10
Performance and Debugging

This chapter discusses features, utilities, and tips to enhance performance of your SQLJ application and to debug your SQLJ source code at runtime. The following topics are discussed:

• Performance Enhancement Features
• SQLJ Debugging Features

Performance Enhancement Features

Oracle SQLJ includes features to enhance your performance by making data access more efficient. These include the following:

• row prefetching--Query results are sent to your application in groups of rows, instead of one at a time.
• update batching--Database updates, inserts, and deletes are sent to the database in batches, instead of one at a time.
• statement caching--Prepared statements are saved in memory and reused, to avoid repeated processing in the server.
• column definitions--Column types and sizes are predefined, possibly saving round trips to the database.
• parameter size definitions--Sizes of host variables are predefined for more efficient memory usage.

For information about underlying Oracle JDBC support for these features, see the Oracle9i JDBC Developer's Guide and Reference.

Your application will likely benefit from the default Oracle-specific code generation (-codegen=oracle). The generated code will be optimized with direct calls to Oracle JDBC, eliminating the overhead of intermediate calls to the SQLJ runtime (which in turn would call JDBC). For information, see "Oracle-Specific Code Generation (No Profiles)".

In addition to the preceding Oracle SQLJ (and JDBC) performance enhancements, you can employ optimizer hints in the SQL operations within a SQLJ program, as you can in any Oracle SQL operations.

Oracle SQL allows you to tune your SQL statements by using "/*+" or "--+" comment notation to pass hints to the Oracle SQL optimizer. The SQLJ translator recognizes and supports these optimizer hints, passing them at runtime as part of your SQL statement.

You can also define cost and selectivity information for a SQLJ stored function, as for any other stored function, using the extensibility features for SQL optimization in Oracle9i. During SQL execution, the optimizer invokes the cost and selectivity methods for the stored function, evaluates alternate strategies for execution, and chooses an efficient execution plan.

For information about the Oracle optimizer, see the Oracle9i SQL Reference.

Note that using Oracle performance extensions in your code requires the following:

• Use one of the Oracle JDBC drivers.
• Use the default Oracle-specific code generation, or customize profiles appropriately.

  For ISO standard code generation, the default customizer, oracle.sqlj.runtime.util.OraCustomizer, is recommended.

• Use the Oracle SQLJ runtime when your application runs.

The Oracle SQLJ runtime and an Oracle JDBC driver are required by your application whenever you customize profiles with the Oracle customizer, even if you do not actually use Oracle extensions in your code.

Row Prefetching

Standard JDBC receives the results of a query one row at a time, with each row requiring a separate round trip to the database. Row prefetching allows you to receive the results more efficiently, in groups of multiple rows each.

Use the setFetchSize() method of an ExecutionContext instance to set the number of rows to be prefetched whenever you execute a SELECT statement (for SQLJ statements using the particular ExecutionContext instance).

The getFetchSize() method of an ExecutionContext instance returns the current prefetch size, as an int value.

Here is an example of setting the prefetch size to 20 by getting the default execution context instance of the default connection context instance and calling the setFetchSize() method:

DefaultContext.getDefaultContext().getExecutionContext().setFetchSize(20);

(It is also possible to set the prefetch size directly on the underlying OracleConnection object, using the JDBC API, but in SQLJ this is discouraged.)

To specify the number of rows to prefetch for queries that use a given connection context instance, use the underlying JDBC connection, cast to a Connection instance. Following is an example that sets the prefetch value to 20 for your default connection:

((Connection)DefaultContext.getDefaultContext().getConnection()).setDefaultRowPrefetch(20);

Each additional connection context instance you use must be set separately, as desired. For example, if ctx is an instance of a declared connection context class, set its prefetch value as follows:

((Connection)ctx.getConnection()).setDefaultRowPrefetch(20);

There is no maximum row-prefetch value. The default is 10 in JDBC, and this is inherited by SQLJ. This value is effective in typical circumstances, although you might want to increase it if you receive a large number of rows.

Statement Caching

SQLJ offers a statement caching feature that improves performance by saving executable statements that are used repeatedly, such as in a loop or in a method that is called repeatedly. The statement cache is located on the client. When a statement is cached before it is re-executed, the code does not have to be reparsed (either on the client or in the server), the statement object does not have to be re-created, and the parameter size definitions do not have to be recalculated. Without this feature, repeated statements would have to be reparsed on the client, and perhaps in the server as well (depending on whether a statement is still available in the general server-side SQL cache when it is encountered again).

For Oracle-specific code generation (the default -codegen=oracle setting), Oracle SQLJ statement caching relies on the Oracle JDBC driver, using the Oracle JDBC explicit caching mechanism. This is distinct from the Oracle JDBC implicit caching mechanism, although there are interdependencies as described below. For more information about these mechanisms, see the Oracle9i JDBC Developer's Guide and Reference. With Oracle-specific code, statement caching is controlled through connection methods.

For ISO code generation (-codegen=iso), SQLJ has its own statement caching mechanism through functionality of the SQLJ runtime. With ISO code, statement caching is controlled through the Oracle customizer stmtcache option.

