25 Performance Extensions

Overview of Update Batching Models

This section compares and contrasts the general models and types of statements supported for standard update batching and Oracle update batching.

Oracle Model versus Standard Model

Oracle update batching uses a batch value that typically results in implicit processing of a batch. The batch value is the number of operations you want to add to a batch for each trip to the database. As soon as that many operations have been added to the batch, the batch is processed. Note the following:

• You can set a default batch for the connection object, which applies to any prepared statement run in that connection.

• For any individual prepared statement object, you can set a statement batch value that overrides the connection batch value.

• You can choose to explicitly process a batch at any time, overriding both the connection batch value and the statement batch value.

Standard update batching is a manual, explicit model. There is no batch value. You manually add operations to the batch, and then, explicitly choose when to process the batch.

Oracle update batching is a more efficient model because the driver knows ahead of time how many operations will be batched. In this sense, the Oracle model is more static and predictable. With the standard model, the driver has no way of knowing in advance how many operations will be batched. In this sense, the standard model is more dynamic in nature.

If you want to use update batching, then you can choose between the two models on the basis of the following:

• Use Oracle update batching if portability is not critical. This will probably result in the greatest performance improvement.

• Use standard update batching if portability is a higher priority than performance.

Types of Statements Supported

As implemented by Oracle, update batching is intended for use with prepared statements, when you are repeating the same statement with different bind variables. Be aware of the following:

• Oracle update batching supports only prepared statement objects. For a callable statement, both the connection default batch value and the statement batch value are overridden with a value of 1. In an Oracle generic statement, there is no statement batch value, and the connection default batch value is overridden with a value of 1.

• To adhere to the JDBC 2.0 standard, Oracle implementation of standard update batching supports callable statements, without OUT parameters, and generic statements, as well as prepared statements. You can migrate standard update batching into an Oracle JDBC application without difficulty.

• You can batch only UPDATE, INSERT, or DELETE operations. Processing a batch that includes an operation that attempts to return a result set will cause an exception.

  Note:

  The Oracle implementation of standard update batching does not implement true batching for generic statements and callable statements. Although Oracle JDBC supports the use of standard batching syntax for Statement and CallableStatement objects, you will see performance improvement for only PreparedStatement objects.

Oracle Update Batching

The Oracle update batching feature associates a batch value with each prepared statement object. With Oracle update batching, instead of the JDBC driver running a prepared statement each time executeUpdate method is called, the driver adds the statement to a batch of accumulated processing requests. The driver will pass all the operations to the database for processing once the batch value is reached. For example, if the batch value is 10, then each batch of 10 operations will be sent to the database and processed in one trip.

A method in the OracleConnection class enables you to set a default batch value for the Oracle connection as a whole, and this batch value applies to any Oracle prepared statement in the connection. For any particular Oracle prepared statement, a method in the OraclePreparedStatement class enables you to set a statement batch value that overrides the connection batch value. You can also override both batch values by choosing to manually process the pending batch.

Oracle Update Batching Characteristics and Limitations

Note the following limitations and implementation details regarding Oracle update batching:

• By default, there is no statement batch value and the connection batch value is 1.

• Batch values between 5 and 30 tend to be the most effective. Setting a very high value might even have a negative effect. It is worth trying different values to verify the effectiveness for your particular application.

• Regardless of the batch value in effect, if any of the bind variables of an Oracle prepared statement is a stream type, then the Oracle JDBC driver sets the batch value to 1 and sends any queued requests to the database for processing.

• The Oracle JDBC driver automatically runs the sendBatch method of an Oracle prepared statement in any of the following circumstances:

  • The connection receives a COMMIT request, either as a result of calling the commit method or as a result of auto-commit mode.

  • The statement receives a close request.

  • The connection receives a close request.

  Note:

  A connection COMMIT request, statement close, or connection close has an effect on a pending batch only if you use Oracle update batching. However, if you use standard update batching, then it has no effect on a pending batch.

