Internal Persistent LOBs, 33 of 42

Trim LOB Data

Figure 9-36 Use Case Diagram: Trim LOB Data

See:
"Use Case Model: Internal Persistent LOBs Basic Operations", for all basic operations of Internal Persistent LOBs.

Purpose

This procedure describes how to trim LOB data.

Usage Notes

Locking the Row Prior to Updating

Prior to updating a LOB value via the PL/SQL DBMS_LOB package or OCI, you must lock the row containing the LOB. While the SQL INSERT and UPDATE statements implicitly lock the row, locking is done explicitly by means of a SQL SELECT FOR UPDATE statement in SQL and PL/SQL programs, or by using an OCI pin or lock function in OCI programs. For more details on the state of the locator after an update, refer to "Updated LObs Via Updated Locators" in Chapter 5, "Advanced Topics".

Syntax

See Chapter 3, "LOB Programmatic Environments" for a list of available functions in each programmatic environment. Use the following syntax references for each programmatic environment:

PL/SQL (DBM_LOB Package): Oracle8i Supplied PL/SQL Packages Reference Chapter 18, "DBMS_LOB" -- TRIM
C (OCI): Oracle Call Interface Programmer's Guide Chapter 15, "Relational Functions" -- LOB Functions, OCILobTrim.
COBOL (Pro*COBOL): Pro*COBOL Precompiler Programmer's Guide Appendix F, "Embedded SQL and Precompiler Directives" -- LOB TRIM
C/C++ (Pro*C/C++): Pro*C/C++ Precompiler Programmer's Guide Appendix F, "Embedded SQL and Precompiler Directives" -- LOB TRIM
Visual Basic (OO4O) (Oracle Objects for OLE (OO4O) Online Help): From Help Topics, Contents tab, select OO4O Automation Server > OBJECTS > Orablob > METHODS > trim
Java (JDBC): Oracle8i JDBC Developer's Guide and Reference Chapter 7, "Working With LOBs" -- Creating and Populating a BLOB or CLOB Column. Oracle8i SQLJ Developer's Guide and Reference Chapter 5, "Type Support", Oracle Type Support, Support for BLOB, CLOB, and BFILE.

Scenario

These examples access text (CLOB data) referenced in the Script column of table Voiceover_tab, and trim it.

Examples

Examples are provided in the following programmatic environments:

PL/SQL (DBMS_LOB Package): Trim LOB Data

/* Note that the example procedure trimLOB_proc is not part of the 
   DBMS_LOB package: */
CREATE OR REPLACE PROCEDURE trimLOB_proc IS
   Lob_loc        CLOB;
BEGIN
   /* Select the LOB, get the LOB locator: */
   SELECT Mtab.Voiced_ref.Script INTO Lob_loc FROM Multimedia_tab Mtab
      WHERE Mtab.Clip_ID = 2
         FOR UPDATE;
   /* Opening the LOB is optional: */
   DBMS_LOB.OPEN (Lob_loc, DBMS_LOB.LOB_READWRITE);
   /* Trim the LOB data: */
   DBMS_LOB.TRIM(Lob_loc,100);
   /* Closing the LOB is mandatory if you have opened it: */
   DBMS_LOB.CLOSE (Lob_loc);
COMMIT;
/* Exception handling: */
EXCEPTION
   WHEN OTHERS THEN
      DBMS_OUTPUT.PUT_LINE('Operation failed');
END;