This section covers the following topics:

• Connection Context Methods for Statement Caching (Oracle-Specific Code)
• Enabling and Disabling Statement Caching (Oracle-Specific Code)
• Key Interactions Between SQLJ/Explicit Caching and JDBC Implicit Caching
• JDBC Support for Statement Caching (ISO Code)
• Oracle Customizer Option for Statement Cache Size (ISO Code)
• Additional Statement Caching Behavior
• Statement Caching Limitations and Notes

  Notes: For Oracle-specific code generation, explicit caching is the only statement caching mechanism that can be manipulated through SQLJ APIs. For the discussion in this document, it will be referred to as "SQLJ/explicit" statement caching. For those who have used statement caching in the past, note that much of the functionality of SQLJ and JDBC statement caching changes significantly as of Oracle9i release 2. Backward compatibility is assured, however.

Connection Context Methods for Statement Caching (Oracle-Specific Code)

If you use Oracle-specific code generation, which is the case with the SQLJ translator default -codegen=oracle setting, use connection context methods for statement caching functionality.

Note that any statement cache size greater than 0 results in SQLJ/explicit statement caching being enabled. By default, it is enabled with a cache size of 5 (five statements).

The following Oracle-specific (non-standard) static methods have been added to the sqlj.runtime.ref.DefaultContext class, and are also included in any connection context classes you declare:

• public static void setDefaultStmtCacheSize(int)

  This sets the default statement cache size for all connection contexts. This becomes the initial statement cache size for any subsequently created instance of any connection context class, not just the class upon which you call the method. The method call does not affect connection context instances that already exist.

• public static int getDefaultStmtCacheSize()

  This retrieves the current default statement cache size for connection contexts.

And the following Oracle-specific instance methods have also been added to the DefaultContext class and are included in any other connection context classes:

• public void setStmtCacheSize(int)
throws java.sql.SQLException

  This sets the statement cache size for the underlying connection of the particular connection context instance (overrides the default).

  Note: If SQLJ/explicit caching is already disabled, setting the size to 0 leaves it disabled. If it is already enabled, setting the size to 0 leaves it "enabled", but renders it non-functional.

• public int getStmtCacheSize()

  This verifies whether SQLJ/explicit statement caching is enabled for the underlying connection of the connection context. If so, it returns the current statement cache size. It can also return either of the following integer constants:

  public static int STMT_CACHE_NOT_ENABLED
  public static int STMT_CACHE_EXCEPTION
  
  

  It is possible for a getStmtCacheSize() call to cause a SQL exception; however, for backward compatibility, this method does not throw the exception directly. When an exception occurs, the method returns the constant STMT_CACHE_EXCEPTION. In this case, you can call the getStmtCacheException() method (described immediately below) to find out what exception occurred.

  If you call getStmtCacheSize() when SQLJ/explicit caching is disabled, then the method returns the constant STMT_CACHE_NOT_ENABLED. As of Oracle9i release 2, this is distinguished from a cache size of 0. Technically, it is possible for SQLJ/explicit caching to be "enabled" (though useless) with a cache size of 0.

• public java.sql.Exception getStmtCacheException()

  See if there is a statement caching exception. There are two scenarios for using this method:

  • Call it if a getStmtCacheSize() call returns STMT_CACHE_EXCEPTION.
  • Call it whenever you create a connection context instance with which you want to use statement caching. This is because of automatic manipulation that occurs with respect to statement cache size whenever you create a connection context instance. (This is described in "Additional Statement Caching Behavior".) If you care about statement caching for the connection context instance, call getStmtCacheException() after creating the instance, to verify there were no problems.

Enabling and Disabling Statement Caching (Oracle-Specific Code)

With Oracle-specific code, to reiterate what was stated earlier, any nonzero statement cache size results in SQLJ/explicit caching being enabled. Because the default size is 5, statement caching is enabled by default.

You cannot explicitly disable SQLJ/explicit statement caching through SQLJ APIs, although you can effectively disable it (render it non-functional) by setting the statement cache size to 0. In this case, the connection context getStmtCacheSize() method might return 0, not STMT_CACHE_NOT_ENABLED.

You can explicitly disable SQLJ/explicit statement caching, or JDBC implicit caching, through JDBC connection APIs. Because SQLJ/explicit caching and JDBC implicit caching use the same cache size, as described in "Key Interactions Between SQLJ/Explicit Caching and JDBC Implicit Caching", there might sometimes be reason to do so. The following methods are available through the OracleConnection class:

• public void setExplicitCachingEnabled(boolean)
• public boolean getExplicitCachingEnabled()
• public void setImplicitCachingEnabled(boolean)
• public boolean getImplicitCachingEnabled()

You have access to these methods if you retrieve the OracleConnection instance from within a SQLJ connection context instance. (See "SQLJ Connection Context and JDBC Connection Interoperability".)