• If the connection receives a ROLLBACK request before sendBatch has been called, then the pending batched operations are not removed. You must explicitly call clearBatch to do this.

Setting the Connection Batch Value

You can specify a default batch value for any Oracle prepared statement in your Oracle connection. To do this, use the setDefaultExecuteBatch method of the OracleConnection object. For example, the following code sets the default batch value to 20 for all prepared statement objects associated with the conn connection object:

((OracleConnection)conn).setDefaultExecuteBatch(20);

Even though this sets the default batch value for all the prepared statements of the connection, you can override it by calling setDefaultBatch on individual Oracle prepared statements.

The connection batch value will apply to statement objects created after this batch value was set.

Note that instead of calling setDefaultExecuteBatch, you can set the defaultBatchValue Java property if you use a Java Properties object in establishing the connection.

Setting the Statement Batch Value

Use the following steps to set the statement batch value for a particular Oracle prepared statement. This will override any connection batch value set using the setDefaultExecuteBatch method of the OracleConnection instance for the connection in which the statement is processed.

1. Write your prepared statement, and specify input values for the first row, as follows:

   PreparedStatement ps = conn.prepareStatement 
                                 ("INSERT INTO dept VALUES (?,?,?)");
   ps.setInt (1,12);
   ps.setString (2,"Oracle");
   ps.setString (3,"USA");
   
   

2. Cast your prepared statement to OraclePreparedStatement, and apply the setExecuteBatch method. In this example, the batch size of the statement is set to 2.

   ((OraclePreparedStatement)ps).setExecuteBatch(2);
   
   

   If you wish, insert the getExecuteBatch method at any point in the program to check the default batch value for the statement, as follows:

   System.out.println (" Statement Execute Batch Value " +
                      ((OraclePreparedStatement)ps).getExecuteBatch());
   
   

3. If you send an execute-update call to the database at this point, then no data will be sent to the database, and the call will return 0.

   // No data is sent to the database by this call to executeUpdate
   System.out.println ("Number of rows updated so far: "
                                     + ps.executeUpdate ());
   
   

4. If you enter a set of input values for a second row and an execute-update, then the number of batch calls to executeUpdate will be equal to the batch value of 2. The data will be sent to the database, and both rows will be inserted in a single round trip.

   ps.setInt (1, 11);
   ps.setString (2, "Applications");
   ps.setString (3, "Indonesia");
   
   int rows = ps.executeUpdate ();
   System.out.println ("Number of rows updated now: " + rows);
   
   ps.close ();
   

Checking the Batch Value

To check the overall connection batch value of an Oracle connection instance, use the OracleConnection class getDefaultExecuteBatch method:

Integer batch_val = ((OracleConnection)conn).getDefaultExecuteBatch();

To check the particular statement batch value of an Oracle prepared statement, use the OraclePreparedStatement class getExecuteBatch method:

Integer batch_val = ((OraclePreparedStatement)ps).getExecuteBatch();

Overriding the Batch Value

If you want to process accumulated operations before the batch value in effect is reached, then use the sendBatch method of the OraclePreparedStatement object.

For this example, presume you set the connection batch value to 20. This sets the default batch value for all prepared statement objects associated with the connection to 20. You can accomplish this by casting your connection to OracleConnection and applying the setDefaultExecuteBatch method for the connection, as follows:

((OracleConnection)conn).setDefaultExecuteBatch (20);

Override the batch value as follows:

1. Write your prepared statement, specify input values for the first row, and then process the statement, as follows:

   PreparedStatement ps =
      conn.prepareStatement ("insert into dept values (?, ?, ?)");
       
   ps.setInt (1, 32);
   ps.setString (2, "Oracle");
   ps.setString (3, "USA");
   
   System.out.println (ps.executeUpdate ()); 
   
   

   The batch is not processed at this point. The ps.executeUpdate method returns 0.

