Performance Extensions

21 Performance Extensions

This chapter describes the Oracle performance extensions to the Java Database Connectivity (JDBC) standard.

This chapter covers the following topics:

Note:

Oracle update batching was deprecated in Oracle Database 12c Release 1 (12.1). Starting in Oracle Database 12c Release 2 (12.2), Oracle update batching is a no operation code (no-op). This means that if you implement Oracle update batching in your application, using the Oracle Database 12c Release 2 (12.2) JDBC driver, then the specified batch size is not set and results in a batch size of 1. With this batch setting, your application processes one row at a time. Oracle strongly recommends that you use the standard JDBC batching if you are using the Oracle Database 12c Release 2 (12.2) JDBC driver.

21.1 Update Batching

This section covers the following topics:

21.1.1 Overview of Update Batching

You can reduce the number of round-trips to the database, thereby improving application performance, by grouping multiple UPDATE, DELETE, or INSERT statements into a single batch and having the whole batch sent to the database and processed in one trip. This is referred to as 'update batching'. This is especially useful with prepared statements, when you are repeating the same statement with different bind variables.

Note:

The JDBC 2.0 specification refers to 'update batching' as 'batch updates'.
To adhere to the JDBC 2.0 standard, Oracle implementation of standard update batching supports callable statements without OUT parameters, generic statements, and prepared statements. You can migrate standard update batching into an Oracle JDBC application without difficulty. However, the Oracle implementation of standard update batching does not implement true batching for generic statements and callable statements and you will see performance improvement for only PreparedStatement objects.

21.1.2 Standard Update Batching

JDBC standard update batching depends on explicitly adding statements to the batch using an addBatch method and explicitly processing the batch using an executeBatch method.

Note:

Disable auto-commit mode when you use update batching. In case an error occurs while you are processing a batch, this provides you the option of committing or rolling back the operations that ran successfully prior to the error.

21.1.2.1 Limitations in the Oracle Implementation of Standard Batching

This section discusses the 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.

21.1.2.2 About 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.

21.1.2.3 About 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.

Note:

If you add too many operations to a batch by calling the addBatch method several times and create a very large batch (for example, with more than or equal to 100,000 rows), then while calling the executeBatch method on the whole batch, you may face severe performance problems in terms of memory. To avoid this issue, the JDBC driver transparently breaks up the large batches into smaller internal batches and makes a roundtrip to the server for each internal batch. This makes your application slightly slower because of each round-trip overhead, but optimizes memory significantly. However, if each bound row is very large in size (for example, more than about 1MB each or so), then this process can impact the overall performance negatively because in such a case, the performance gained in terms of memory will be less than the performance lost in terms of time.

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();
...

21.1.2.4 Row Count per Iteration for Array DMLs

Starting from Oracle Database 12c Release 1 (12.1), the executeBatch method has been improved so that it returns an int array of size that is the same as the number of records in the batch and each item in the return array is the number of database table rows affected by the corresponding record of the batch. For example, if the batch size is 5, then the executeBatch method returns an array of size 5. In case of an error in between execution of the batch, the executeBatch method cannot return a value, instead it throws a BatchUpdateException. In this case, the exception itself carries an int array of size n as its data, where n is the number of successful record executions. For example, if the batch is of size 5 and the error occurs at the 4th record, then the BatchUpdateException has an array of size 3 (3 records executed successfully) and each item in the array represents how many rows were affected by each of them.

21.1.2.5 About 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 nonbatched 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.

21.1.2.6 About 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.

Keep the following things in mind:

When a batch is processed, operations are performed in the order in which they were batched.
After calling addBatch, you must call either executeBatch or clearBatch before a call to executeUpdate, otherwise there will be a SQL exception.
A clearBatch or executeBatch call resets the statement batch to empty.
The statement batch is not reset to empty if the connection receives a ROLLBACK request. You must explicitly call clearBatch to reset it.
Invoking clearBatch method after a rollback works for all releases.
An executeBatch call closes the current result set of the statement object, if one exists.
Nothing is returned by the clearBatch method.

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();
   ...
}

21.1.2.7 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, the array contains the actual update counts indicating the number of rows affected by each operation.
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 the Oracle implementation of standard batching.
For a callable statement batch, the array contains the actual update counts indicating the number of 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 21-1 illustrates the use of standard update batching.

Example 21-1 Standard Update Batching

This example combines the sample fragments in the previous sections, accomplishing the following steps:

Disabling auto-commit mode, which you should always perform when using update batching
Creating a prepared statement object
Adding operations to the batch associated with the prepared statement object
Processing the batch
Committing the operations from the batch

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();
...

You can process the update counts array to determine if the batch processed successfully.

