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Oracle® Call Interface Programmer's Guide
11g Release 2 (11.2)

E10646-15
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7 LOB and BFILE Operations

This chapter contains these topics:

Using OCI Functions for LOBs

OCI includes a set of functions for performing operations on large objects (LOBs) in a database. Persistent LOBs (BLOBs, CLOBs, NCLOBs) are stored in the database tablespaces in a way that optimizes space and provides efficient access. These LOBs have the full transactional support of the Oracle database. BFILEs are large data objects stored in the server's operating system files outside the database tablespaces.

OCI also provides support for temporary LOBs, which can be used like local variables for operating on LOB data.

BFILEs are read-only. Oracle Database supports only binary BFILEs.

See Also:

Creating and Modifying Persistent LOBs

LOB instances can be either persistent (stored in the database) or temporary (existing only in the scope of your application). Do not confuse the concept of a persistent LOB with a persistent object.

There are two ways of creating and modifying persistent LOBs:

  • Using the data interface

    You can create a LOB by inserting character data into a CLOB column or RAW data into a BLOB column directly. You can also modify LOBs by using a SQL UPDATE statement, to bind character data into a CLOB column or RAW data into a BLOB column.

    Insert, update, and select of remote LOBs (over a dblink) is supported because neither the remote server nor the local server is of a release earlier than Oracle Database 10g Release 2. The data interface only supports data size up to 2 GB – 1, the maximum size of an sb4 data type.

    See Also:

    Oracle Database SecureFiles and Large Objects Developer's Guide chapter about data interface for persistent LOBs for more information and examples
  • Using the LOB locator

    You create a new internal LOB by initializing a new LOB locator using OCIDescriptorAlloc(), calling OCIAttrSet() to set it to empty (using the OCI_ATTR_LOBEMPTY attribute), and then binding the locator to a placeholder in an INSERT statement. Doing so inserts the empty locator into a table with a LOB column or attribute. You can then perform a SELECT...FOR UPDATE operation on this row to get the locator, and write to it using one of the OCI LOB functions.

    Note:

    To modify a LOB column or attribute (write, copy, trim, and so forth), you must lock the row containing the LOB. One way to do this is to use a SELECT...FOR UPDATE statement to select the locator before performing the operation.

See Also:

"Binding LOB Data" for usage and examples for both INSERT and UPDATE

For any LOB write command to be successful, a transaction must be open. If you commit a transaction before writing the data, you must lock the row again (by reissuing the SELECT...FOR UPDATE statement, for example), because the commit closes the transaction.

Associating a BFILE in a Table with an Operating System File

The BFILENAME function can be used in an INSERT statement to associate an external server-side (operating system) file with a BFILE column or attribute in a table. Using BFILENAME in an UPDATE statement associates the BFILE column or attribute with a different operating system file. OCILobFileSetName() can also be used to associate a BFILE in a table with an operating system file. BFILENAME is usually used in an INSERT or UPDATE statement without bind variables, and OCILobFileSetName() is used for bind variables.

See Also:

LOB Attributes of an Object

An OCI application can use the OCIObjectNew() function to create a persistent or transient object with a LOB attribute.

Writing to a LOB Attribute of an Object

It is possible to use OCI to create a new persistent object with a LOB attribute and write to that LOB attribute. The application would follow these steps when using a LOB locator:

  1. Call OCIObjectNew() to create a persistent object with a LOB attribute.

  2. Mark the object as "dirty" (modified).

  3. Flush the object, thereby inserting a row into the table.

  4. Repin the latest version of the object (or refresh the object), thereby retrieving the object from the database and acquiring a valid locator for the LOB.

  5. Call OCILobWrite2() using the LOB locator in the object to write the data.

    See Also:

    Chapter 11 and the chapters that follow it for more information about objects

There is a second way of writing to a LOB attribute. When using the data interface, you can bind or define character data for a CLOB attribute or RAW data for a BLOB attribute.

See Also:

Transient Objects with LOB Attributes

An application can call OCIObjectNew() and create a transient object with an internal LOB (BLOB, CLOB, NCLOB) attribute. However, you cannot perform any operations, such as read or write, on the LOB attribute because transient objects with LOB attributes are not supported. Calling OCIObjectNew() to create a transient internal LOB type does not fail, but the application cannot use any LOB operations with the transient LOB.

An application can, however, create a transient object with a BFILE attribute and use the BFILE attribute to read data from a file stored in the server's file system. The application can also call OCIObjectNew() to create a transient BFILE.

Array Interface for LOBs

You can use the OCI array interface with LOBs, just as with any other data type. There are two ways of using the array interface.

  • Using the data interface

    You can bind or define arrays of character data for a CLOB column or RAW data for a BLOB column. You can use array bind and define interfaces to insert and select multiple rows with LOBs in one round-trip to the server.

    See Also:

  • Using the LOB locator

    When using the LOB locator you must allocate the descriptors, as shown in Example 7-1.

    Example 7-1 Using the LOB Locator and Allocating the Descriptors

    /* First create an array of OCILobLocator pointers: */
    OCILobLocator *lobp[10];
    
    for (i=0; i < 10; i++)
    { OCIDescriptorAlloc (...,&lobp[i],...);
    
    /* Then bind the descriptor as follows */
      OCIBindByPos(... &lobp[i], ...);
    

Using LOBs of Size Greater than 4 GB

Starting with Oracle Database 10g Release 1 of OCI, functions were introduced to support LOBs of size greater than 4 GB. These new functions can also be used in new applications for LOBs of less than 4 GB.

Oracle Database enables you to create tablespaces with block sizes different from the database block size. The maximum size of a LOB depends on the size of the tablespace blocks. The tablespace block size in which the LOB is stored controls the value of CHUNK, which is a parameter of LOB storage. When you create a LOB column, you specify a value for CHUNK, which is the number of bytes to be allocated for LOB manipulation. The value must be a multiple of the tablespace block size, or Oracle Database rounds up to the next multiple. (If the tablespace block size equals the database block size, then CHUNK is also a multiple of the database block size.) The default CHUNK size is one tablespace block, and the maximum value is 32 KB.