2. If you enter a set of input values for a second operation and call executeUpdate again, then the data will still not be sent to the database, because the batch value in effect for the statement is the connection batch value, which is 20.

   ps.setInt (1, 33);
   ps.setString (2, "Applications");
   ps.setString (3, "Indonesia");
   
   // this batch is still not executed at this point
   int rows = ps.executeUpdate ();  
       
   System.out.println ("Number of rows updated before calling sendBatch: "
                           + rows);
   
   

   Note that the value of rows in the println statement is 0.

3. If you apply the sendBatch method at this point, then the two previously batched operations will be sent to the database in a single round trip. The sendBatch method also returns the total number of updated rows. This property of sendBatch is used by println to print the number of updated rows.

   // Execution of both previously batched executes will happen
   // at this point. The number of rows updated will be
   // returned by sendBatch.
   rows = ((OraclePreparedStatement)ps).sendBatch ();
   
   System.out.println ("Number of rows updated by calling sendBatch: "
                           + rows);
   ps.close ();
   
   

Committing the Changes in Oracle Batching

After you process the batch, you must still commit the changes, presuming auto-commit is disabled as recommended.

Calling commit on the connection object in Oracle batching not only commits operations in batches that have been processed, but also issues an implicit sendBatch call to process all pending batches. So commit effectively commits changes for all operations that have been added to a batch.

Update Counts in Oracle Batching

In a non-batching situation, the executeUpdate method of an OraclePreparedStatement object will return the number of database rows affected by the operation.

In an Oracle batching situation, this method returns the number of rows affected at the time the method is invoked, as follows:

• If an executeUpdate call results in the operation being added to the batch, then the method returns a value of 0, because nothing was written to the database yet.

• If an executeUpdate call results in the batch value being reached and the batch being processed, then the method will return the total number of rows affected by all operations in the batch.

Similarly, the sendBatch method of an OraclePreparedStatement object returns the total number of rows affected by all operations in the batch.

Example 25-1 illustrates the use of Oracle update batching.

...
OracleDataSource ods = new OracleDataSource();
ods.setURL("jdbc:oracle:oci);
ods.setUser("scott");
ods.setPassword("tiger");

Connection conn = ods.getConnection();
conn.setAutoCommit(false);

PreparedStatement ps = 
  conn.prepareStatement("insert into dept values (?, ?, ?)"); 
     
//Change batch size for this statement to 3 
((OraclePreparedStatement)ps).setExecuteBatch (3);
 
ps.setInt(1, 23); 
ps.setString(2, "Sales"); 
ps.setString(3, "USA"); 
ps.executeUpdate(); //JDBC queues this for later execution 
 
ps.setInt(1, 24); 
ps.setString(2, "Blue Sky"); 
ps.setString(3, "Montana"); 
ps.executeUpdate(); //JDBC queues this for later execution 
 
ps.setInt(1, 25); 
ps.setString(2, "Applications"); 
ps.setString(3, "India"); 
ps.executeUpdate(); //The queue size equals the batch value of 3 
                    //JDBC sends the requests to the database

ps.setInt(1, 26); 
ps.setString(2, "HR"); 
ps.setString(3, "Mongolia"); 
ps.executeUpdate(); //JDBC queues this for later execution
 
((OraclePreparedStatement)ps).sendBatch(); // JDBC sends the queued request
conn.commit();

ps.close();
...

UPDATE emp SET name = "Sue" WHERE name = "Bob";
SELECT name FROM emp WHERE name = "Sue";

Standard Update Batching

Oracle implements the standard update batching model according to the JDBC 2.0 specification.

This model, unlike the Oracle update batching model, depends on explicitly adding statements to the batch using an addBatch method and explicitly processing the batch using an executeBatch method. In the Oracle model, you call executeUpdate as in a non-batching situation, but whether an operation is added to the batch or the whole batch is processed is typically determined implicitly, depending on whether a pre-determined batch value is reached.