21.1.2.8 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 pr epared statement batch, in case of an error in between execution of the batch, the executeBatch method cannot return a value, instead it throws a BatchUpdateException. In this case, the exception itself carries an int array of size n as its data, where n is the number of successful record executions. For example, if the batch is of size 5 and the error occurs at the 4th record, then the BatchUpdateException has an array of size 3 (3 records executed successfully) and each item in the array represents how many rows were affected by each of them.
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.

21.1.2.9 About Intermixing Batched Statements and Nonbatched Statements

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

However, you can intermix batched operations and nonbatched operations in a single statement object if you process nonbatched 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 valid 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 nonbatched 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 regard to update batching operations. A COMMIT request will affect all nonbatched operations and all successful operations in processed batches, but will not affect any pending batches.

21.1.3 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", "HR"); 
info.setProperty("passwd", "hr"); 
// 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();

Note:

The AccumulateBatchResult property is set to true by default.

Example 21-2 illustrates premature batch flushing.

Example 21-2 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();

21.2 Additional Oracle Performance Extensions

In addition to update batching, Oracle JDBC drivers support the following extensions that improve performance by reducing round-trips to the database:

Prefetching rows

This reduces round-trips to the database by fetching multiple rows of data each time data is fetched. The extra data is stored in client-side buffers for later access by the client. The number of rows to prefetch can be set as desired.
Specifying column types

This avoids an inefficiency in the standard JDBC protocol for performing and returning the results of queries.
Suppressing database metadata TABLE_REMARKS columns

This avoids an expensive outer join operation.

Oracle provides several extensions to connection properties objects to support these performance extensions. These extensions enable you to set the remarksReporting flag and default values for row prefetching and update batching.

This section covers the following topics:

21.2.1 About Prefetching LOB Data

For the JDBC drivers prior to Oracle Database 11g Release 2 JDBC drivers, if you want to retrieve LOB data in one round trip, then you have to fetch the data as VARCHAR2 type, that is, you have to use OracleTypes.VARCHAR or OracleTypes.LONGVARCHAR with the JDBC defineColumnType method. The limitation of this approach is that when LOB data is fetched as CHAR type, the locator cannot be fetched along with the data. So, if the application wants to get the LOB data at a later point of time, or if the application wants to perform other LOB operations, then one more round trip is required to get the LOB locator, as LOB locator is not available to the application.

Note:

Array operations on LOB locators are not supported in the JDBC APIs.

Starting from Oracle Database 11g Release 2 JDBC drivers, the number of round trips is reduced by prefetching frequently used metadata, such as the LOB length and the chunk size as well as the beginning of the LOB data along with the locator during regular fetch operations. For small LOBs, the data may be totally prefetched in one single round trip, that is, the select parse, execution, and fetch occurs in one round trip, and performance is improved greatly. For large LOBs that are larger than 5 times the prefetch size, the performance improvement is not very significant as only the round trip for retrieving the chunk size is not needed.

defaultLobPrefetchSize Connection Property

Starting from Oracle Database 11g Release 2, there is a new connection property oracle.jdbc.defaultLobPrefetchSize that can be used to set the default LOB prefetch size for the connection. This connection property is defined as the following constant: OracleConnection.CONNECTION_PROPERTY_DEFAULT_LOB_PREFETCH_SIZE. The value of this property is used as the default LOB prefetch size for the current connection. The default value of this connection property is 4000. If you want to change the default value at the statement level, then use the setLobPrefetchSize method defined in oracle.jdbc.OracleStatement interface. You can change the default value to:

-1 to disable LOB prefetch for the current connection
0 to enable LOB prefetch for metadata only
Any value greater than 0 to specify the number of bytes for BLOBs and the number of characters for CLOBs to be prefetched along with the locator during fetch operations

Use getLobPrefetchSize method defined in oracle.jdbc.OracleStatement interface to retrieve the LOB prefetch size.

You can also set the value of LOB prefetch size at the column level by using the defineColumnType method. The column-level value overrides any value that is set at the connection or statement level.

Note:

If LOB prefetch is not disabled at the connection level or statement level, it cannot be disabled at the column level.

21.2.2 Oracle Row-Prefetching Limitations

There is no maximum prefetch setting. The default value is 10. Larger or smaller values may be appropriate depending on the number of rows and columns expected from the query. You can set the default connection row-prefetch value using a Properties object.

When a statement object is created, it receives the default row-prefetch setting from the associated connection. Subsequent changes to the default connection row-prefetch setting will have no effect on the statement row-prefetch setting.

If a column of a result set is of data type LONG, LONG RAW or LOBs returned through the data interface, that is, the streaming types, then JDBC changes the statement row-prefetch setting to 1, even if you never actually read a value of either of these types.