In this guide, 4 GB is defined as 4 gigabytes – 1, or 4,294,967,295 bytes. The maximum size of a LOB, persistent or temporary, is (4 gigabytes – 1) * (CHUNK). The maximum LOB size can range from 8 terabytes (TB) to 128 TB.

For example, suppose that your database block size is 32 KB and you create a tablespace with a nonstandard block size of 8 KB. Further suppose that you create a table with a LOB column and specify a CHUNK size of 16 KB (which is a multiple of the 8 KB tablespace block size). Then the maximum size of a LOB in this column is (4 gigabytes – 1) * 16 KB.

The maximum size of a BFILE is the maximum file size allowed in the operating system, or UB8MAXVAL, whichever is smaller.

Older LOB functions use ub4 as the data types of some parameters, and the ub4 data type can only hold up to 4 GB. The newer functions use parameters of 8-byte length, oraub8, which is a data type defined in oratypes.h. The data types oraub8 and orasb8 are mapped to appropriate 64-bit native data types depending on the compiler and operating system. Macros are used to not define oraub8 and orasb8 if compiling in 32-bit mode with strict ANSI option.

OCILobGetChunkSize() returns the usable chunk size in bytes for BLOBs, CLOBs, and NCLOBs. The number of bytes stored in a chunk is actually less than the size of the CHUNK parameter due to internal storage overhead. The function OCILobGetStorageLimit() is provided to return the maximum size in bytes of internal LOBs in the current installation.

Note:

Oracle Database does not support BFILEs larger than 4 gigabytes in any programmatic environment. An additional file size limit imposed by your operating system also applies to BFILEs.

Functions to Use for the Increased LOB Sizes

Eight functions with names that end in "2" and that use the data type oraub8 in place of the data type ub4 were introduced in Oracle Database 10g Release 1. Other changes were made in the read and write functions (OCILobRead2(), OCILobWrite2(), and OCILobWriteAppend2()) to solve several problems:

Problem: Before Oracle Database 10g Release 1, the parameter amtp assumed either byte or char length for LOBs based on the locator type and character set. It was complicated and users did not have the flexibility to use byte length or char length according to their requirements.

Solution: Read/Write calls should take both byte_amtp and char_amtp parameters as replacement for the amtp parameter. The char_amtp parameter is preferred for CLOB and NCLOB, and the byte_amtp parameter is only considered as input if char_amtp is zero. On output for CLOB and NCLOB, both byte_amtp and char_amtp parameters are filled. For BLOB and BFILE, the char_ampt parameter is ignored for both input and output.

Problem: For OCILobRead2(), there is no flag to indicate polling mode. There is no easy way for the users to say "I have a 100-byte buffer. Fill it as much as you can." Previously, they had to estimate how many characters to specify for the amount. If they guessed too much, they were forced into polling mode unintentionally. The user code thus can get trapped in the polling mode and subsequent OCI calls are all blocked.

Solution: This call should take piece as an input parameter and if OCI_ONE_PIECE is passed, it should fill the buffer as much as possible and come out even if the amount indicated by the byte_amtp parameter or char_amtp parameter is more than the buffer length. The value of bufl is used to specify the maximum amount of bytes to read.

Problem: After calling for a LOB write in polling mode, users do not know how many chars or bytes are actually fetched till the end of the polling.

Solution: Both the byte_amtp and char_amtp parameters must be updated after each call in polling mode.

Problem: While reading or writing data in streaming mode with callback, users must use the same buffer for each piece of data.

Solution: The callback function must have two new parameters to provide the buffer and the buffer length. Callback functions can set the buffer parameter to NULL to follow old behavior: to use the default buffer passed in the first call for all the pieces.

Compatibility and Migration

Existing OCI programs can be enhanced to process larger amounts of LOB data that are greater than 4 GB. Table 7-1 summarizes compatibility issues in this table, "old" refers to releases before Oracle Database 10g Release 1, and NA means not applicable.

Table 7-1 LOB Functions Compatibility and Migration

LOB Function Old Client/New or Old ServerFoot 1  New Client/Old Server New Client/New Server

OCILobArrayRead()

Foot 2 NA

OK until piece size and offset are < 4 GB.

OK

OCILobArrayWrite()

NA

OK until piece size and offset are < 4 GB.

OK

OCILobCopy2()

NA

OK until LOB size, piece size (amount) and offset are < 4 GB.

OK

OCILobCopy()

OK; limit is 4 GB.

OK

OK; limit is 4 GB.

OCILobErase2()

NA

OK until piece size and offset are < 4 GB.

OK

OCILobErase()

OK; limit is 4 GB.

OK

OK; limit is 4 GB.

OCILobGetLength2()

NA

OK

OK

OCILobGetLength()

OK; limit is 4 GB.

OK

OK; OCI_ERROR if LOB size > 4 GB.

OCILobLoadFromFile2()

NA

OK until LOB size, piece size (amount), and offset are < 4 GB.

OK

OCILobLoadFromFile()

OK; limit is 4 GB.

OK

OK; limit is 4 GB.

OCILobRead2()

NA

OK until LOB size, piece size (amount), and offset are < 4 GB.

OK

OCILobRead()

OK; limit 4 GB.

With new server: OCI_ERROR is returned if you try to read any amount >= 4 GB from any offset < 4 GB. This is because when you read any amount >= 4 GB, that results in an overflow of returned value in *amtp, and so it is flagged as an error.

Note:

  • If you read up to 4 GB – 1 from offset, that is not flagged as an error.

  • When you use streaming mode with polling, no error is returned if no attempt is made to use piece size > 4 GB (you can read data > 4 GB in this case).

OK

OK.

OCI_ERROR is returned if you try to read any amount >= 4 GB from any offset < 4 GB. This is because when you read any amount >= 4 GB, that results in an overflow of returned value in *amtp, and so it is flagged as an error.