Limitations in the Oracle Implementation of Standard Batching

Note the following limitations and implementation details regarding the Oracle implementation of standard update batching:

• In Oracle JDBC applications, update batching is intended for use with prepared statements that are being processed repeatedly with different sets of bind values.

  The Oracle implementation of standard update batching does not implement true batching for generic statements and callable statements. Even though Oracle JDBC supports the use of standard batching for Statement and CallableStatement objects, you are unlikely to see performance improvement.

• The Oracle implementation of standard update batching does not support stream types as bind values. Any attempt to use stream types will result in an exception.

Adding Operations to the Batch

When any statement object is first created, its statement batch is empty. Use the standard addBatch method to add an operation to the statement batch. This method is specified in the standard java.sql.Statement, PreparedStatement, and CallableStatement interfaces, which are implemented by the oracle.jdbc.OracleStatement, OraclePreparedStatement, and OracleCallableStatement interfaces, respectively.

For a Statement object, the addBatch method takes a Java String with a SQL operation as input. For example:

...
Statement stmt = conn.createStatement();

stmt.addBatch("INSERT INTO emp VALUES(1000, 'Joe Jones')");
stmt.addBatch("INSERT INTO dept VALUES(260, 'Sales')");
stmt.addBatch("INSERT INTO emp_dept VALUES(1000, 260)");
...

At this point, three operations are in the batch.

For prepared statements, update batching is used to batch multiple runs of the same statement with different sets of bind parameters. For a PreparedStatement or OraclePreparedStatement object, the addBatch method takes no input. It simply adds the operation to the batch using the bind parameters last set by the appropriate setXXX methods. This is also true for CallableStatement or OracleCallableStatement objects, but remember that in the Oracle implementation of standard update batching, you will probably see no performance improvement in batching callable statements.

For example:

...
PreparedStatement pstmt = 
          conn.prepareStatement("INSERT INTO employees VALUES(?, ?)");

pstmt.setInt(1, 2000);
pstmt.setString(2, "Milo Mumford");
pstmt.addBatch();

pstmt.setInt(1, 3000);
pstmt.setString(2, "Sulu Simpson");
pstmt.addBatch();
...

At this point, two operations are in the batch.

Because a batch is associated with a single prepared statement object, you can batch only repeated runs of a single prepared statement, as in this example.

Processing the Batch

To process the current batch of operations, use the executeBatch method of the statement object. This method is specified in the standard Statement interface, which is extended by the standard PreparedStatement and CallableStatement interfaces.

Following is an example that repeats the prepared statement addBatch calls shown previously and then processes the batch:

...
PreparedStatement pstmt = 
          conn.prepareStatement("INSERT INTO employees VALUES(?, ?)");

pstmt.setInt(1, 2000);
pstmt.setString(2, "Milo Mumford");
pstmt.addBatch();

pstmt.setInt(1, 3000);
pstmt.setString(2, "Sulu Simpson");
pstmt.addBatch();

int[] updateCounts = pstmt.executeBatch();
...

The executeBatch method returns an int array, typically one element per batched operation, indicating success or failure in processing the batch and sometimes containing information about the number of rows affected.

Committing the Changes in the Oracle Implementation of Standard Batching

After you process the batch, you must still commit the changes, presuming auto-commit is disabled as recommended.

Calling commit, commits non-batched operations and batched operations for statement batches that have been processed, but for the Oracle implementation of standard batching, has no effect on pending statement batches that have not been processed.

Clearing the Batch

To clear the current batch of operations instead of processing it, use the clearBatch method of the statement object. This method is specified in the standard Statement interface, which is extended by the standard PreparedStatement and CallableStatement interfaces.