C (OCI): Trim LOB Data

/* Select the locator into a locator variable */
sb4 select_lock_voice_locator(Lob_loc, errhp, svchp, stmthp)
OCILobLocator *Lob_loc;
OCIError      *errhp;
OCISvcCtx     *svchp;
OCIStmt       *stmthp;     
{
  text  *sqlstmt = 
      (text *) "SELECT Mtab.Voiced_ref.Script \
           FROM Multimedia_tab Mtab WHERE Mtab.Clip_ID = 2 FOR UPDATE";
  OCIDefine *defnp1;
  checkerr (errhp, OCIStmtPrepare(stmthp, errhp, sqlstmt, 
                                  (ub4)strlen((char *)sqlstmt),
                                  (ub4) OCI_NTV_SYNTAX, (ub4) OCI_DEFAULT));
    checkerr (errhp, OCIDefineByPos(stmthp, &defnp1, errhp, (ub4) 1,
                                  (dvoid *)&Lob_loc, (sb4)0, 
                                  (ub2) SQLT_CLOB,(dvoid *) 0, 
                                  (ub2 *) 0, (ub2 *) 0, (ub4) OCI_DEFAULT));
 
  /* Execute the select and fetch one row */
  checkerr(errhp, OCIStmtExecute(svchp, stmthp, errhp, (ub4) 1, (ub4) 0,
                                 (CONST OCISnapshot*) 0, (OCISnapshot*) 0,  
                                 (ub4) OCI_DEFAULT));
  return 0;
}
void trimLob(envhp, errhp, svchp, stmthp)
OCIEnv    *envhp;
OCIError  *errhp;
OCISvcCtx *svchp;
OCIStmt   *stmthp;
{
  OCILobLocator *Lob_loc;
  unsigned int trimLength;
  (void) OCIDescriptorAlloc((dvoid *) envhp, (dvoid **) &Lob_loc, 
                            (ub4)OCI_DTYPE_LOB, (size_t) 0, (dvoid **) 0);

  /* Select the CLOB */
  printf( " select a voice LOB\n");
  select_lock_voice_locator(Lob_loc, errhp, svchp, stmthp);

  /* Open the CLOB */
  checkerr (errhp, (OCILobOpen(svchp, errhp, Lob_loc, OCI_LOB_READWRITE)));

  /* Trim the LOB to its new length */
  trimLength = 100;                      /* <New truncated length of the LOB>*/

  printf (" trim the lob to %d bytes\n", trimLength);
  checkerr (errhp, OCILobTrim (svchp, errhp, Lob_loc, trimLength ));

  /* Closing the CLOB is mandatory if you have opened it */
  checkerr (errhp, OCILobClose(svchp, errhp, Lob_loc));

  /* Free resources held by the locators*/
  (void) OCIDescriptorFree((dvoid *) Lob_loc, (ub4) OCI_DTYPE_LOB);
 }

COBOL (Pro*COBOL): Trim LOB Data

       IDENTIFICATION DIVISION.
       PROGRAM-ID. TRIM-CLOB.
       ENVIRONMENT DIVISION.
       DATA DIVISION.
       WORKING-STORAGE SECTION.

       01  CLOB1          SQL-CLOB.
       01  NEW-LEN        PIC S9(9) COMP.
      * Define the source and destination position and location: 
       01  SRC-POS        PIC S9(9) COMP.
       01  DEST-POS       PIC S9(9) COMP.
       01  SRC-LOC        PIC S9(9) COMP.
       01  DEST-LOC       PIC S9(9) COMP.
       01  USERID   PIC X(11) VALUES "SAMP/SAMP".
           EXEC SQL INCLUDE SQLCA END-EXEC.
        
       PROCEDURE DIVISION.
       TRIM-CLOB.
           EXEC SQL WHENEVER SQLERROR DO PERFORM SQL-ERROR END-EXEC.
           EXEC SQL CONNECT :USERID END-EXEC.

      * Allocate and initialize the CLOB locators: 
           EXEC SQL ALLOCATE :CLOB1 END-EXEC.
           EXEC SQL WHENEVER NOT FOUND GOTO END-OF-CLOB END-EXEC.
           EXEC SQL 
                SELECT MTAB.STORY INTO :CLOB1
                FROM MULTIMEDIA_TAB MTAB
                WHERE MTAB.CLIP_ID = 2 FOR UPDATE END-EXEC.
 
      * Open the CLOB: 
           EXEC SQL LOB OPEN :CLOB1 READ WRITE END-EXEC.

