23
DBMS_LOB

The DBMS_LOB package provides subprograms to operate on BLOBs, CLOBs, NCLOBs, BFILEs, and temporary LOBs. You can use DBMS_LOB to access and manipulation specific parts of a LOB or complete LOBs.

This package must be created under SYS (connect internal). Operations provided by this package are performed under the current calling user, not under the package owner SYS.

DBMS_LOB can read and modify BLOBs, CLOBs, and NCLOBs; it provides read-only operations for BFILEs. The bulk of the LOB operations are provided by this package.

This chapter discusses the following topics:

• LOB Locators for DBMS_LOB
• Datatypes, Constants, and Exceptions for DBMS_LOB
• Security for DBMS_LOB
• Rules and Limitations for DBMS_LOB
• Temporary LOBs
• Summary of DBMS_LOB Subprograms

LOB Locators for DBMS_LOB

All DBMS_LOB subprograms work based on LOB locators. For the successful completion of DBMS_LOB subprograms, you must provide an input locator that represents a LOB that already exists in the database tablespaces or external file system. See also Chapter 1 of Oracle9i Application Developer's Guide - Large Objects (LOBs).

To use LOBs in your database, you must first use SQL data definition language (DDL) to define the tables that contain LOB columns.

Internal LOBs

To populate your table with internal LOBs after LOB columns are defined in a table, you use the SQL data manipulation language (DML) to initialize or populate the locators in the LOB columns.

External LOBs

For an external LOB to be represented by a LOB locator, you must:

• Ensure that a DIRECTORY object representing a valid, existing physical directory has been defined, and that physical files (the LOBs you plan to add) exist with read permission for Oracle. If your operating system uses case-sensitive path names, then be sure you specify the directory in the correct format.
• Pass the DIRECTORY object and the filename of the external LOB you are adding to the BFILENAME() function to create a LOB locator for your external LOB.

Once you have completed these tasks, you can insert or update a row containing a LOB column using the given LOB locator.

After the LOBs are defined and created, you can then SELECT from a LOB locator into a local PL/SQL LOB variable and use this variable as an input parameter to DBMS_LOB for access to the LOB value.

For details on the different ways to do this, you must refer to the section of the Oracle9i Application Developer's Guide - Large Objects (LOBs) that describes Accessing External LOBs (BFILEs).

Temporary LOBs

For temporary LOBs, you must use the OCI, PL/SQL, or another programmatic interface to create or manipulate them. Temporary LOBs can be either BLOBs, CLOBs, or NCLOBs.

Datatypes, Constants, and Exceptions for DBMS_LOB

Datatypes

Parameters for the DBMS_LOB subprograms use these datatypes:

Table 23-1 DBMS_LOB Datatypes

BLOB

RAW

CLOB

VARCHAR2

INTEGER

BFILE

The DBMS_LOB package defines no special types. NCLOB is a special case of CLOBs for fixed-width and varying-width, multibyte national character sets. The clause ANY_CS in the specification of DBMS_LOB subprograms for CLOBs enables them to accept a CLOB or NCLOB locator variable as input.

Constants

DBMS_LOB defines the following constants:

file_readonly CONSTANT BINARY_INTEGER := 0;
lob_readonly  CONSTANT BINARY_INTEGER := 0;
lob_readwrite CONSTANT BINARY_INTEGER := 1;
lobmaxsize    CONSTANT INTEGER        := 4294967295;
call          CONSTANT PLS_INTEGER    := 12;
session       CONSTANT PLS_INTEGER    := 10;

Oracle supports a maximum LOB size of 4 gigabytes (2³²). However, the amount and offset parameters of the package can have values between 1 and 4294967295 (2³²-1).

The PL/SQL 3.0 language specifies that the maximum size of a RAW or VARCHAR2 variable is 32767 bytes.