Note:

  • If you read up to 4 GB – 1 from offset, that is not to be flagged as an error.

  • When you use streaming mode with polling, no error is returned if no attempt is made to use piece size > 4 GB.

OCILobTrim2()

NA

OK

OK

OCILobTrim()

OK; limit 4 GB.

OK

OK; limit 4 GB.

OCILobWrite2()

NA

OK until LOB size, piece size (amount) and offset are < 4 GB.

OK

OCILobWrite()

OK; limit 4 GB.

With new server:

OCI_ERROR is returned if you write any amount >= 4 GB (from any offset < 4 GB) because that results an in overflow of returned value in *amtp.

Note: Updating a LOB of 10 GB from any offset up to 4 GB –1 by up to 4 GB –1 amount of data is not flagged as an error.

OK

OK.

OCI_ERROR is returned if you write any amount > =4 GB (from any offset < 4 GB) because that results in an overflow of returned value in *amtp.

Note: Updating a LOB of 10 GB from any offset up to 4 GB –1 by up to 4 GB –1 amount of data is not flagged as an error.

OCILobWriteAppend2()

NA

OK until LOB size and piece size are <4 GB.

OK

OCILobWriteAppend()

OK; limit 4 GB.

With new server: OCI_ERROR is returned if you append any amount >= 4 GB of data because that results in an overflow of returned value in *amtp.

OK

OK; limit 4 GB.

OCI_ERROR is returned if you append any amount >= 4 GB of data because that results in an overflow of returned value in *amtp.

OCILobGetStorageLimit()

NA

Error

OK


Footnote 1 The term "old" refers to releases before Oracle Database 10g Release 1.

Footnote 2 NA means not applicable.

Use the functions that end in "2" when using the current server and current client. Mixing deprecated functions with functions that end in "2" can result in unexpected situations, such as data written using OCILobWrite2() being greater than 4 GB if the application tries to read it with OCILobRead() and gets only partial data (if a callback function is not used). In most cases, the application gets an error message when the size crosses 4 GB and the deprecated functions are used. However, there is no issue if you use those deprecated functions for LOBs of size smaller than 4 GB.

LOB and BFILE Functions in OCI

In all LOB operations that involve offsets into the data, the offset begins at 1. For LOB operations, such as OCILobCopy2(), OCILobErase2(), OCILobLoadFromFile2(), and OCILobTrim2(), the amount parameter is in characters for CLOBs and NCLOBs, regardless of the client-side character set.

These LOB operations refer to the amount of LOB data on the server. When the client-side character set is of varying width, the following general rules apply to the amount and offset parameters in LOB calls:

  • amount - When the amount parameter refers to the server-side LOB, the amount is in characters. When the amount parameter refers to the client-side buffer, the amount is in bytes.

  • offset - Regardless of whether the client-side character set is varying-width, the offset parameter is always in characters for CLOBs or NCLOBs and in bytes for BLOBs or BFILEs.

Exceptions to these general rules are noted in the description of the specific LOB call.

Improving LOB Read/Write Performance

Here are some hints to improve performance.

Using Data Interface for LOBs

You can bind or define character data for a CLOB column or RAW data for a BLOB column. This requires only one round-trip for inserting or selecting a LOB, as opposed to the traditional LOB interface that requires multiple round-trips.

See Also:

Using OCILobGetChunkSize()

OCILobGetChunkSize() returns the usable chunk size in bytes for BLOBs, CLOBs, and NCLOBs. You can use the OCILobGetChunkSize() call to improve the performance of LOB read and write operations for BasicFile LOBs. When a read or write is done on BasicFile LOB data whose size is a multiple of the usable chunk size and the operation starts on a chunk boundary, performance is improved. There is no requirement for SecureFile LOBs to be written or read with OCILobGetChunkSize() alignment.

Calling OCILobGetChunkSize() returns the usable chunk size of the LOB, so that an application can batch a series of write operations for the entire chunk, rather than issuing multiple LOB write calls for the same chunk.

Using OCILobWriteAppend2()

OCI provides a shortcut for more efficient writing of data to the end of a LOB. The OCILobWriteAppend2() call appends data to the end of a LOB without first requiring a call to OCILobGetLength2() to determine the starting point for an OCILobWrite2() operation. OCILobWriteAppend2() does both steps.

Using OCILobArrayRead() and OCILobArrayWrite()

You can improve performance by using by using OCILobArrayRead() to read LOB data for multiple LOB locators and OCILobArrayWrite() to write LOB data for multiple LOB locators. These functions, which were introduced in Oracle Database 10g Release 2, reduce the number of round-trips for these operations.

See Also:

Oracle Database SecureFiles and Large Objects Developer's Guide, sections "LOB Array Read" and "LOB Array Write" for more information and code examples that show how to use these functions with callback functions and in piecewise mode

LOB Buffering Functions

OCI provides several calls for controlling LOB buffering for small reads and writes of internal LOB values:

These functions enable applications that are using internal LOBs (BLOB, CLOB, NCLOB) to buffer small reads and writes in client-side buffers. This reduces the number of network round-trips and LOB versions, thereby improving LOB performance significantly.

See Also:

Functions for Opening and Closing LOBs

OCI provides functions to explicitly open a LOB, OCILobOpen(), to close a LOB, OCILobClose(), and to test whether a LOB is open, OCILobIsOpen(). These functions mark the beginning and end of a series of LOB operations so that specific processing, such as updating indexes, can be performed when a LOB is closed.

For internal LOBs, the concept of openness is associated with a LOB and not its locator. The locator does not store any information about the state of the LOB. It is possible for more than one locator to point to the same open LOB. However, for BFILEs, being open is associated with a specific locator. Hence, more than one open call can be performed on the same BFILE by using different locators.

If an application does not wrap LOB operations within a set of OCILobOpen() and OCILobClose() calls, then each modification to the LOB implicitly opens and closes the LOB, thereby firing any triggers associated with changes to the LOB.