      * Move some value to  NEW-LEN: 
           MOVE 3 TO NEW-LEN.
           EXEC SQL 
                LOB TRIM :CLOB1 TO :NEW-LEN END-EXEC.

           EXEC SQL LOB CLOSE :CLOB1 END-EXEC.
       END-OF-CLOB.
           EXEC SQL WHENEVER NOT FOUND CONTINUE END-EXEC.
           EXEC SQL FREE :CLOB1 END-EXEC.
           EXEC SQL ROLLBACK WORK RELEASE END-EXEC.
           STOP RUN.

       SQL-ERROR.
           EXEC SQL WHENEVER SQLERROR CONTINUE END-EXEC.
           DISPLAY " ".
           DISPLAY "ORACLE ERROR DETECTED:".
           DISPLAY " ".
           DISPLAY SQLERRMC.
           EXEC SQL ROLLBACK WORK RELEASE END-EXEC.
           STOP RUN.

C/C++ (Pro*C/C++): Trim LOB Data

/*In addition to the data structures set up above in the section "Examples", you 
should use DML like this:
INSERT INTO multimedia_tab VALUES (2, 'The quick brown fox jumped over the lazy 
dog',  empty_clob(), NULL, empty_blob(), empty_blob(), NULL, NULL, NULL, NULL);
INSERT INTO voiceover_tab VALUES (voiced_typ('hello', (SELECT story FROM 
multimedia_tab WHERE clip_id = 2), 'world', 1, NULL))
UPDATE multimedia_tab SET voiced_ref = (SELECT REF(r) FROM voiceover_tab r WHERE 
r.take = 1) WHERE clip_id = 2
Then create this text file, pers_trim.typ, containing:
case=lower
type voiced_typ
Then run this Object Type Translator command:
ott intyp=pers_trim.typ outtyp=pers_trim_o.typ 
hfile=pers_trim.h code=c user=samp/samp
*/

#include "pers_trim.h"
#include <stdio.h>
#include <sqlca.h>
void Sample_Error()
{
  EXEC SQL WHENEVER SQLERROR CONTINUE;
  printf("sqlcode = %ld\n", sqlca.sqlcode);
  printf("%.*s\n", sqlca.sqlerrm.sqlerrml, sqlca.sqlerrm.sqlerrmc);
  EXEC SQL ROLLBACK WORK RELEASE;
  exit(1);
}

void trimLOB_proc()
{
  voiced_typ_ref *vt_ref;
  voiced_typ *vt_typ;
  OCIClobLocator *Lob_loc;
  unsigned int Length, trimLength;

  EXEC SQL WHENEVER SQLERROR DO Sample_Error();
  EXEC SQL ALLOCATE :Lob_loc;
  EXEC SQL ALLOCATE :vt_ref;
  EXEC SQL ALLOCATE :vt_typ;

  /* Retrieve the REF using Associative SQL */
  EXEC SQL SELECT Mtab.Voiced_ref INTO :vt_ref
           FROM Multimedia_tab Mtab WHERE Mtab.Clip_ID = 2 FOR UPDATE;

  /* Dereference the Object using the Navigational Interface */
  EXEC SQL OBJECT DEREF :vt_ref INTO :vt_typ FOR UPDATE;
  Lob_loc = vt_typ->script;

  /* Opening the LOB is Optional */
  EXEC SQL LOB OPEN :Lob_loc READ WRITE;
  EXEC SQL LOB DESCRIBE :Lob_loc GET LENGTH INTO :Length;
  printf("Old length was %d\n", Length);
  trimLength = (unsigned int)(Length / 2);

  /* Trim the LOB to its new length */
  EXEC SQL LOB TRIM :Lob_loc TO :trimLength;

  /* Closing the LOB is mandatory if it has been opened */
  EXEC SQL LOB CLOSE :Lob_loc;

  /* Mark the Object as Modified (Dirty) */
  EXEC SQL OBJECT UPDATE :vt_typ;