Key Interactions Between SQLJ/Explicit Caching and JDBC Implicit Caching

With regard to statement caching in Oracle-specific code, this document naturally emphasizes SQLJ/explicit caching rather than JDBC implicit caching. If you do not use JDBC code in your application, SQLJ/explicit caching is the only statement caching that is relevant. However, there are situations where you might want to use both SQLJ and JDBC code in your application, and in these circumstances you might also want to use implicit caching.

As described earlier, SQLJ/explicit caching and JDBC implicit caching are enabled independently of each other. Furthermore, you do not have access to the implicit cache through SQLJ.

There is, however, a key interaction between the two--they share the same cache size. If, for example, the statement cache size is 5, then you can have a maximum total of five statements cached for SQLJ/explicit caching and implicit caching combined.

An important point related to this is that if you choose to effectively disable SQLJ/explicit statement caching by setting the cache size to 0, then you have also effectively disabled implicit caching.

Also be aware that if SQLJ/explicit caching is disabled, changing the cache size to a value greater than 0 will enable it, but this does not affect whether implicit caching is enabled.

JDBC Support for Statement Caching (ISO Code)

With ISO standard code generation, specified through the SQLJ translator -codegen=iso setting, statement caching is a standard SQLJ feature that does not require any particular JDBC driver. However, using a driver that implements the interface sqlj.runtime.profile.ref.ClientDataSupport allows more robust caching. Oracle9i JDBC drivers implement this interface, providing the following features:

• a separate cache for each database connection, instead of a single static cache for the entire application
• the ability to share cached statements between multiple instances of a connection context class that share the same underlying connection

When a single cache is used, as is the case with a generic JDBC driver that does not implement ClientDataSupport, a statement executed in one connection can cause a cached statement from another connection to be flushed (if the statement cache size--the maximum number of statements that can be cached--is exceeded).

Oracle Customizer Option for Statement Cache Size (ISO Code)

With ISO standard code generation, statement caching is enabled in your application by default with a cache size of 5 (the same default size as with Oracle-specific code) when you use the Oracle customizer, which is typically executed as part of Oracle SQLJ translation.

You can alter the statement cache size as desired, or effectively disable statement caching with a cache size of 0, through the Oracle customizer stmtcache option. This is set as -P-Cstmtcache=n, where n is an integer. For information, see "Oracle Customizer Statement Cache Size Option (stmtcache)".

If you use multiple connection context classes and, therefore, have multiple profiles, you can set their statement cache sizes individually by running SQLJ (actually, the customizer) separately for each profile.

At runtime, the appropriate SQLJ profile determines the statement cache size for a connection. This would be the profile that corresponds to the first connection context class instantiated for this connection. Its statement cache size setting, if any, is determined according to how you set the Oracle customizer stmtcache option when you customized the profile. The runtime statement cache size for a connection is set when the first statement on that connection is executed.

Additional Statement Caching Behavior

This section describes additional behaviors and interactions of statement caching, applicable with either Oracle-specific or ISO code generation.

Automatic Manipulation of JDBC Statement Cache Size

When a SQLJ connection context object is instantiated, if the statement cache size on the underlying JDBC connection is smaller than the default size for the connection context class, then the SQLJ runtime will attempt to increase the JDBC statement cache size to the connection context default value. This manipulation occurs even with ISO code generation, enabling Oracle JDBC explicit statement caching in the process, although this is actually of no relevance in the ISO code case.

If, on the other hand, the actual JDBC statement cache size is larger, then the SQLJ runtime will not attempt to perform a change in the cache size. The SQLJ runtime checks the actual JDBC cache size against the default size set whenever it creates a SQLJ connection context instance.

It is important to note that these methods have the same effect regardless of the context class on which they are issued, since they modify or report the same underlying static field.

As an example, assume the following connection context class declarations:

#sql context CtxtA;
#sql context CtxtB;

In this case, each of the following three code instructions has the same effect--that whenever a new SQLJ connection context instance is subsequently created, it will not try to enable SQLJ/explicit statement caching:

sqlj.runtime.ref.DefaultContext.setDefaultStmtCacheSize(0);

or:

CtxtA.setDefaultStmtCacheSize(0);

or:

CtxtB.setDefaultStmtCacheSize(0);

Statement Caching Semantics

SQLJ/explicit caching and JDBC implicit caching functionality have different semantics and behaviors. As noted earlier, SQLJ statement caching applies only to single statements used repeatedly, such as in a loop or through repeated calls to the same method. Consider the following example:

#sql { same SQL operaton }; // occurrence #1
...
Java code
...
#sql { same SQL operaton }; // occurrence #2
...
Java code
...
#sql { same SQL operaton }; // occurrence #3
...

Assume the three SQL operations are identical, including white space.

SQLJ caching would consider these three occurrences of the same SQL operation to be three different statements. They will occupy three separate slots in the cache. JDBC implicit caching, however, would recognize these as identical statements, using only a single cache slot for all three. The statement would be reused for occurrence #2 and occurrence #3.