Setting the prefetch size can affect the performance of an application. Increasing the prefetch size will reduce the number of round-trips required to get all the data, but will increase memory usage. This will depend on the number and size of the columns in the query and the number of rows expected to be returned. It will also depend on the memory and CPU loading of the JDBC client machine. The optimum for a standalone client application will be different from a heavily loaded application server. The speed and latency of the network connection should also be considered.

Note:

Starting from Oracle Database 11g Release 1, the Thin driver can fetch the first prefetch_size number of rows from the server in the very first round-trip. This saves one round-trip in SELECT statements.

If you are migrating an application from earlier releases of Oracle JDBC drivers to 10g Release 1 (10.1) or later releases of Oracle JDBC drivers, then you should revisit the optimizations that you had done earlier, because the memory usage and performance characteristics may have changed substantially.

A common situation that you may encounter is, say, you have a query that selects a unique key. The query will return only zero or one row. Setting the prefetch size to 1 will decrease memory and CPU usage and cannot increase round-trips. However, you must be careful to avoid the error of requesting an extra fetch by writing while(rs.next()) instead of if(rs.next()).

If you are using the JDBC Thin driver, then use the useFetchSizeWithLongColumn connection property, because it will perform PARSE, EXECUTE, and FETCH in a single round-trip.

Tuning of the prefetch size should be done along with tuning of memory management in your JVM under realistic loads of the actual application.

Note:

Do not mix the JDBC 2.0 fetch size application programming interface (API) and the Oracle row-prefetching API in your application. You can use one or the other, but not both.
Be aware that setting the Oracle fetch size value can affect not only queries, but also explicitly refetching rows in a result set through the result set refreshRow method, which is relevant for scroll-sensitive/read-only, scroll-sensitive/updatable, and scroll-insensitive/updatable result sets, and the window size of a scroll-sensitive result set, affecting how often automatic refetches are performed. However, the Oracle fetch size value will be overridden by any setting of the fetch size.

21.2.3 About Defining Column Types

Note:

Starting from Oracle Database 12c Release 1 (12.1), the defineColumnType method is deprecated.

The implementation of defineColumnType changed significantly since 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. Starting from 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 driver 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:

If necessary, cast your statement object to OracleStatement, OraclePreparedStatement, or OracleCallableStatement, as applicable.
If necessary, use the clearDefines method of your Statement object to clear any previous column definitions for this Statement object.
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);
```
If the column is VARCHAR or equivalent and you know the length limit:
```
stmt.defineColumnType(column_index, typeCode, max_size);
```
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 );
```
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.

Note:

It is no longer necessary to specify a data type for each column of the expected result set.

The following example illustrates the use of this feature. It assumes you have imported the oracle.jdbc.* interfaces.

Example 21-3 Defining Column Types

OracleDataSource ods = new OracleDataSource();
ods.setURL("jdbc:oracle:thin:@localhost:5221:orcl");
ods.setUser("HR");
ods.setPassword("hr");
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.

The following table lists the valid column definition arguments that you can use in the defineColumnType method.

Table 21-1 Valid Column Type Specifications

If the column has Oracle SQL type:	You can use defineColumnType to define it as:
`NUMBER`, `VARNUM`	`BIGINT`, `TINYINT`, `SMALLINT`, `INTEGER`, `FLOAT`, `REAL`, `DOUBLE`, `NUMERIC`, `DECIMAL`, `CHAR`, `VARCHAR`
`CHAR`, `VARCHAR2`	`CHAR`, `VARCHAR`
`LONG`	`CHAR`, `VARCHAR`, `LONGVARCHAR`
`LONGRAW`	`LONGVARBINARY`, `VARBINARY`, `BINARY`
`RAW`	`VARBINARY`, `BINARY`
`DATE`	`DATE`, `TIME`, `TIMESTAMP`, `CHAR`, `VARCHAR`
`ROWID`	`ROWID`
`BLOB`	`VARBINARY`, `BINARY`
`CLOB`	`LONG`, `CHAR`, `VARCHAR`

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

21.2.4 About Reporting DatabaseMetaData TABLE_REMARKS

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.

The following code snippet illustrates how to enable TABLE_REMARKS reporting:

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

Here, conn is the name of your standard Connection object, the following statement enables TABLE_REMARKS reporting:

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.OracleConnection)conn ).setIncludeSynonyms(true)

This will cause all subsequent getColumns method calls 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, 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 the percent sign (%). Otherwise, the catalog parameter should be a package name pattern, with SQL wild cards, if desired.
schemaPattern

All objects within Oracle database 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 percent sign (%).