  /* Flush the changes to the LOB in the Object Cache */
  EXEC SQL OBJECT FLUSH :vt_typ;

  /* Display the new (modified) length */
  EXEC SQL SELECT Mtab.Voiced_ref.Script INTO :Lob_loc
           FROM Multimedia_tab Mtab WHERE Mtab.Clip_ID = 2;
  EXEC SQL LOB DESCRIBE :Lob_loc GET LENGTH INTO :Length;
  printf("New length is now %d\n", Length);

  /* Free the Objects and the LOB Locator */
  EXEC SQL FREE :vt_ref;
  EXEC SQL FREE :vt_typ;
  EXEC SQL FREE :Lob_loc;
}

void main()
{
  char *samp = "samp/samp";
  EXEC SQL CONNECT :samp;
  trimLOB_proc();
  EXEC SQL ROLLBACK WORK RELEASE;
}

Visual Basic (OO4O): Trim LOB Data

Dim MySession As OraSession
Dim OraDb As OraDatabase
Dim OraDyn As OraDynaset, OraSound1 As OraBlob, OraSoundClone As OraBlob

Set MySession = CreateObject("OracleInProcServer.XOraSession")
Set OraDb = MySession.OpenDatabase("exampledb", "samp/samp", 0&)
Set OraDyn = OraDb.CreateDynaset(
   "SELECT * FROM Multimedia_tab ORDER BY clip_id", ORADYN_DEFAULT)
Set OraSound1 = OraDyn.Fields("Sound").Value

OraDyn.Edit
OraSound1.Trim 10
OraDyn.Update

Java (JDBC): Trim LOB Data

// Java IO classes: 
import java.io.InputStream;
import java.io.OutputStream;

// Core JDBC classes: 
import java.sql.DriverManager;
import java.sql.Connection;
import java.sql.Types;
import java.sql.Statement;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

// Oracle Specific JDBC classes: 
import oracle.sql.*;
import oracle.jdbc.driver.*;

public class Ex2_141
{
  static final int MAXBUFSIZE = 32767;
  public static void main (String args [])
       throws Exception
  {
    // Load the Oracle JDBC driver: 
    DriverManager.registerDriver(new oracle.jdbc.driver.OracleDriver());

    // Connect to the database: 
    Connection conn =
      DriverManager.getConnection ("jdbc:oracle:oci8:@", "samp", "samp");

    // It's faster when auto commit is off: 
    conn.setAutoCommit (false);

    // Create a Statement: 
    Statement stmt = conn.createStatement ();
    try
    {
     CLOB lob_loc = null;

     ResultSet rset = stmt.executeQuery (
        "SELECT mtab.voiced_ref.script FROM multimedia_tab mtab 
            WHERE mtab.clip_id = 2 FOR UPDATE");
   if (rset.next())
   {
     lob_loc = ((OracleResultSet)rset).getCLOB (1);
   }

   // Open the LOB for READWRITE: 
       OracleCallableStatement cstmt = (OracleCallableStatement)
          conn.prepareCall ("BEGIN DBMS_LOB.OPEN(?,DBMS_LOB.LOB_READWRITE);  
          END;");
   cstmt.setCLOB(1, lob_loc);
   cstmt.execute();

   // Trim the LOB to length of 400: 
   cstmt = (OracleCallableStatement) 
      conn.prepareCall ("BEGIN DBMS_LOB.TRIM(?, 400); END;");
   cstmt.setCLOB(1, lob_loc);
   cstmt.execute();

   // Close the LOB: 
   cstmt = (OracleCallableStatement) conn.prepareCall (
      "BEGIN DBMS_LOB.CLOSE(?); END;");
   cstmt.setCLOB(1, lob_loc);
   cstmt.execute();

   stmt.close();
   cstmt.close();
   conn.commit();
   conn.close();
    }
    catch (SQLException e)
    {
       e.printStackTrace();
    }
  }
}