Statement Caching Limitations and Notes

Using a statement cache, even of size 1, will improve the performance of almost any SQLJ application. Be aware of the following, however:

• There is no benefit if each statement is executed only once.
• Try to avoid interleaving statements executed once with statements executed multiple times. The statements being executed only once would needlessly take up space in the statement cache, which becomes an issue when you reach the statement cache size limit. As an alternative, if you use ISO code generation you can use a separate connection context class for statements that are executed only once, and disable statement caching for that connection context class.
• Distinct statements with identical SQL operations are treated the same way as any distinct statements--each is processed and cached separately. As an alternative, put the SQL operation in a method, and call the method repeatedly, instead of using distinct statements.
• Be careful in choosing an appropriate statement cache size. If it is too small, then the cache might fill up, resulting in statements being flushed before they are re-executed. If it is too large, then database resources or program resources may be exhausted.

Also be aware of the following general notes regarding statement caching.

• With Oracle-specific code generation, using separate SQLJ connection context instances to have separate statement caching behavior will not work if the connection contexts share the same underlying JDBC connection instance. This is because under Oracle-specific code generation, SQLJ uses the JDBC statement cache.
• For Oracle applications, the statement cache size plus the maximum number of open JDBC statements in your application (both directly and through SQLJ) should total less than the maximum number of cursors available for a session, because the maximum number of cursors defines the maximum number of statements that can be open simultaneously.
• Using a statement cache generally does not change the execution semantics of an operation itself, although there are some scenarios where it does. For example, if you have a statement that throws an exception when its resources are released, then using a cache would mean that the exception would not be thrown until the connection is closed or the statement is flushed from the cache (which happens when the cache size is exceeded).

Update Batching

Update batching (referred to as batch updates in the Sun Microsystems JDBC 2.0 specification) allows UPDATE, DELETE, and INSERT statements that are batchable and compatible (as discussed below) to be collected into a batch and sent to the database for execution at once, saving round trips to the database. This feature is included in the JDBC 2.0 and SQLJ specifications, and is therefore supported by both Oracle9i JDBC and Oracle9i SQLJ. Update batching is typically used for an operation that is executed repeatedly within a loop.

In SQLJ, update batching is tied to execution context usage. This feature is enabled or disabled in each execution context, independently of any other execution context, and each execution context instance maintains its own batch.

This section covers the following topics:

• Batchable and Compatible Statements
• Enabling and Disabling Update Batching
• Explicit and Implicit Batch Execution
• Canceling a Batch
• Execution Context Update Counts
• Setting a Batch Limit
• Batching Incompatible Statements
• Using Implicit Execution Contexts for Update Batching
• General Cautions Regarding Update Batching
• Error Conditions During Batch Execution
• Recursive Call-ins and Update Batching

  Important: Be aware of the following for update batching: You must use the default Oracle-specific code generation or, for ISO code generation, customize your application with the Oracle customizer. It is highly advisable to disable auto-commit mode. This gives you control of what to commit and what to roll back in case of an error during batch execution.

Batchable and Compatible Statements

Two criteria determine whether a statement can be added to an existing batch of statements:

• Is it batchable? You cannot batch some kinds of statements under any circumstances.
• Is it compatible with statements in the existing batch?

Batchable

For Oracle9i SQLJ, the following kinds of statements are batchable:

• UPDATE
• INSERT
• DELETE

Note the following restriction, however:

• UPDATE and INSERT statements with one or more stream host expressions are not batchable.

In future releases or other implementations of SQLJ, additional kinds of statements might be batchable (such as stored procedure calls or DDL statements).

Compatible

In Oracle9i SQLJ, only multiple instances of the same statement are compatible. This can occur in one of two circumstances:

• A statement is executed repeatedly in a loop.
• A statement is executed in a method, and the method is called repeatedly.

In future releases or other implementations of SQLJ, additional kinds of statements might be compatible (such as instances of different statements that have no host expressions).

Enabling and Disabling Update Batching

SQLJ performs update batching separately for each execution context instance. Each one can have update batching enabled independently of your other execution context instances, and each maintains its own batch.

To enable or disable update batching for a particular execution context instance, use the setBatching() method of that execution context instance. This method takes boolean input, as follows:

...
ExecutionContext ec = new ExecutionContext();
ec.setBatching(true);
...

or:

...
ExecutionContext ec = new ExecutionContext();
ec.setBatching(false);
...

Update batching is disabled by default.

Use the isBatching() method of an execution context instance to determine if update batching is enabled for that execution context:

ExecutionContext ec = new ExecutionContext();
...
boolean batchingOn = ec.isBatching();

This does not, however, indicate whether a batch is currently pending.

Explicit and Implicit Batch Execution

You can explicitly execute a pending update batch as desired, but it might also be implicitly executed under certain circumstances.

Explicit

Use the executeBatch() method of the execution context instance to explicitly execute an update batch. This method returns an int array of update counts with meanings as described in "Execution Context Update Counts".