If LOB operations are not wrapped within open and close calls, any extensible indexes on the LOB are updated as LOB modifications are made, and thus are always valid and may be used at any time. If the LOB is modified within a set of OCILobOpen() and OCILobClose() calls, triggers are not fired for individual LOB modifications. Triggers are only fired after the OCILobClose() call, so indexes are not updated until after the close call and thus are not valid within the open and close calls. OCILobIsOpen() can be used with internal LOBs and BFILEs.

An error is returned when you commit the transaction before closing all opened LOBs that were opened by the transaction. When the error is returned, the LOB is no longer marked as open, but the transaction is successfully committed. Hence, all the changes made to the LOB and non-LOB data in the transaction are committed, but the domain and functional indexing are not updated. If this happens, rebuild your functional and domain indexes on the LOB column.

A LOB opened when there is no transaction must be closed before the end of the session. If there are LOBs open at the end of session, the LOB is no longer marked as open and the domain and functional indexing is not updated. If this happens, rebuild your functional and domain indexes on the LOB column.

Restrictions on Opening and Closing LOBs

The LOB opening and closing mechanism has the following restrictions:

  • An application must close all previously opened LOBs before committing a transaction. Failing to do so results in an error. If a transaction is rolled back, all open LOBs are discarded along with the changes made. Because the LOBs are not closed, so the associated triggers are not fired.

  • Although there is no limit to the number of open internal LOBs, there is a limit on the number of open files. See the SESSION_MAX_OPEN_FILES parameter in Oracle Database Reference. Assigning an already opened locator to another locator does not count as opening a new LOB.

  • It is an error to open or close the same internal LOB twice within the same transaction, either with different locators or the same locator.

  • It is an error to close a LOB that has not been opened.

    Note:

    The definition of a transaction within which an open LOB value must be closed is one of the following:
    • Between SET TRANSACTION and COMMIT

    • Between DATA MODIFYING DML or SELECT ... FOR UPDATE and COMMIT.

    • Within an autonomous transaction block

    See Also:

LOB Read and Write Callbacks

OCI supports read and write callback functions. The following sections describe the use of callbacks in more detail.

Callback Interface for Streaming

User-defined read and write callback functions for inserting or retrieving data provide an alternative to the polling methods for streaming LOBs. These functions are implemented by you and registered with OCI through the OCILobRead2(), OCILobWriteAppend2(), and OCILobWrite2() calls. These callback functions are called by OCI whenever they are required.

Reading LOBs by Using Callbacks

The user-defined read callback function is registered through the OCILobRead2() function. The callback function should have the following prototype:

CallbackFunctionName ( void *ctxp, CONST void *bufp, oraub8 len, ub1 piece,
                       void **changed_bufpp, oraub8 *changed_lenp);

The first parameter, ctxp, is the context of the callback that is passed to OCI in the OCILobRead2() function call. When the callback function is called, the information provided by you in ctxp is passed back to you (OCI does not use this information on the way IN). The bufp parameter in OCILobRead2()) is the pointer to the storage where the LOB data is returned and bufl is the length of this buffer. It tells you how much data has been read into the buffer provided.

If the buffer length provided in the original OCILobRead2() call is insufficient to store all the data returned by the server, then the user-defined callback is called. In this case, the piece parameter indicates whether the information returned in the buffer is the first, next, or last piece.

The parameters changed_bufpp and changed_lenp can be used inside the callback function to change the buffer dynamically. The changed_bufpp parameter should point to the address of the changed buffer and the changed_lenp parameter should point to the length of the changed buffer. The changed_bufpp and changed_lenp parameters need not be used inside the callback function if the application does not change the buffer dynamically.

Example 7-2 shows a code fragment that implements read callback functions using OCILobRead2(). Assume that lobl is a valid locator that has been previously selected, svchp is a valid service handle, and errhp is a valid error handle. In the example, the user-defined function cbk_read_lob() is repeatedly called until all the LOB data has been read.

Example 7-2 Implementing Read Callback Functions Using OCILobRead2()

...
oraub8   offset = 1;
oraub8   loblen = 0; 
oraub8   byte_amt = 0;
oraub8   char_amt = 0
ub1      bufp[MAXBUFLEN];

sword retval; 
byte_amtp = 4294967297;     /* 4 gigabytes plus 1 */ 

if (retval = OCILobRead2(svchp, errhp, lobl, &byte_amt, &char_amt, offset,
  (void *) bufp, (oraub8) MAXBUFLEN, (void *) 0, OCI_FIRST_PIECE, 
  cbk_read_lob, (ub2) 0, (ub1) SQLCS_IMPLICIT))
{
     (void) printf("ERROR: OCILobRead2() LOB.\n");
     report_error();
}
...
sb4 cbk_read_lob(ctxp, bufxp, len, piece, changed_bufpp, changed_lenp)
void       *ctxp;
CONST void *bufxp;
oraub8      len;
ub1         piece;
void       **changed_bufpp;
oraub8      *changed_lenp; 
{ 
    static ub4 piece_count = 0; 
    piece_count++; 

    switch (piece)
   {
      case OCI_LAST_PIECE:     /*--- buffer processing code goes here ---*/ 
          (void) printf("callback read the %d th piece\n\n", piece_count);
          piece_count = 0;
          break;
      case OCI_FIRST_PIECE:   /*--- buffer processing code goes here ---*/ 
          (void) printf("callback read the %d th piece\n", piece_count);
          /* --Optional code to set changed_bufpp and changed_lenp if the
              buffer must be changed dynamically --*/
          break;
      case OCI_NEXT_PIECE:   /*--- buffer processing code goes here ---*/
          (void) printf("callback read the %d th piece\n", piece_count);
         /* --Optional code to set changed_bufpp and changed_lenp if the
             buffer must be changed dynamically --*/
          break;
      default:
          (void) printf("callback read error: unknown piece = %d.\n", piece);
           return OCI_ERROR;
     } 
    return OCI_CONTINUE;
}

Writing LOBs by Using Callbacks

Similar to read callbacks, the user-defined write callback function is registered through the OCILobWrite2() function. The callback function should have the following prototype:

CallbackFunctionName ( void *ctxp, void *bufp, oraub8 *lenp, ub1 *piecep,
                        void **changed_bufpp, oraub8 *changed_lenp);

The first parameter, ctxp, is the context of the callback that is passed to OCI in the OCILobWrite2() function call. The information provided by you in ctxp is passed back to you when the callback function is called by OCI (OCI does not use this information on the way IN). The bufp parameter is the pointer to a storage area; you provide this pointer in the call to OCILobWrite2().