Exceptions

Table 23-2 DBMS_LOB Exceptions

invalid_argval

access_error

noexist_directory

nopriv_directory

invalid_directory

operation_failed

unopened_file

open_toomany

Security for DBMS_LOB

Any DBMS_LOB subprogram called from an anonymous PL/SQL block is executed using the privileges of the current user. Any DBMS_LOB subprogram called from a stored procedure is executed using the privileges of the owner of the stored procedure.

With Oracle8i, when creating the procedure, users can set the AUTHID to indicate whether they want definer's rights or invoker's rights. For example:

CREATE PROCEDURE proc1 authid definer ...
 

or

CREATE PROCEDURE proc1 authid current_user ....

You can provide secure access to BFILEs using the DIRECTORY feature discussed in BFILENAME function in the Oracle9i Application Developer's Guide - Large Objects (LOBs) and the Oracle9i SQL Reference.

Rules and Limitations for DBMS_LOB

• The following rules apply in the specification of subprograms in this package:
  • length and offset parameters for subprograms operating on BLOBs and BFILEs must be specified in terms of bytes.
  • length and offset parameters for subprograms operating on CLOBs must be specified in terms of characters.
  • offset and amount parameters are always in characters for CLOBs/NCLOBs and in bytes for BLOBs/BFILEs.
• A subprogram raises an INVALID_ARGVAL exception if the following restrictions are not followed in specifying values for parameters (unless otherwise specified):
  1. Only positive, absolute offsets from the beginning of LOB data are permitted: Negative offsets from the tail of the LOB are not permitted.
  2. Only positive, nonzero values are permitted for the parameters that represent size and positional quantities, such as amount, offset, newlen, nth, and so on. Negative offsets and ranges observed in Oracle SQL string functions and operators are not permitted.
  3. The value of offset, amount, newlen, nth must not exceed the value lobmaxsize (4GB-1) in any DBMS_LOB subprogram.
  4. For CLOBs consisting of fixed-width multibyte characters, the maximum value for these parameters must not exceed (lobmaxsize/character_width_in_bytes) characters.

     For example, if the CLOB consists of 2-byte characters, such as:

     JA16SJISFIXED
     
     

     Then, the maximum amount value should not exceed:

     4294967295/2 = 2147483647 characters.
     
     

• PL/SQL language specifications stipulate an upper limit of 32767 bytes (not characters) for RAW and VARCHAR2 parameters used in DBMS_LOB subprograms. For example, if you declare a variable to be:

  charbuf VARCHAR2(3000)
  
  

  Then, charbuf can hold 3000 single byte characters or 1500 2-byte fixed width characters. This has an important consequence for DBMS_LOB subprograms for CLOBs and NCLOBs.

• The %CHARSET clause indicates that the form of the parameter with %CHARSET must match the form of the ANY_CS parameter to which it refers.

  For example, in DBMS_LOB subprograms that take a VARCHAR2 buffer parameter, the form of the VARCHAR2 buffer must match the form of the CLOB parameter. If the input LOB parameter is of type NCLOB, then the buffer must contain NCHAR data. Conversely, if the input LOB parameter is of type CLOB, then the buffer must contain CHAR data.

  For DBMS_LOB subprograms that take two CLOB parameters, both CLOB parameters must have the same form; that is, they must both be NCLOBs, or they must both be CLOBs.

• If the value of amount plus the offset exceeds 4 GB (that is, lobmaxsize+1) for BLOBs and BFILEs, and (lobmaxsize/character_width_in_bytes)+1 for CLOBs in calls to update subprograms (that is, APPEND, COPY, TRIM, WRITE and WRITEAPPEND subprograms), then access exceptions are raised.

  Under these input conditions, read subprograms, such as READ, COMPARE, INSTR, and SUBSTR, read until End of Lob/File is reached. For example, for a READ operation on a BLOB or BFILE, if the user specifies offset value of 3 GB and an amount value of 2 GB, then READ reads only ((4GB-1)-3GB) bytes.