Following is an example of explicitly executing a batch:

...
ExecutionContext ec = new ExecutionContext();
ec.setBatching(true);
...
double[] sals = ...;
String[] empnos = ...;
for (int i = 0; i < empnos.length; i++)
{
   #sql [ec] { UPDATE emp SET sal = :(sals[i]) WHERE empno = :(empnos[i]) };
}
int[] updateCounts = ec.executeBatch();
...

Implicit

When a pending update batch exists, it is implicitly executed in the following circumstances:

• An executable statement is encountered that is not batchable. In this case the existing batch is executed first, then the non-batchable statement is executed.
• An update statement is encountered that is batchable, but is not compatible with the statements in the existing batch (in other words, is not an instance of the same statement). In this case the batch is executed, then a new batch is created, starting with the incompatible statement.
• A predefined batch limit--that is, a specified number of statements--is reached. This is discussed in "Setting a Batch Limit".

Following is an example. First one batch is created and executed implicitly when an unbatchable statement is encountered, then a new batch is created and executed implicitly when a batchable, but incompatible, statement is encountered:

ExecutionContext ec = new ExecutionContext();
ec.setBatching(true);
...
/* Statements in the following loop will be placed in a batch */
double[] sals = ...;
String[] empnos = ...; 
for (int i = 0; i < empnos.length; i++) 
{
   #sql [ec] { UPDATE emp SET sal = :(sals[i]) WHERE empno = :(empnos[i]) };
}

/* a SELECT is unbatchable so causes the batch to be executed */
double avg;
#sql [ec] { SELECT avg(sal) INTO :avg FROM emp };

/* Statements in the following loop will be placed in a new batch */
double[] comms = ...;
for (int i = 0; i < empnos.length; i++) 
{
   #sql [ec] { UPDATE emp SET comm = :(comms[i]) WHERE empno = :(empnos[i]) };
}

/* the following update is incompatible with the second batch, so causes it to 
be executed */
int smithdeptno = ...;
#sql [ec] { UPDATE emp SET deptno = :smithdeptno WHERE ename = 'Smith' };

To obtain the update count array for a batch executed implicitly, invoke the getBatchUpdateCounts() method of the execution context instance. This returns the update counts for the last batch to be executed successfully in this execution context instance. The following code statement could be inserted after the SELECT and after the last UPDATE:

int[] updateCounts = ec.getBatchUpdateCounts();

The meanings of these update counts are described in "Execution Context Update Counts".

Canceling a Batch

To cancel the batch that is pending in an execution context, use the cancel() method of the execution context instance. You can, for example, cancel a batch that has been executed, but not yet committed, in the event that an exception occurred during batch execution. Following is an example:

...
ExecutionContext ec = new ExecutionContext();
ec.setBatching(true);
...
double[] sals = ...;
String[] empnos = ...;
for (int i = 0; i < empnos.length; i++)
{
   #sql [ec] { UPDATE emp SET sal = :(sals[i]) WHERE empno = :(empnos[i]) };
   if (!check(sals[i], empnos[i])) //assume "check" is a user-supplied function
   {
      ec.cancel();
      throw new SQLException("Process canceled.");
   }
}

try 
{ 
   int[] updateCounts = ec.executeBatch();
} catch ( SQLException exception) { ec.cancel(); }
...

When you cancel a batch, the next batchable statement will start a new batch.

Execution Context Update Counts

In Oracle9i SQLJ, the array of update counts returned by the executeBatch() method or the getBatchUpdateCounts() method of an execution context instance does not contain counts of the number of rows updated by the batched statements, but simply values indicating whether each statement was successful. So its functionality differs from that of the single update count returned by the getUpdateCount() method of the execution context instance when batching is not enabled, which is described in "Status Methods".

As statements are batched, and after batch execution, the single update count returned by getUpdateCount() is also affected.

Value Available from getUpdateCount()

In a batch-enabled environment, the value available from the getUpdateCount() method of the execution context instance is modified after each statement is encountered. It will be updated with one of several ExecutionContext class static int constant values, as follows:

• NEW_BATCH_COUNT--Indicates that a new batch was created for the last statement encountered.
• ADD_BATCH_COUNT--Indicates that the last statement encountered was added to an existing batch.
• EXEC_BATCH_COUNT--Indicates that the pending batch was executed, either explicitly or implicitly, after the last statement was encountered.

If you refer to these constants, use the qualified names:

ExecutionContext.NEW_BATCH_COUNT
ExecutionContext.ADD_BATCH_COUNT
ExecutionContext.EXEC_BATCH_COUNT

Values Available from executeBatch() or getBatchUpdateCounts()

After a batch has been executed, either explicitly or implicitly, the array of values returned by executeBatch() or getBatchUpdateCounts() indicates only whether the statements executed successfully. There is an array element for each batched statement. In accordance with the JDBC 2.0 specification, a value of -2 for an array element indicates that the corresponding statement completed successfully, but that the number of rows it affected is unknown.

Checking all the array values after execution of a batch would not be meaningful. As currently implemented, the only useful test of this array would be to verify the number of statements that were in the batch prior to execution, by checking the number of elements in the array after a successful execution (essentially, after a batch execution that does not produce an exception).