After inserting the data provided in the call to OCILobWrite2() any data remaining is inserted by the user-defined callback. In the callback, provide the data to insert in the storage indicated by bufp and also specify the length in lenp. You also indicate whether it is the next (OCI_NEXT_PIECE) or the last (OCI_LAST_PIECE) piece using the piecep parameter. You must ensure that the storage pointer that is provided by the application does not write more than the allocated size of the storage.

The parameters changed_bufpp and changed_lenp can be used inside the callback function to change the buffer dynamically. The changed_bufpp parameter should point to the address of the changed buffer and the changed_lenp parameter should point to the length of the changed buffer. The changed_bufpp and changed_lenp parameters need not be used inside the callback function if the application does not change the buffer dynamically.

Example 7-3 shows a code fragment that implements write callback functions using OCILobWrite2(). Assume that lobl is a valid locator that has been locked for updating, svchp is a valid service handle, and errhp is a valid error handle. The user-defined function cbk_write_lob() is repeatedly called until the piecep parameter indicates that the application is providing the last piece.

Example 7-3 Implementing Write Callback Functions Using OCILobWrite2()

...
 
ub1      bufp[MAXBUFLEN];
oraub8   byte_amt = MAXBUFLEN * 20;
oraub8   char_amt = 0;
oraub8   offset = 1;
oraub8   nbytes = MAXBUFLEN; 

/*-- code to fill bufp with data goes here. nbytes should reflect the size and
 should be less than or equal to MAXBUFLEN --*/
if (retval = OCILobWrite2(svchp, errhp, lobl, &byte_amt, &char_amt, offset,
    (void*)bufp, (ub4)nbytes, OCI_FIRST_PIECE, (void *)0, cbk_write_lob, 
    (ub2) 0, (ub1) SQLCS_IMPLICIT)) 
{
   (void) printf("ERROR: OCILobWrite2().\n");
   report_error();
   return;
}
... 
sb4 cbk_write_lob(ctxp, bufxp, lenp,  piecep, changed_bufpp, changed_lenp)
void    *ctxp; 
void    *bufxp; 
oraub8  *lenp; 
ub1       *piecep;
void  **changed_bufpp;
oraub8  *changed_lenp; 
{ 
    /*-- code to fill bufxp with data goes here. *lenp should reflect the
        size and should be less than or equal to MAXBUFLEN -- */
    /* --Optional code to set changed_bufpp and changed_lenp if the
        buffer must be changed dynamically --*/
    if (this is the last data buffer) 
        *piecep = OCI_LAST_PIECE; 
    else
        *piecep = OCI_NEXT_PIECE; 
    return OCI_CONTINUE;
}

Temporary LOB Support

OCI provides functions for creating and freeing temporary LOBs, OCILobCreateTemporary() and OCILobFreeTemporary(), and a function for determining whether a LOB is temporary, OCILobIsTemporary().

Temporary LOBs are not permanently stored in the database, but act like local variables for operating on LOB data. OCI functions that operate on standard (persistent) LOBs can also be used on temporary LOBs.

As with persistent LOBs, all functions operate on the locator for the temporary LOB, and the actual LOB data is accessed through the locator.

Temporary LOB locators can be used as arguments to the following types of SQL statements:

  • UPDATE - The temporary LOB locator can be used as a value in a WHERE clause when testing for nullity or as a parameter to a function. The locator can also be used in a SET clause.

  • DELETE - The temporary LOB locator can be used in a WHERE clause when testing for nullity or as a parameter to a function.

  • SELECT - The temporary LOB locator can be used in a WHERE clause when testing for nullity or as a parameter to a function. The temporary LOB can also be used as a return variable in a SELECT...INTO statement when selecting the return value of a function.

Note:

If you select a permanent locator into a temporary locator, the temporary locator is overwritten with the permanent locator. In this case, the temporary LOB is not implicitly freed. You must explicitly free the temporary LOB before the SELECT...INTO operation. If the temporary LOB is not freed explicitly, it is not freed until the end of its specified duration. Unless you have another temporary locator pointing to the same LOB, you no longer have a locator pointing to the temporary LOB, because the original locator was overwritten by the SELECT...INTO operation.

Creating and Freeing Temporary LOBs

You create a temporary LOB with the OCILobCreateTemporary() function. The parameters passed to this function include a value for the duration of the LOB. The default duration is for the length of the current session. All temporary LOBs are deleted at the end of the duration. Users can reclaim temporary LOB space by explicitly freeing the temporary LOB with the OCILobFreeTemporary() function. A temporary LOB is empty when it is created.

When creating a temporary LOB, you can also specify whether the temporary LOB is read into the server's buffer cache.

To make a temporary LOB permanent, use OCILobCopy2() to copy the data from the temporary LOB into a permanent one. You can also use the temporary LOB in the VALUES clause of an INSERT statement, as the source of the assignment in an UPDATE statement, or assign it to a persistent LOB attribute and then flush the object. Temporary LOBs can be modified using the same functions that are used for standard LOBs.

Note:

The most efficient way to insert an empty LOB is to bind a temporary LOB with no value assigned to it. This uses less resources than the following method.
INSERT INTO tab1 VALUES(EMPTY_CLOB())

Temporary LOB Durations

OCI supports several predefined durations for temporary LOBs, and a set of functions that the application can use to define application-specific durations. The predefined durations and their associated attributes are:

  • Call, OCI_DURATION_CALL, only on the server side

  • Session, OCI_DURATION_SESSION

The session duration expires when the containing session or connection ends. The call duration expires at the end of the current OCI call.