Following is an example that repeats the prepared statement addBatch calls shown previously but then clears the batch under certain circumstances:

...
PreparedStatement pstmt = 
          conn.prepareStatement("INSERT INTO employees VALUES(?, ?)");

pstmt.setInt(1, 2000);
pstmt.setString(2, "Milo Mumford");
pstmt.addBatch();

pstmt.setInt(1, 3000);
pstmt.setString(2, "Sulu Simpson");
pstmt.addBatch();

if (...condition...)
{
   int[] updateCounts = pstmt.executeBatch();
   ...
}
else
{
   pstmt.clearBatch();
   ...
}

Update Counts in the Oracle Implementation of Standard Batching

If a statement batch is processed successfully, then the integer array, or update counts array, returned by the statement executeBatch call will always have one element for each operation in the batch. In the Oracle implementation of standard update batching, the values of the array elements are as follows:

• For a prepared statement batch, it is not possible to know the number of rows affected in the database by each individual statement in the batch. Therefore, all array elements have a value of -2. According to the JDBC 2.0 specification, a value of -2 indicates that the operation was successful but the number of rows affected is unknown.

• For a generic statement batch the array contains the actual update counts indicating the number of rows affected by each operation. The actual update counts can be provided only in the case of generic statements in Oracle implementation of standard batching.

• For a callable statement batch, the server always returns the value 1 as the update count, irrespective of the number rows affected by each operation.

In your code, upon successful processing of a batch, you should be prepared to handle either -2, 1, or true update counts in the array elements. For a successful batch processing, the array contains either all -2, 1, or all positive integers.

Example 25-2 illustrates the use of standard update batching.

conn.setAutoCommit(false);

PreparedStatement pstmt = 
          conn.prepareStatement("INSERT INTO employees VALUES(?, ?)");

pstmt.setInt(1, 2000);
pstmt.setString(2, "Milo Mumford");
pstmt.addBatch();

pstmt.setInt(1, 3000);
pstmt.setString(2, "Sulu Simpson");
pstmt.addBatch();

int[] updateCounts = pstmt.executeBatch();

conn.commit();

pstmt.close();
...

Error Handling in the Oracle Implementation of Standard Batching

If any one of the batched operations fails to complete successfully or attempts to return a result set during an executeBatch call, then the processing stops and a java.sql.BatchUpdateException is generated.

After a batch exception, the update counts array can be retrieved using the getUpdateCounts method of the BatchUpdateException object. This returns an int array of update counts, just as the executeBatch method does. In the Oracle implementation of standard update batching, contents of the update counts array are as follows, after a batch is processed:

• For a prepared statement batch, it is not possible to know which operation failed. The array has one element for each operation in the batch, and each element has a value of -3. According to the JDBC 2.0 specification, a value of -3 indicates that an operation did not complete successfully. In this case, it was presumably just one operation that actually failed, but because the JDBC driver does not know which operation that was, it labels all the batched operations as failures.

  You should always perform a ROLLBACK operation in this situation.

• For a generic statement batch or callable statement batch, the update counts array is only a partial array containing the actual update counts up to the point of the error. The actual update counts can be provided because Oracle JDBC cannot use true batching for generic and callable statements in the Oracle implementation of standard update batching.

  For example, if there were 20 operations in the batch, the first 13 succeeded, and the 14th generated an exception, then the update counts array will have 13 elements, containing actual update counts of the successful operations.

  You can either commit or roll back the successful operations in this situation, as you prefer.

In your code, upon failed processing of a batch, you should be prepared to handle either -3 or true update counts in the array elements when an exception occurs. For a failed batch processing, you will have either a full array of -3 or a partial array of positive integers.

Intermixing Batched Statements and Non-Batched Statements

You cannot call executeUpdate for regular, non-batched processing of an operation if the statement object has a pending batch of operations.

However, you can intermix batched operations and non-batched operations in a single statement object if you process non-batched operations either prior to adding any operations to the statement batch or after processing the batch. Essentially, you can call executeUpdate for a statement object only when its update batch is empty. If the batch is non-empty, then an exception will be generated.