Note that the update counts array is not modified as statements are batched, only as the batch is executed.

Setting a Batch Limit

You can specify that each update batch be executed after a predefined number of statements have been batched, before the next statement would be added. Use the setBatchLimit() method of the execution context instance, inputting a positive, nonzero integer as follows:

...
ExecutionContext ec = new ExecutionContext();
ec.setBatching(true);
ec.setBatchLimit(10);
...
double[] sals = ...;
String[] empnos = ...; 
for (int i = 0; i < 20; i++) 
{
   #sql [ec] { UPDATE emp1 SET sal = :(sals[i]) WHERE empno = :(empnos[i]) };
}

This loop is executed 20 times, with the statements being batched and the batch being executed during the 11th time through the loop, before an 11th statement would be added to the batch. Note that the batch would not be executed a second time in the loop, however. When your application exits the loop, the last ten statements would still be in the batch and would not be executed until another statement is encountered or you execute the batch explicitly.

You can use two special static int constants of the ExecutionContext class as input to the setBatchLimit() method:

• AUTO_BATCH--Allows the SQLJ runtime to determine the batch limit.
• UNLIMITED_BATCH (default)--Specifies that there is no batch limit.

For example:

...
ExecutionContext ec = new ExecutionContext();
ec.setBatching(true);
ec.setBatchLimit(ExecutionContext.AUTO_BATCH);
...

or:

ec.setBatchLimit(ExecutionContext.UNLIMITED_BATCH);
...

To check the current batch limit, use the getBatchLimit() method of the execution context instance.

Batching Incompatible Statements

If you want to batch a statement that is incompatible with statements in an existing batch, without implicitly executing the existing batch, you will have to use a separate execution context instance. Following is an example:

...
ExecutionContext ec1 = new ExecutionContext();
ec1.setBatching(true);
ExecutionContext ec2 = new ExecutionContext();
ec2.setBatching(true);
...
double[] sals = ...;
String[] empnos = ...; 
for (int i = 0; i < empnos.length; i++) 
{
   #sql [ec1] { UPDATE emp1 SET sal = :(sals[i]) WHERE empno = :(empnos[i]) };
   #sql [ec2] { UPDATE emp2 SET sal = :(sals[i]) WHERE empno = :(empnos[i]) };
}
int[] updateCounts1 = ec1.executeBatch();
int[] updateCounts2 = ec2.executeBatch();
...

An alternative is to use a single execution context and separate loops so that all the EMP1 updates are batched and executed prior to the EMP2 updates:

...
ExecutionContext ec = new ExecutionContext();
ec.setBatching(true);
...
double[] sals = ...;
String[] empnos = ...; 
for (int i = 0; i < empnos.length; i++) 
{
   #sql [ec] { UPDATE emp1 SET sal = :(sals[i]) WHERE empno = :(empnos[i]) };
}
for (int i = 0; i < empnos.length; i++) 
{
   #sql [ec] { UPDATE emp2 SET sal = :(sals[i]) WHERE empno = :(empnos[i]) };
}
ec.executeBatch();
...

This example executes the first batch implicitly and the second batch explicitly.

Using Implicit Execution Contexts for Update Batching

All the update batching examples so far have created and specified explicit execution context instances. This is not necessary, however, given that every connection context instance has an implicit execution context instance. For example, you can access the implicit execution context instance of the default connection as follows:

DefaultContext.getDefaultContext().getExecutionContext().setBatching(true);
...
double[] sals = ...;
String[] empnos = ...; 
for (int i = 0; i < empnos.length; i++) 
{
   #sql { UPDATE emp SET sal = :(sals[i]) WHERE empno = :(empnos[i]) };
}
// implicitly execute the batch and commit
#sql { COMMIT };

or, alternatively, you could execute the batch explicitly:

DefaultContext.getDefaultContext().getExecutionContext().executeBatch();

General Cautions Regarding Update Batching

If you use update batching, especially if you mix statements using an unbatched execution context instance with statements using a batched execution context instance, remember the following points:

• If an unbatched statement depends on a batched statement, be sure the batch is executed prior to the unbatched statement.
• A JDBC COMMIT or ROLLBACK operation--that is, an auto-commit or any explicit use of the commit() method or rollback() method of a JDBC Connection instance--does not execute pending statements in a batch.

  It is important to note, however, that using a SQLJ COMMIT or ROLLBACK statement, such as follows, will execute pending statements in a batch:

  #sql { COMMIT };
  
  

  or:

  #sql { ROLLBACK };
  
  

  This is another reason that you should always commit or roll back changes using #sql syntax, which cleans up both SQLJ resources and JDBC resources.

• When a batch is implicitly executed as a result of an unbatchable or incompatible statement being encountered, the batch is executed before the unbatchable/incompatible statement is executed, but after the input parameters of that statement have been evaluated and passed to the statement.
• If you no longer intend to use a particular batch-enabled execution context instance, then explicitly execute or cancel its pending batch to free resources.