When you run in object mode, you can also define application-specific durations. An application-specific duration, also referred to as a user duration, is defined by specifying the start of a duration using OCIDurationBegin() and the end of the duration using OCIDurationEnd().

Note:

User-defined durations are only available if an application has been initialized in object mode.

Each application-specific duration has a duration identifier that is returned by OCIDurationBegin() and is guaranteed to be unique until OCIDurationEnd() is called. An application-specific duration can be as long as a session duration.

At the end of a duration, all temporary LOBs associated with that duration are freed. The descriptor associated with the temporary LOB must be freed explicitly with the OCIDescriptorFree() call.

User-defined durations can be nested; one duration can be defined as a child duration of another user duration. It is possible for a parent duration to have child durations that have their own child durations.

Note:

When a duration is started with OCIDurationBegin(), one of the parameters is the identifier of a parent duration. When a parent duration is ended, all child durations are also ended.

Freeing Temporary LOBs

Any time that your OCI program obtains a LOB locator from SQL or PL/SQL, use the OCILobIsTemporary() function to check that the locator is temporary. If it is, then free the locator when your application is finished with it by using the OCILobFreeTemporary() call. The locator can be from a define during a select or an out bind. A temporary LOB duration is always upgraded to a session duration when it is shipped to the client side. The application must do the following before the locator is overwritten by the locator of the next row:

OCILobIsTemporary(env, err, locator, is_temporary);
if(is_temporary)
    OCILobFreeTemporary(svc, err, locator);

Take Care When Assigning Pointers

Special care must be taken when assigning OCILobLocator pointers. Pointer assignments create a shallow copy of the LOB. After the pointer assignment, source and target LOBs point to the same copy of data. This behavior is different from using LOB APIs, such as OCILobAssign() or OCILobLocatorAssign(), to perform assignments. When the APIs are used, the locators logically point to independent copies of data after assignment.

For temporary LOBs, before pointer assignments, you must ensure that any temporary LOB in the target LOB locator is freed by OCILobFreeTemporary(). When OCILobLocatorAssign() is used, the original temporary LOB in the target LOB locator variable, if any, is freed before the assignment happens.

Before an out-bind variable is reused in executing a SQL statement, you must free any temporary LOB in the existing out-bind LOB locator buffer by using the OCILobFreeTemporary() call.

See Also:

Temporary LOB Example

Example 7-4 shows how temporary LOBs can be used.

Example 7-4 Using Temporary LOBs

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <oci.h>

/* Function Prototype */
static void checkerr (/*_ OCIError *errhp, sword status _*/);
sb4 select_and_createtemp (OCILobLocator *lob_loc,
                           OCIError      *errhp,
                           OCISvcCtx     *svchp,
                           OCIStmt       *stmthp,
                           OCIEnv        *envhp);
/* This function reads in a single video frame from the print_media table.
Then it creates a temporary LOB. The temporary LOB that is created is read
through the CACHE, and is automatically cleaned up at the end of the user's
session, if it is not explicitly freed sooner. This function returns OCI_SUCCESS
if it completes successfully or OCI_ERROR if it fails. */