For example, it is legal to have a sequence, such as the following:

...
PreparedStatement pstmt = 
          conn.prepareStatement("INSERT INTO employees VALUES(?, ?)");

pstmt.setInt(1, 2000);
pstmt.setString(2, "Milo Mumford");

int scount = pstmt.executeUpdate();   // OK; no operations in pstmt batch

pstmt.setInt(1, 3000);
pstmt.setString(2, "Sulu Simpson");
pstmt.addBatch();                    // Now start a batch

pstmt.setInt(1, 4000);
pstmt.setString(2, "Stan Leland");
pstmt.addBatch();

int[] bcounts = pstmt.executeBatch();

pstmt.setInt(1, 5000);
pstmt.setString(2, "Amy Feiner");

int scount = pstmt.executeUpdate();   // OK; pstmt batch was executed
...

Intermixing non-batched operations on one statement object and batched operations on another statement object within your code is permissible. Different statement objects are independent of each other with regards to update batching operations. A COMMIT request will affect all non-batched operations and all successful operations in processed batches, but will not affect any pending batches.

Premature Batch Flush

Premature batch flush happens due to a change in cached metadata. Cached metadata can be changed due to various reasons, such as the following:

• The initial bind was null and the following bind is not null

• A scalar type is initially bound as string and then bound as scalar type or the reverse

The premature batch flush count is summed to the return value of the next executeUpdate or sendBatch method.

The old functionality lost all these batch flush values which can be obtained now. To switch back to the old functionality, you can set the AccumulateBatchResult property to false, as follows:

java.util.Properties info = new java.util.Properties(); 
info.setProperty("user", "SCOTT"); 
info.setProperty("passwd", "TIGER"); 
// other properties 
... 

// property: batch flush type 
info.setProperty("AccumulateBatchResult", "false");

OracleDataSource ods = new OracleDataSource();
ods.setConnectionProperties(info);
ods.setURL("jdbc:oracle:oci:@"");
Connection conn = ods.getConnection(); 

Example 25-3 illustrates premature batch flushing.

((OraclePreparedStatement)pstmt).setExecuteBatch (2); 

pstmt.setNull (1, OracleTypes.NUMBER); 
pstmt.setString (2, "test11"); 
int count = pstmt.executeUpdate (); // returns 0 

/* 
* Premature batch flush happens here. 
*/ 
pstmt.setInt (1, 22); 
pstmt.setString (2, "test22"); 
int count = pstmt.executeUpdate (); // returns 0 

pstmt.setInt (1, 33); 
pstmt.setString (2, "test33"); 
/* 
*  returns 3 with the new batching scheme where as, 
*  returns 2 with the old batching scheme. 
*/ 
int count = pstmt.executeUpdate ();

Oracle Row Prefetching

Oracle JDBC drivers include extensions that enable you to set the number of rows to prefetch into the client while a result set is being populated during a query. This feature reduces the number of round trips to the server.

This section covers the following topics:

• Setting the Oracle Prefetch Value

• Oracle Row-Prefetching Limitations

• Considerations for getProcedures and getProcedureColumns Methods

OracleDataSource ods = new OracleDataSource();ods.setURL("jdbc:oracle:oci:");ods.setUser("scott");ods.setPassword("tiger");Connection conn = ods.getConnection();

//Set the default row-prefetch setting for this connection 
((OracleConnection)conn).setDefaultRowPrefetch(7); 

/* The following statement gets the default row-prefetch value for
   the connection, that is, 7.
 */
Statement stmt = conn.createStatement(); 

/* Subsequent statements look the same, regardless of the row
   prefetch value. Only execution time changes. 
 */
ResultSet rset = stmt.executeQuery("SELECT ename FROM emp");  
System.out.println( rset.next () ); 

while( rset.next () ) 
    System.out.println( rset.getString (1) ); 