Error Conditions During Batch Execution

In the event that a statement causes an exception in the middle of a batch execution, be aware of the following:

• Batched statements following the statement that caused the exception are not executed.
• Batched statements that had already been executed prior to the exception are not rolled back.
• If the batch where the exception occurred was executed implicitly as the result of another (unbatchable or incompatible) statement being encountered, that statement is not executed.

  Note: Presumably you have disabled auto-commit mode when using update batching. This gives you commit/rollback control in case of an error during batch execution.

When an exception occurs during batch execution under JDBC 2.0 or higher, it is typically an instance of the standard java.sql.BatchUpdateException class, a subclass of the java.sql.SQLException class. (Under JDK 1.1.x, the Oracle JDBC drivers support update-batching exceptions with the oracle.jdbc2.BatchUpdateException class.)

The BatchUpdateException class has a getUpdateCounts() method that, for batched statements successfully executed before the exception occurred, returns an array of update counts equivalent to what would be returned by the ExecutionContext class executeBatch() or getBatchUpdateCounts() method.

Recursive Call-ins and Update Batching

As discussed in "Recursive SQLJ Calls in the Server", execution of SQLJ stored procedures where one calls the other can result in situations where the two procedures are simultaneously using the same execution context instance. The update-batching flag (set using the setBatching() method of the execution context instance) would behave in the same way as other execution context attributes--regardless of which stored procedure sets it, it would affect the next executable statement in either stored procedure.

For this reason, update batching is automatically disabled in the server whenever a recursive call-in occurs. The pending batch is executed, and no batching occurs in the recursively invoked procedure.

To avoid this behavior, use explicit execution context instances in batch-enabled stored procedures.

Column Definitions

Oracle SQLJ reflects Oracle JDBC support for column type and size definitions. Depending on the driver implementation, which differs somewhat among the different Oracle JDBC drivers, registering column types and sizes can save a trip to the database for each query. In particular, this is true for the Oracle JDBC Thin driver and use of positional iterators.

Oracle SQLJ Implementation of Column Definitions

If you enable column definitions, Oracle SQLJ takes the following steps to automatically register column types and sizes:

• During customization, or during translation when the default Oracle-specific code generation is used (-codegen=oracle), Oracle SQLJ connects to a specified database schema to determine types and sizes of columns being retrieved. With ISO standard SQLJ code generation, the column defaults become part of the SQLJ profile. This can be accomplished during the customization step of source code translation, or during separate customization of an existing profile.
• When your application executes, the SQLJ runtime will use the column information to register the column types and sizes with the JDBC driver, using a call to the defineColumnType() method available in the Oracle JDBC statement classes.

Customizer and Translator Options for Column Definitions

To enable column definitions, set SQLJ options as follows:

• Enable the optcols flag. For Oracle-specific code generation, use the SQLJ translator -optcols option. For ISO standard code generation, use either the translator option or the Oracle customizer option (-P-Coptcols on the SQLJ command line).
• Set the user, password, and URL for a database connection. For Oracle-specific code generation, this is through the SQLJ translator -user, -password, and -url options. For ISO standard code generation, this can be through the translator options or you can separately use the customizer options (-P-user, -P-password, and -P-url on the SQLJ command line). In addition, set the JDBC driver class (-P-driver on the SQLJ command line) if you are not using the default OracleDriver class.

For information about the translator options, see "Column Definitions (-optcols)" and "Connection Options".

For information about the customizer options, see the optcols section under "Overview of Customizer-Specific Options", and the user, password, url, and driver sections under "Overview of Customizer Harness Options".

Parameter Size Definitions

Oracle JDBC and Oracle SQLJ allow you to optimize JDBC resource allocation by defining parameter sizes--sizes of Java host variables used as any of the following:

• input or output parameters in stored procedure or function calls
• return values from stored function calls
• input or output parameters in SET statements
• input or output parameters in PL/SQL blocks

Oracle SQLJ Implementation of Parameter Size Definitions

Oracle SQLJ implements parameter size definitions through option settings, in combination with "hints" embedded in source code comments. For ISO standard SQLJ code generation, Oracle customizer options are available. For the default Oracle-specific code generation, equivalent SQLJ translator options are available.

Use options and hints as follows:

• Enable parameter size definitions through the SQLJ translator or Oracle customizer parameter definition flag.
• Specify default sizes for particular datatypes through the SQLJ translator or Oracle customizer parameter default size option.
• Override datatype default sizes for particular parameters by embedding hints in source code comments, following a prescribed format.

For any given host variable, when parameter size definitions are enabled, resources are allocated according to the source code hint if there is one. If there is no source code hint, then the default size for the corresponding datatype is used if one was specified. If there is no source code hint or appropriate default size, then maximum resources are allocated according to the JDBC implementation.

When your application executes, the parameter sizes are registered through calls to the defineParameterType() and registerOutParameter() methods available in the Oracle JDBC statement classes.