sb4 select_and_createtemp (OCILobLocator *lob_loc,
                           OCIError      *errhp,
                           OCISvcCtx     *svchp,
                           OCIStmt       *stmthp,
                           OCIEnv        *envhp)
{
  OCIDefine     *defnp1;
  OCIBind       *bndhp;
  text          *sqlstmt;
  int rowind =1;
  ub4 loblen = 0;
  OCILobLocator *tblob;
  printf ("in select_and_createtemp \n");
    if(OCIDescriptorAlloc((void*)envhp, (void **)&tblob,
                          (ub4)OCI_DTYPE_LOB, (size_t)0, (void**)0))
  {
    printf("failed in OCIDescriptor Alloc in select_and_createtemp \n");
    return OCI_ERROR;
 }
  /* arbitrarily select where Clip_ID =1 */
  sqlstmt=(text *)"SELECT Frame FROM print_media WHERE product_ID = 1 FOR UPDATE";
  if (OCIStmtPrepare(stmthp, errhp, sqlstmt, (ub4) strlen((char *)sqlstmt),
                     (ub4) OCI_NTV_SYNTAX, (ub4) OCI_DEFAULT))
  {
      (void) printf("FAILED: OCIStmtPrepare() sqlstmt\n");
      return OCI_ERROR;
  }
  /* Define for BLOB */
  if (OCIDefineByPos(stmthp, &defnp1, errhp, (ub4)1, (void *) &lob_loc, (sb4)0,
             (ub2) SQLT_BLOB, (void *)0, (ub2 *)0, (ub2 *)0, (ub4) OCI_DEFAULT))
  {
    (void) printf("FAILED: Select locator: OCIDefineByPos()\n");
    return OCI_ERROR;
  }
  /* Execute the select and fetch one row */
  if (OCIStmtExecute(svchp, stmthp, errhp, (ub4) 1, (ub4) 0,
                   (CONST OCISnapshot*) 0, (OCISnapshot*) 0, (ub4) OCI_DEFAULT))
  {
    (void) printf("FAILED: OCIStmtExecute() sqlstmt\n");
    return OCI_ERROR;
  }
  if(OCILobCreateTemporary(svchp, errhp, tblob, (ub2)0, SQLCS_IMPLICIT,
                         OCI_TEMP_BLOB, OCI_ATTR_NOCACHE, OCI_DURATION_SESSION))
  {
    (void) printf("FAILED: CreateTemporary() \n");
    return OCI_ERROR;
  }
  if (OCILobGetLength(svchp, errhp, lob_loc, &loblen) != OCI_SUCCESS)
  {
    printf("OCILobGetLength FAILED\n");
    return OCI_ERROR;
  }
  if (OCILobCopy(svchp, errhp, tblob,lob_loc,(ub4)loblen, (ub4) 1, (ub4) 1))
  {
    printf( "OCILobCopy FAILED \n");
  }
  if(OCILobFreeTemporary(svchp,errhp,tblob))
  {
    printf ("FAILED: OCILobFreeTemporary call \n");
    return OCI_ERROR;
  }
     return OCI_SUCCESS;
}
int main(char *argv, int argc)
{
  /* OCI Handles */
  OCIEnv        *envhp;
  OCIServer     *srvhp;
  OCISvcCtx     *svchp;
  OCIError      *errhp;
  OCISession    *authp;
  OCIStmt       *stmthp;
  OCILobLocator *clob, *blob;
  OCILobLocator *lob_loc;
  int type =1;
  /* Initialize and Log on */
  OCIEnvCreate(&envhp, OCI_DEFAULT, (void *)0, 0, 0, 0,
        (size_t)0, (void *)0);
  (void) OCIHandleAlloc( (void *) envhp, (void **) &errhp, OCI_HTYPE_ERROR,
                          (size_t) 0, (void **) 0);
  /* server contexts */
  (void) OCIHandleAlloc( (void *) envhp, (void **) &srvhp, OCI_HTYPE_SERVER,
                         (size_t) 0, (void **) 0);
   /* service context */
  (void) OCIHandleAlloc( (void *) envhp, (void **) &svchp, OCI_HTYPE_SVCCTX,
                         (size_t) 0, (void **) 0);
    /* attach to Oracle Database */
  (void) OCIServerAttach( srvhp, errhp, (text *)"", strlen(""), 0);
    /* set attribute server context in the service context */
  (void) OCIAttrSet ((void *) svchp, OCI_HTYPE_SVCCTX,
                     (void *)srvhp, (ub4) 0,
                     OCI_ATTR_SERVER, (OCIError *) errhp);
   (void) OCIHandleAlloc((void *) envhp,
                        (void **)&authp, (ub4) OCI_HTYPE_SESSION,
                        (size_t) 0, (void **) 0);
   (void) OCIAttrSet((void *) authp, (ub4) OCI_HTYPE_SESSION,
                    (void *) "scott", (ub4)5,
                    (ub4) OCI_ATTR_USERNAME, errhp);
  (void) OCIAttrSet((void *) authp, (ub4) OCI_HTYPE_SESSION,
                    (void *) "password", (ub4) 5,
                    (ub4) OCI_ATTR_PASSWORD, errhp);
  /* Begin a User Session */
  checkerr(errhp, OCISessionBegin ( svchp,  errhp, authp, OCI_CRED_RDBMS,
                                    (ub4) OCI_DEFAULT));
  (void) OCIAttrSet((void *) svchp, (ub4) OCI_HTYPE_SVCCTX,
                    (void *) authp, (ub4) 0,
                    (ub4) OCI_ATTR_SESSION, errhp);
  /* ------- Done logging in ----------------------------------*/
   /* allocate a statement handle */
  checkerr(errhp, OCIHandleAlloc( (void *) envhp, (void **) &stmthp,
                             OCI_HTYPE_STMT, (size_t) 0, (void **) 0));
  checkerr(errhp, OCIDescriptorAlloc((void *)envhp, (void **)&lob_loc,
                   (ub4) OCI_DTYPE_LOB, (size_t) 0, (void **) 0));
  /* Subroutine calls begin here */
  printf("calling select_and_createtemp\n");
  select_and_createtemp (lob_loc, errhp, svchp,stmthp,envhp);
  return 0;
}
void checkerr(errhp, status)
OCIError *errhp;
sword status;
{
  text errbuf[512];
  sb4 errcode = 0;
  switch (status)
  {
  case OCI_SUCCESS:
    break;
  case OCI_SUCCESS_WITH_INFO:
    (void) printf("Error - OCI_SUCCESS_WITH_INFO\n");
    break;
  case OCI_NEED_DATA:
    (void) printf("Error - OCI_NEED_DATA\n");
    break;
  case OCI_NO_DATA:
    (void) printf("Error - OCI_NODATA\n");
    break;
  case OCI_ERROR:
    (void) OCIErrorGet((void *)errhp, (ub4) 1, (text *) NULL, &errcode,
                        errbuf, (ub4) sizeof(errbuf), OCI_HTYPE_ERROR);
    (void) printf("Error - %.*s\n", 512, errbuf);
    break;
  case OCI_INVALID_HANDLE:
    (void) printf("Error - OCI_INVALID_HANDLE\n");
    break;
  case OCI_STILL_EXECUTING:
    (void) printf("Error - OCI_STILL_EXECUTE\n");
    break;
  case OCI_CONTINUE:
    (void) printf("Error - OCI_CONTINUE\n");
    break;
  default:
    break;
  }
}

Prefetching of LOB Data, Length, and Chunk Size

To improve OCI access of smaller LOBs, LOB data can be prefetched and cached while also fetching the locator. This applies to internal LOBs, temporary LOBs, and BFILEs. Take the following steps to prepare your application:

  1. Set the OCI_ATTR_DEFAULT_LOBPREFETCH_SIZE attribute for the session handle. The value of this attribute indicates the default prefetch data size for a LOB locator. This attribute value enables prefetching for all the LOB locators fetched in the session. The default value for this attribute is zero (no prefetch of LOB data). This option relieves the application developer from setting the prefetch LOB size for each define handle. You can either set this attribute or set (in Step 3) OCI_ATTR_LOBPREFETCH_SIZE.

  2. Perform the prepare and define steps for the statement to be executed.

  3. You can override the default prefetch size, if required, for the LOB locators to be fetched, by setting OCI_ATTR_LOBPREFETCH_SIZE attribute for the define handle. This optional attribute provides control of the prefetch size for the locators fetched from a particular column.