• Functions with NULL or invalid input values for parameters return a NULL. Procedures with NULL values for destination LOB parameters raise exceptions.
• Operations involving patterns as parameters, such as COMPARE, INSTR, and SUBSTR do not support regular expressions or special matching characters (such as % in the LIKE operator in SQL) in the pattern parameter or substrings.
• The End Of LOB condition is indicated by the READ procedure using a NO_DATA_FOUND exception. This exception is raised only upon an attempt by the user to read beyond the end of the LOB/FILE. The READ buffer for the last read contains 0 bytes.
• For consistent LOB updates, you must lock the row containing the destination LOB before making a call to any of the procedures (mutators) that modify LOB data.
• Unless otherwise stated, the default value for an offset parameter is 1, which indicates the first byte in the BLOB or BFILE data, and the first character in the CLOB or NCLOB value. No default values are specified for the amount parameter -- you must input the values explicitly.
• You must lock the row containing the destination internal LOB before calling any subprograms that modify the LOB, such as APPEND, COPY, ERASE, TRIM, or WRITE. These subprograms do not implicitly lock the row containing the LOB.

BFILE-Specific Rules and Limitations

• The subprograms COMPARE, INSTR, READ, SUBSTR, FILECLOSE, FILECLOSEALL and LOADFROMFILE operate only on an opened BFILE locator; that is, a successful FILEOPEN call must precede a call to any of these subprograms.
• For the functions FILEEXISTS, FILEGETNAME and GETLENGTH, a file's open/close status is unimportant; however, the file must exist physically, and you must have adequate privileges on the DIRECTORY object and the file.
• DBMS_LOB does not support any concurrency control mechanism for BFILE operations.
• In the event of several open files in the session whose closure has not been handled properly, you can use the FILECLOSEALL subprogram to close all files opened in the session and resume file operations from the beginning.
• If you are the creator of a DIRECTORY, or if you have system privileges, then use the CREATE OR REPLACE, DROP, and REVOKE statements in SQL with extreme caution.

  If you, or other grantees of a particular directory object, have several open files in a session, then any of the preceding commands can adversely affect file operations. In the event of such abnormal termination, your only choice is to invoke a program or anonymous block that calls FILECLOSEALL, reopen your files, and restart your file operations.

• All files opened during a user session are implicitly closed at the end of the session. However, Oracle strongly recommends that you close the files after both normal and abnormal termination of operations on the BFILE.

  In the event of normal program termination, proper file closure ensures that the number of files that are open simultaneously in the session remains less than SESSION_MAX_OPEN_FILES.

  In the event of abnormal program termination from a PL/SQL program, it is imperative that you provide an exception handler that ensures closure of all files opened in that PL/SQL program. This is necessary because after an exception occurs, only the exception handler has access to the BFILE variable in its most current state.

  After the exception transfers program control outside the PL/SQL program block, all references to the open BFILEs are lost. The result is a larger open file count which may or may not exceed the SESSION_MAX_OPEN_FILES value.

  For example, consider a READ operation past the end of the BFILE value, which generates a NO_DATA_FOUND exception:

  DECLARE 
         fil BFILE; 
         pos INTEGER; 
         amt BINARY_INTEGER; 
         buf RAW(40); 
  BEGIN 
         SELECT f_lob INTO fil FROM lob_table WHERE key_value = 21; 
         dbms_lob.open(fil, dbms_lob.lob_readonly);   
         amt := 40; pos := 1 + dbms_lob.getlength(fil); buf := ''; 
         dbms_lob.read(fil, amt, pos, buf); 
         dbms_output.put_line('Read F1 past EOF: '|| 
             utl_raw.cast_to_varchar2(buf));
         dbms_lob.close(fil); 
  END;
        
  ORA-01403: no data found 
  ORA-06512: at "SYS.DBMS_LOB", line 373 
  ORA-06512: at line 10 
  
  

  After the exception has occurred, the BFILE locator variable file goes out of scope, and no further operations on the file can be done using that variable. Therefore, the solution is to use an exception handler:

