10 PL/SQL Semantics for LOBs

VARCHAR2 and CLOB in PL/SQL Built-In Functions

CLOB and VARCHAR2 are still two distinct types. But 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. Please 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 should be raised and no operation will be 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 will raise 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 will not be 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 will no longer be 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.

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/function/procedure. You can choose to free the temporary LOBs to reclaim system resources and temporary tablespace by calling DBMS_LOB.FREETEMPORARY on the CLOB variable.

Freeing Temporary LOBs Automatically and Manually

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.

CLOBs Follow the VARCHAR2 Collating Sequence

As a rule, CLOBs follow the same collating sequence as VARCHAR2s. That is, when a CLOB is compared, the result is consistent with if the CLOB data content is retrieved into a VARCHAR2 buffer and the VARCHAR2 is compared. The rule applies to all cases including comparisons between CLOB and CLOB, CLOB and VARCHAR2, and CLOB and CHAR.

It makes no sense to compare CLOBs with non-character data, or with BLOBs. An error is returned in these cases.