//Override the default row-prefetch setting for this statement
( (OracleStatement)stmt ).setRowPrefetch (2); 

ResultSet rset = stmt.executeQuery("SELECT ename FROM emp");  
System.out.println( rset.next () ); 

while( rset.next() ) 
   System.out.println( rset.getString (1) ); 

stmt.close(); 

Defining Column Types

The implementation of defineColumnType has changed significantly in Oracle Database 10g. Previously, defineColumnType was used both as a performance optimization and to force data type conversion. In previous releases, all of the drivers benefited from calls to defineColumnType. In Oracle Database 10g, the JDBC Thin driver no longer needs the information provided. The JDBC Thin driver achieves maximum performance without calls to defineColumnType. The JDBC Oracle Call Interface (OCI) and server-side internal drivers still get better performance when the application uses defineColumnType.

If your code is used with both the JDBC Thin and OCI drivers, you can disable the defineColumnType method when using the Thin by setting the connection property disableDefineColumnType to true. Doing this makes defineColumnType have no effect. Do not set this connection property to true when using the JDBC OCI or server-side internal drivers.

You can also use defineColumnType to control how much memory the client side allocates or to limit the size of variable-length data.

Follow these general steps to define column types for a query:

1. If necessary, cast your statement object to OracleStatement, OraclePreparedStatement, or OracleCallableStatement, as applicable.

2. If necessary, use the clearDefines method of your Statement object to clear any previous column definitions for this Statement object.

3. On each column, call the defineColumnType method of your Statement object, passing it these parameters:

   • Column index (integer)

   • Type code (integer)

     Use the static constants of the java.sql.Types class or oracle.jdbc.OracleTypes class, such as Types.INTEGER, Types.FLOAT, Types.VARCHAR, OracleTypes.VARCHAR, and OracleTypes.ROWID. Type codes for standard types are identical in these two classes.

   • Type name (string)

     For structured objects, object references, and arrays, you must also specify the type name. For example, Employee, EmployeeRef, or EmployeeArray.

   • Maximum field size (integer)

     Optionally specify a maximum data length for this column.

     You cannot specify a maximum field size parameter if you are defining the column type for a structured object, object reference, or array. If you try to include this parameter, it will be ignored.

   • Form-of-use (short)

     Optionally specify a form of use for the column. This can be OraclePreparedStatement.FORM_CHAR to use the database character set or OraclePreparedStatement.FORM_NCHAR to use the national character set. If this parameter is omitted, the default is FORM_CHAR.

   For example, assuming stmt is an Oracle statement, use:

   stmt.defineColumnType(column_index, typeCode);
   
   

   or, if the column is VARCHAR or equivalent and you know the length limit:

   stmt.defineColumnType(column_index, typeCode, max_size);
   
   

   or, for an NVARCHAR column where the original maximum length is desired and conversion to the database character set is requested:

   stmt.defineColumnType(column_index, typeCode, 0,
      OraclePreparedStatement.FORM_CHAR );
   
   

   or, for structured object, object reference, and array columns:

   stmt.defineColumnType(column_index, typeCode, typeName);
   
   

   Set a maximum field size if you do not want to receive the full default length of the data. Calling the setMaxFieldSize method of the standard JDBC Statement class sets a restriction on the amount of data returned. Specifically, the size of the data returned will be the minimum of the following:

   • The maximum field size set in defineColumnType

   • The maximum field size set in setMaxFieldSize

   • The natural maximum size of the data type

After you complete these steps, use the executeQuery method of the statement to perform the query.

Example 25-5 illustrates the use of this feature. It assumes you have imported the oracle.jdbc.* interfaces.