  DECLARE
       fil BFILE; 
       pos INTEGER;
       amt BINARY_INTEGER; 
       buf RAW(40); 
  BEGIN 
       SELECT f_lob INTO fil FROM lob_table WHERE key_value = 21;
       dbms_lob.open(fil, dbms_lob.lob_readonly);   
       amt := 40; pos := 1 + dbms_lob.getlength(fil); buf := ''; 
       dbms_lob.read(fil, amt, pos, buf); 
       dbms_output.put_line('Read F1 past EOF: '|| 
            utl_raw.cast_to_varchar2(buf)); 
       dbms_lob.close(fil); 
       exception 
       WHEN no_data_found 
       THEN 
         BEGIN 
           dbms_output.put_line('End of File reached. Closing file'); 
           dbms_lob.fileclose(fil); 
      -- or dbms_lob.filecloseall if appropriate 
         END; 
  END; 
       / 
  
  Statement processed. 
  End of File reached. Closing file
  

  In general, you should ensure that files opened in a PL/SQL block using DBMS_LOB are closed before normal or abnormal termination of the block.

Temporary LOBs

Oracle8i supports the definition, creation, deletion, access, and update of temporary LOBs. Your temporary tablespace stores the temporary LOB data. Temporary LOBs are not permanently stored in the database. Their purpose is mainly to perform transformations on LOB data.

A temporary LOB is empty when it is created. By default, all temporary LOBs are deleted at the end of the session in which they were created. If a process dies unexpectedly or if the database crashes, then temporary LOBs are deleted, and the space for temporary LOBs is freed.

In Oracle8i, there is also an interface to let you group temporary LOBs together into a logical bucket. The duration represents this logical store for temporary LOBs. Each temporary LOB can have separate storage characteristics, such as CACHE/ NOCACHE. There is a default store for every session into which temporary LOBs are placed if you don't specify a specific duration. Additionally, you are able to perform a free operation on durations, which causes all contents in a duration to be freed.

There is no support for consistent read (CR), undo, backup, parallel processing, or transaction management for temporary LOBs. Because CR and rollbacks are not supported for temporary LOBs, you must free the temporary LOB and start over again if you encounter an error.

Because CR, undo, and versions are not generated for temporary LOBs, there is potentially a performance impact if you assign multiple locators to the same temporary LOB. Semantically, each locator should have its own copy of the temporary LOB.

A copy of a temporary LOB is created if the user modifies the temporary LOB while another locator is also pointing to it. The locator on which a modification was performed now points to a new copy of the temporary LOB. Other locators no longer see the same data as the locator through which the modification was made. A deep copy was not incurred by permanent LOBs in these types of situations, because CR snapshots and version pages enable users to see their own versions of the LOB cheaply.

You can gain pseudo-REF semantics by using pointers to locators in OCI and by having multiple pointers to locators point to the same temporary LOB locator, if necessary. In PL/SQL, you must avoid using more than one locator for each temporary LOB. The temporary LOB locator can be passed by reference to other procedures.

Because temporary LOBs are not associated with any table schema, there are no meanings to the terms in-row and out-of-row temporary LOBs. Creation of a temporary LOB instance by a user causes the engine to create and return a locator to the LOB data. The PL/SQL DBMS_LOB package, PRO*C, OCI, and other programmatic interfaces operate on temporary LOBs through these locators just as they do for permanent LOBs.

There is no support for client side temporary LOBs. All temporary LOBs reside in the server.

Temporary LOBs do not support the EMPTY_BLOB or EMPTY_CLOB functions that are supported for permanent LOBs. The EMPTY_BLOB function specifies the fact that the LOB is initialized, but not populated with any data.

A temporary LOB instance can only be destroyed by using OCI or the DBMS_LOB package by using the appropriate FREETEMPORARY or OCIDurationEnd statement.

A temporary LOB instance can be accessed and modified using appropriate OCI and DBMS_LOB statements, just as for regular permanent internal LOBs. To make a temporary LOB permanent, you must explicitly use the OCI or DBMS_LOB COPY command, and copy the temporary LOB into a permanent one.

