usage, a CLOB can be passed to SQL and PL/SQL VARCHAR2 built-in functions, used exactly like a VARCHAR2. Or the variable can be passed into DBMS_LOB APIs, acting like a LOB locator. See the following combined example,"CLOB Variables in PL/SQL".

PL/SQL VARCHAR2 functions and operators can take CLOBs as arguments or operands.

When the size of a VARCHAR2 variable is not large enough to contain the result from a function that returns a CLOB, or a SELECT on a CLOB column, an error is raised and no operation is performed. This is consistent with VARCHAR2 semantics.

CLOB Variables in PL/SQL

1 declare
2   myStory CLOB;
3   revisedStory CLOB;
4   myGist VARCHAR2(100);
5   revisedGist VARCHAR2(100);
6 BEGIN
7  -- select a CLOB column into a CLOB variable
8  SELECT Story INTO myStory FROM print_media WHERE product_id=10;
9  -- perform VARCHAR2 operations on a CLOB variable
10 revisedStory := UPPER(SUBSTR(myStory, 100, 1)); 
11 -- revisedStory is a temporary LOB
12 -- Concat a VARCHAR2 at the end of a CLOB
13 revisedStory := revisedStory || myGist;
14 -- The following statement raises an error because myStory is 
15 -- longer than 100 bytes
16 myGist := myStory;
17 END;

Please note that in line 10 of "CLOB Variables in PL/SQL", a temporary CLOB is implicitly created and is pointed to by the revisedStory CLOB locator. In the current interface the line can be expanded as: 

buffer VARCHAR2(32000)
DBMS_LOB.CREATETEMPORARY(revisedStory);
buffer := UPPER(DBMS_LOB.SUBSTR(myStory,100,1));
DBMS_LOB.WRITE(revisedStory,length(buffer),1, buffer);

In line 13, myGist is appended to the end of the temporary LOB, which has the same effect of: 

DBMS_LOB.WRITEAPPEND(revisedStory, myGist, length(myGist));

In some occasions, implicitly created temporary LOBs in PL/SQL statements can change the representation of LOB locators previously defined. Consider the next example.

Change in Locator-Data Linkage

1 declare
2 myStory CLOB;
3 amt number:=100;
4 buffer VARCHAR2(100):='some data';
5 BEGIN
6 -- select a CLOB column into a CLOB variable
7 SELECT Story INTO myStory FROM print_media WHERE product_id=10;
8 DBMS_LOB.WRITE(myStory, amt, 1, buf);
9 -- write to the persistent LOB in the table
10
11 myStory:= UPPER(SUBSTR(myStory, 100, 1));
12 -- perform VARCHAR2 operations on a CLOB variable, temporary LOB created.
13 -- Changes are not reflected in the database table from this point on.
14 
15 update print_media set Story = myStory WHERE product_id = 10;
16 -- an update is necessary to synchronize the data in the table.
17 END;

After line 7, myStory represents a persistent LOB in print_media.

The DBMS_LOB.WRITE call in line 8 directly writes the data to the table.

No UPDATE statement is necessary. Subsequently in line 11, a temporary LOB is created and assigned to myStory because myStory is now used like a local VARCHAR2 variable. The LOB locator myStory now points to the newly-created temporary LOB.

Therefore, modifications to myStory are no longer reflected in the database. To propagate the changes to the database table, an UPDATE statement becomes necessary now. Note again that for the previous persistent LOB, the UPDATE is not required.

Note:

If the SQL statement returns a LOB or a LOB is an OUT parameter for a PL/SQL function or procedure, you must test if it is a temporary LOB, and if it is, then free it after you are done with it.

Freeing Temporary LOBs Automatically and Manually

Temporary LOBs created in a program block as a result of a SELECT or an assignment are freed automatically at the end of the PL/SQL block or function or procedure. You must also free the temporary LOBs that were created with DBMS_LOB.CREATETEMPORARY to reclaim system resources and temporary tablespace. Do this by calling DBMS_LOB.FREETEMPORARY on the CLOB variable. 

declare
   Story1 CLOB;
   Story2 CLOB;
   StoryCombined CLOB;
   StoryLower CLOB;
BEGIN
   SELECT Story INTO Story1 FROM print_media WHERE product_ID = 1;
   SELECT Story INTO Story2 FROM print_media WHERE product_ID = 2;
   StoryCombined := Story1 || Story2; -- StoryCombined is a temporary LOB
   -- Free the StoryCombined manually to free up space taken
   DBMS_LOB.FREETEMPORARY(StoryCombined);
   StoryLower := LOWER(Story1) || LOWER(Story2);
END; -- At the end of block, StoryLower is freed.

8.4 PL/SQL Functions for Remote LOBs and BFILEs

See Also:

PL/SQL Functions for Remote LOBs and BFILEs for PL/SQL functions that support remote LOBs and BFILEs