Customizer and Translator Options for Parameter Size Definitions

Use the following SQLJ options for parameter size definitions:

• Use the optparams flag to enable parameter size definitions. For Oracle-specific code generation, use the SQLJ translator -optparams option. For ISO standard code generation, use either the translator option or the Oracle customizer option (-P-Coptparams on the SQLJ command line).
• Use optparamdefaults to set default sizes for particular datatypes. For Oracle-specific code generation, use the SQLJ translator -optparamdefaults=xxxx option. For ISO standard code generation, use either the translator option or the Oracle customizer option, -P-Coptparamdefaults=xxxx on the SQLJ command line.

For information about the translator options, see "Parameter Definitions (-optparams)" and "Parameter Default Size (-optparamdefaults)".

For information about the customizer options, see the appropriate sections under "Overview of Customizer-Specific Options".

Source Code Hints for Parameter Size Definitions

Embed source code hints for parameter size definitions within your SQLJ statements in the following format (you can add white space within the comment, as desired):

/*(size)*/

The size is in bytes. Hints are ignored if the optparams flag is disabled.

You can override the default parameter size, without specifying a new size (leaving size allocation to the JDBC implementation), as follows:

/*()*/

Here is an example:

byte[] hash;
String name=Tyrone;
String street=2020 Meryl Street;
String city=Wichita;
String state=Kansas;
String zipcode=77777;
#sql hash = { /* (5) */ VALUES (ADDR_HASH(:name /* (20) */, :street /* () */, 
                               :city, :state, :INOUT zipcode /* (10) */ )) };

A hint for a result expression, such as the result expression hash in the example, must be the first item appearing inside the brackets of the SQLJ statement, as shown. Hints for input and output host variables must immediately follow the variables, as shown.

The example sets parameter sizes as follows:

• hash--5 bytes
• name--20 bytes
• street--override default, but with no setting (leave allocation up to JDBC)
• city--none (use appropriate datatype default, if any)
• state--none (use appropriate datatype default, if any)
• zipcode--10 bytes

  Note: If any parameter size is altered such that its actual size exceeds its registered size at runtime, a SQL exception will be thrown.

SQLJ Debugging Features

This section summarizes debugging features in Oracle SQLJ and refers to other parts of this manual where these features are described in more detail.

• If you run SQLJ from the command line, the -linemap option (or -jdblinemap option if you are using the jdb debugger) can aid in debugging your SQLJ code.
• The embedded server-side translator has an option that will aid in debugging your Java code in general, but not your SQLJ code in particular.
• If you use ISO standard code generation, you can use the specialized customizer AuditorInstaller to install "auditors" for use in debugging.
• SQLJ is integrated into the Oracle9i JDeveloper integrated development environment, allowing access to JDeveloper debugging facilities.

SQLJ -linemap Flag for Debugging

The -linemap flag instructs SQLJ to map line numbers from a SQLJ source code file to locations in the corresponding .class file. This will be the .class file created during compilation of the .java file generated by the SQLJ translator. As a result of this, when Java runtime errors occur, the line number reported by the Java virtual machine (JVM) is the line number in the SQLJ source code, making it much easier to debug.

If you are using the Sun Microsystems jdb debugger, then use the -jdblinemap option instead of the -linemap option. The options are equivalent, except that -jdblinemap does some special processing, necessitated by the fact that jdb does not support Java source files with file name extensions other than the .java extension.

For more information, see "Line-Mapping to SQLJ Source File (-linemap)" and "Line-Mapping to SQLJ Source File for jdb Debugger (-jdblinemap)".

Server-Side debug Option

If you are loading SQLJ source into the server and using the server-side embedded translator to translate it, the server-side debug option instructs the server-side compiler to output debugging information when a .sqlj or .java source file is compiled in the server. This is equivalent to using the -g option when running the standard javac compiler on a client. This does not aid in debugging your SQLJ code in particular, but aids in debugging your Java code in general.

See "Option Support in the Server Embedded Translator" for more information about this option and information about how to set options in the server.

For general information about debugging in the Oracle JVM, see the Oracle9i Java Developer's Guide.

Introduction to the AuditorInstaller Specialized Customizer

For ISO code generation, SQLJ provides a special customizer, AuditorInstaller. This customizer will insert sets of debugging statements, known as auditors, into profiles specified on the SQLJ command line. These profiles must already exist from previous customization.

The debugging statements will execute during SQLJ runtime (when someone runs your application), displaying a trace of method calls and values returned.

Use the customizer harness debug option, preceded by -P- as with any general customization option, to insert the debugging statements.

AuditorInstaller is described in detail in "AuditorInstaller Customizer for Debugging".

Introduction to Developing and Debugging in Oracle9i JDeveloper

Oracle SQLJ is fully integrated into the Oracle9i JDeveloper visual programming tool.

JDeveloper also includes an integrated debugger that supports SQLJ. SQLJ statements, as with standard Java statements, can be debugged in-line as your application executes. Reported line numbers are according to the line numbers in your SQLJ source code (as opposed to in the generated Java code).

See "SQLJ in Oracle9i JDeveloper and Other IDEs" for an introduction to JDeveloper.