Security is provided through the LOB locator. Only the user who created the temporary LOB is able to see it. Locators are not expected to be able to pass from one user's session to another. Even if someone did pass a locator from one session to another, they would not access the temporary LOBs from the original session. Temporary LOB lookup is localized to each user's own session. Someone using a locator from somewhere else is only able to access LOBs within his own session that have the same LOB ID. Users should not try to do this, but if they do, they are not able to affect anyone else's data.

Oracle keeps track of temporary LOBs for each session in a v$ view called V$TEMPORARY_LOBS, which contains information about how many temporary LOBs exist for each session. V$ views are for DBA use. From the session, Oracle can determine which user owns the temporary LOBs. By using V$TEMPORARY_LOBS in conjunction with DBA_SEGMENTS, a DBA can see how much space is being used by a session for temporary LOBs. These tables can be used by DBAs to monitor and guide any emergency cleanup of temporary space used by temporary LOBs.

Temporary LOBs Usage Notes

1. All functions in DBMS_LOB return NULL if any of the input parameters are NULL. All procedures in DBMS_LOB raise an exception if the LOB locator is input as NULL.
2. Operations based on CLOBs do not verify if the character set IDs of the parameters (CLOB parameters, VARCHAR2 buffers and patterns, and so on) match. It is the user's responsibility to ensure this.
3. Data storage resources are controlled by the DBA by creating different temporary tablespaces. DBAs can define separate temporary tablespaces for different users, if necessary.
4. Temporary LOBs still adhere to value semantics in order to be consistent with permanent LOBs and to try to conform to the ANSI standard for LOBs. As a result, each time a user does an OCILobLocatatorAssign, or the equivalent assignment in PL/SQL, the database makes a copy of the temporary LOB.

   Each locator points to its own LOB value. If one locator is used to create a temporary LOB, and then is assigned to another LOB locator using OCILobLOcatorAssign in OCI or through an assignment operation in PL/SQL, then the database copies the original temporary LOB and causes the second locator to point to the copy.

   In order for users to modify the same LOB, they must go through the same locator. In OCI, this can be accomplished fairly easily by using pointers to locators and assigning the pointers to point to the same locator. In PL/SQL, the same LOB variable must be used to update the LOB to get this effect.

   The following example shows a place where a user incurs a copy, or at least an extra roundtrip to the server.

   DECLARE 
     a blob; 
     b blob; 
   BEGIN 
     dbms_lob.createtemporary(b, TRUE); 
     -- the following assignment results in a deep copy 
     a := b; 
   END; 
    
   

   The PL/SQL compiler makes temporary copies of actual arguments bound to OUT or IN OUT parameters. If the actual parameter is a temporary LOB, then the temporary copy is a deep (value) copy.

   The following PL/SQL block illustrates the case where the user incurs a deep copy by passing a temporary LOB as an IN OUT parameter.

   DECLARE 
     a blob; 
     procedure foo(parm IN OUT blob) is 
     BEGIN 
      ... 
     END; 
   BEGIN 
     dbms_lob.createtemporary(a, TRUE); 
     -- the following call results in a deep copy of the blob a 
     foo(a); 
   END; 
    
   

   To minimize deep copies on PL/SQL parameter passing, use the NOCOPY compiler hint where possible.

   The duration parameter passed to dbms_lob.createtemporary() is a hint. The duration of the new temp LOB is the same as the duration of the locator variable in PL/SQL. For example, in the preceding program block, the program variable a has the duration of the residing frame. Therefore at the end of the block, memory of a will be freed at the end of the function.

   If a PL/SQL package variable is used to create a temp LOB, it will have the duration of the package variable, which has a duration of SESSION.

   BEGIN 
      y clob; 
     END; 
   /
   BEGIN 
      dbms_lob.createtemporary(package.y, TRUE); 
   END; 
   

   See Also: . PL/SQL User's Guide and Reference for more information on NOCOPY syntax

Exceptions

Table 23-3 DBMS_LOB Exceptions

INVALID_ARGVAL

ACCESS_ERROR

NO_DATA_FOUND

VALUE_ERROR