  4. Set the OCI_ATTR_LOBPREFETCH_LENGTH attribute to the prefetch LOB length and chunk size.

  5. Execute the statement.

  6. Call OCILobRead2() or OCILobArrayRead() with individual LOB locators; OCI takes the data from the prefetch buffer, does the necessary character conversion, and copies the data into the LOB read buffer (no change in LOB semantic). If the data requested is bigger than the prefetch buffer, then it will require additional round-trips.

  7. Call OCILobGetLength2() and OCILobGetChunkSize() to obtain the length and chunk size without making round-trips to the server.

Note that the prefetch size is in number of bytes for BLOBs and BFILEs and in number of characters for CLOBs.

Example 7-5 shows a code fragment illustrating these steps.

Example 7-5 Prefetching of LOB Data, Length, and Chunk Size

...
ub4 default_lobprefetch_size = 2000;                  /* Set default size to 2K */
... 
/* set LOB prefetch attribute to session */
OCIAttrSet (sesshp, (ub4) OCI_HTYPE_SESSION,
            (void *)&default_lobprefetch_size,               /* attribute value */
            0,                      /* attribute size; not required to specify; */
            (ub4) OCI_ATTR_DEFAULT_LOBPREFETCH_SIZE,
            errhp);
...
/* select statement */
char *stmt = "SELECT lob1 FROM lob_table";
...
/* declare and allocate LOB locator */
OCILobLocator * lob_locator;
lob_locator = OCIDescriptorAlloc(..., OCI_DTYPE_LOB, ...);
 
OCIDefineByPos(..., 1, (void *) &lob_locator, ..., SQLT_CLOB, ...);
...
/* Override the default prefetch size to 4KB */
ub4 prefetch_size = 4000;
OCIAttrSet (defhp,  OCI_HTYPE_DEFINE,
            (void *) &prefetch_size                             /* attr value */,
            0                  /* restricting prefetch size to be ub4 max val */,
            OCI_ATTR_LOBPREFETCH_SIZE                            /* attr type */,
            errhp);
...
/* Set prefetch length attribute */
boolean prefetch_length = TRUE;
OCIAttrSet( defhp,  OCI_HTYPE_DEFINE,
            (dvoid *) &prefetch_length  /* attr value */,
            0,
            OCI_ATTR_LOBPREFETCH_LENGTH  /* attr type */,
            errhp );
...
/* execute the statement. 4KB of data for the LOB is read and
 * cached in descriptor cache buffer.
 */
OCIStmtExecute (svchp, stmthp, errhp,
                1,               /* iters */
                0,               /* row offset */
                NULL,            /* snapshot IN */
                NULL,            /* snapshot out */
                OCI_DEFAULT);    /* mode */
...
oraub8 char_amtp = 4000;
oraub8 lob_len;
ub4 chunk_size;
 
/* LOB chunk size, length, and data are read from cache. No round-trip. */
 
OCILobGetChunkSize (svchp, errhp, lob_locator, &chunk_size);
 
OCILobGetLength2(svchp, errhp, lob_locator, &lob_len );
 
OCILobRead2(svchp, errhp, lob_locator, NULL, &char_amtp, ...);
...

Prefetch cache allocation: The prefetch cache buffer for a descriptor is allocated while fetching a LOB locator. The allocated buffer size is determined by the OCI_ATTR_LOBPREFETCH_SIZE attribute for the define handle; the default value of this attribute is indicated by the OCI_ATTR_DEFAULT_LOBPREFETCH_SIZE attribute value of the session handle. If the cache buffer is already allocated, then it is resized if required.

For the following two LOB APIs, if the source locator has cached data, then the destination locator cache is allocated or resized and cached data is copied from source to destination.

Once allocated, the cache buffer memory for a descriptor is released when the descriptor itself is freed.

Prefetch cache invalidation: The cache for a descriptor gets invalidated when LOB data is updated using the locator. Meaning the cache is no longer used for reading data and the next OCILobRead2() call on the locator makes a round-trip.

The following LOB APIs invalidate the prefetch cache for the descriptor used:

The following LOB APIs invalidate the cache for the destination LOB locator:

Performance Tuning: The prefetch buffer size must be decided upon based on average LOB size and client-side memory. If a large amount of data is prefetched, you must ensure the memory availability. Performance gain may not be significant for prefetching large LOBs, because the cost of fetching data is much higher compared to the cost of a round-trip to the server.

You must have a fair idea of the LOB data size to be able to make best use of this LOB prefetch feature. Because the parameters are part of application design, the application must be rebuilt if any parameter value must be modified.

Upgrading: LOB prefetching cannot be used against a pre-11.1 release server or in a pre-11.1 client against an 11.1 or later server. When you use a pre-11.1 server with an 11.1 or later client, OCIAttrSet() returns an error or an error-with-information saying that "server does not support this functionality."

Options of SecureFiles LOBs

For SecureFiles (LOBs with the STORE AS SECUREFILE option, which were introduced in Oracle Database 11g Release 1) you can specify the SQL parameter DEDUPLICATE in CREATE TABLE and ALTER TABLE statements. This parameter value enables you to specify that LOB data that is identical in two or more rows in a LOB column shares the same data blocks, thus saving disk space. KEEP_DUPLICATES turns off this capability. The following options are also used with SECUREFILE:

The parameter COMPRESS turns on LOB compression. NOCOMPRESS turns LOB compression off.

The parameter ENCRYPT turns on LOB encryption and optionally selects an encryption algorithm. NOENCRYPT turns off LOB encryption. Each LOB column can have its own encryption specification, independent of the encryption of other LOB or non-LOB columns. Valid algorithms are 3DES168, AES128, AES192, and AES256.

The LOBs paradigm used before release 11.1 is the default. This default LOBs paradigm is also now explicitly set by the option STORE AS BASICFILE.

The following OCI functions are used with the SECUREFILE features:

See Also:

Oracle Database SecureFiles and Large Objects Developer's Guide for complete details of relevant SQL functions and cross-references to PL/SQL packages and information about migrating to SecureFiles