OracleDataSource ods = new OracleDataSource();ods.setURL("jdbc:oracle:thin:@localhost:1502:orcl");ods.setUser("scott");ods.setPassword("tiger");Connection conn = ods.getConnection();

Statement stmt = conn.createStatement();// Allocate only 2 chars for this column (truncation will happen)((OracleStatement)stmt).defineColumnType(1, Types.VARCHAR, 2);ResultSet rset = stmt.executeQuery("select ename from emp");while (rset.next() )  System.out.println(rset.getString(1));stmt.close();

As this example shows, you must cast the Statement object, stmt, to OracleStatement in the invocation of the defineColumnType method. The createStatement method of the connection returns an object of type java.sql.Statement, which does not have the defineColumnType and clearDefines methods. These methods are provided only in the OracleStatement implementation.

The define-extensions use JDBC types to specify the desired types. The allowed define types for columns depend on the internal Oracle type of the column.

All columns can be defined to their natural JDBC types. In most cases, they can be defined to the Types.CHAR or Types.VARCHAR type code.

Table 25-1 lists the valid column definition arguments you can use in the defineColumnType method.

It is always valid to use defineColumnType with the original data type of the column.

DatabaseMetaData TABLE_REMARKS Reporting

The getColumns, getProcedureColumns, getProcedures, and getTables methods of the database metadata classes are slow if they must report TABLE_REMARKS columns, because this necessitates an expensive outer join. For this reason, the JDBC driver does not report TABLE_REMARKS columns by default.

You can enable TABLE_REMARKS reporting by passing a true argument to the setRemarksReporting method of an OracleConnection object.

Equivalently, instead of calling setRemarksReporting, you can set the remarksReporting Java property if you use a Java Properties object in establishing the connection.

If you are using a standard java.sql.Connection object, you must cast it to OracleConnection to use setRemarksReporting.

Example 25-6 illustrates how to enable TABLE_REMARKS reporting.

( (oracle.jdbc.OracleConnection)conn ).setRemarksReporting(true);

Considerations for getColumns

By default, the getColumns method does not retrieve information about the columns if a synonym is specified. To enable the retrieval of information if a synonym is specified, you must call the setIncludeSynonyms method on the connection as follows:

( (oracle.jdbc.driver.OracleConnection)conn ).setIncludeSynonyms(true)

This will cause all subsequent getColumns method call on the connection to include synonyms. This is similar to setRemarksReporting. Alternatively, you can set the includeSynonyms connection property. This is similar to the remarksReporting connection property.

However, you need to bear in mind that if includeSynonyms is true, then the name of the object returned in the table_name column will be the synonym name, if a synonym exists. This is true even if you pass the table name to getColumns.

Considerations for getProcedures and getProcedureColumns Methods

According to JDBC versions 1.1 and 1.2, the methods getProcedures and getProcedureColumns treat the catalog, schemaPattern, columnNamePattern, and procedureNamePattern parameters in the same way. In the Oracle definition of these methods, the parameters are treated differently:

• catalog

  Oracle does not have multiple catalogs, but it does have packages. Consequently, the catalog parameter is treated as the package name. This applies both on input, which is the catalog parameter, and the output, which is the catalog column in the returned ResultSet. On input, the construct " ", which is an empty string, retrieves procedures and arguments without a package, that is, standalone objects. A null value means to drop from the selection criteria, that is, return information about both standalone and packaged objects. That is, it has the same effect as passing in %. Otherwise the catalog parameter should be a package name pattern, with SQL wild cards, if desired.

• schemaPattern

  All objects within Oracle must have a schema, so it does not make sense to return information for those objects without one. Thus, the construct " ", which is an empty string, is interpreted on input to mean the objects in the current schema, that is, the one to which you are currently connected. To be consistent with the behavior of the catalog parameter, null is interpreted to drop the schema from the selection criteria. That is, it has the same effect as passing in %. It can also be used as a pattern with SQL wild cards.

• procedureNamePattern and columnNamePattern

  The empty string (" ") does not make sense for either parameter, because all procedures and arguments must have names. Thus, the construct " " will raise an exception. To be consistent with the behavior of other parameters, null has the same effect as passing in %.