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Oracle9i JDBC Developer's Guide and Reference
Release 2 (9.2)
Part Number A96654-01
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13
Result Set Enhancements

Standard JDBC 2.0 features in JDK 1.2.x include enhancements to result set functionality--processing forward or backward, positioning relatively or absolutely, seeing changes to the database made internally or externally, and updating result set data and then copying the changes to the database.

This chapter discusses these features, including the following topics:

The Oracle JDBC drivers also include extensions to support these features in a JDK 1.1.x environment.

For more general and conceptual information about JDBC 2.0 result set enhancements, refer to the Sun Microsystems JDBC 2.0 API specification.

Overview

This section provides an overview of JDBC 2.0 result set functionality and categories, and some discussion of implementation requirements for the Oracle JDBC drivers.

Result Set Functionality and Result Set Categories Supported in JDBC 2.0

Result set functionality in JDBC 2.0 includes enhancements for scrollability and positioning, sensitivity to changes by others, and updatability.

Specify the desired result set type and concurrency type when you create the statement object that will produce the result set.

Together, the various result set types and concurrency types provide for six different categories of result set.

This section provides an overview of these enhancements, types, and categories.

Scrollability, Positioning, and Sensitivity

Scrollability refers to the ability to move backward as well as forward through a result set. Associated with scrollability is the ability to move to any particular position in the result set, through either relative positioning or absolute positioning.

Relative positioning allows you to move a specified number of rows forward or backward from the current row. Absolute positioning allows you to move to a specified row number, counting from either the beginning or the end of the result set.

Under JDBC 1.0 (in JDK 1.1.x) you can scroll only forward, using the next() method as described in "Process the Result Set", and there is no positioning functionality. You can start only at the beginning and iterate row-by-row until the end.

Under JDBC 2.0 (in JDK 1.2.x), scrollable/positionable result sets are also available.

When creating a scrollable/positionable result set, you must also specify sensitivity. This refers to the ability of a result set to detect and reveal changes made to the underlying database from outside the result set.

A sensitive result set can see changes made to the database while the result set is open, providing a dynamic view of the underlying data. Changes made to the underlying columns values of rows in the result set are visible.

An insensitive result set is not sensitive to changes made to the database while the result set is open, providing a static view of the underlying data. You would need to retrieve a new result set to see changes made to the database.

Sensitivity is not an option in a JDBC 1.0/non-scrollable result set.

Result Set Types for Scrollability and Sensitivity

When you create a result set under JDBC 2.0 functionality, you must choose a particular result set type to specify whether the result set is scrollable/positional and sensitive to underlying database changes.

If the JDBC 1.0 functionality is all you desire, JDBC 2.0 continues to support this through the forward-only result set type. A forward-only result set cannot be sensitive.

If you want a scrollable result set, you must also specify sensitivity. Specify the scroll-sensitive type for the result set to be scrollable and sensitive to underlying changes. Specify the scroll-insensitive type for the result set to be scrollable but not sensitive to underlying changes.

To summarize, the following three result set types are available with JDBC 2.0:

Updatability

Updatability refers to the ability to update data in a result set and then (presumably) copy the changes to the database. This includes inserting new rows into the result set or deleting existing rows.

Updatability might also require database write locks to mediate access to the underlying database. Because you cannot have multiple write locks concurrently, updatability in a result set is associated with concurrency in database access.

Result sets can optionally be updatable under JDBC 2.0, but not under JDBC 1.0.

Note:

Updatability is independent of scrollability and sensitivity, although it is typical for an updatable result set to also be scrollable so that you can position it to particular rows that you want to update or delete.

Concurrency Types for Updatability

The concurrency type of a result set determines whether it is updatable. Under JDBC 2.0, the following concurrency types are available:

Summary of Result Set Categories

Because scrollability and sensitivity are independent of updatability, the three result set types and two concurrency types combine for a total of six result set categories:

Oracle JDBC Implementation Overview for Result Set Enhancements

This section discusses key aspects of the Oracle JDBC implementation of result set enhancements for scrollability--through use of a client-side cache--and for updatability--through use of ROWIDs.

It is permissible for customers to implement their own client-side caching mechanism, and Oracle provides an interface to use in doing so.

Oracle JDBC Implementation for Result Set Scrollability

Because the underlying server does not support scrollable cursors, Oracle JDBC must implement scrollability in a separate layer.

It is important to be aware that this is accomplished by using a client-side memory cache to store rows of a scrollable result set.

Important:

Because all rows of any scrollable result set are stored in the client-side cache, a situation where the result set contains many rows, many columns, or very large columns might cause the client-side Java virtual machine to fail. Do not specify scrollability for a large result set.

Scrollable cursors in the Oracle server, and therefore a server-side cache, will be supported in a future Oracle release.

Oracle JDBC Implementation for Result Set Updatability

To support updatability, Oracle JDBC uses ROWIDs to uniquely identify database rows that appear in a result set. For every query into an updatable result set, the Oracle JDBC driver automatically retrieves the ROWID along with the columns you select.

Note:

Client-side caching is not required by updatability in and of itself. In particular, a forward-only updatable result set will not require a client-side cache.

Implementing a Custom Client-Side Cache for Scrollability

There is some flexibility in how to implement client-side caching in support of JDBC 2.0 scrollable result sets.

Although Oracle JDBC provides a complete implementation, it also supplies an interface, OracleResultSetCache, that you can implement as desired:

public interface OracleResultSetCache 
{ 
  /** 
   * Save the data in the i-th row and j-th column. 
   */ 
  public void put (int i, int j, Object value) throws IOException; 

  /** 
   * Return the data stored in the i-th row and j-th column. 
   */ 
  public Object get (int i, int j) throws IOException; 

  /** 
   * Remove the i-th row. 
   */ 
  public void remove (int i) throws IOException; 

  /** 
   * Remove the data stored in i-th row and j-th column 
   */ 
  public void remove (int i, int j) throws IOException; 

  /** 
   * Remove all data from the cache. 
   */ 
  public void clear () throws IOException; 

  /** 
   * Close the cache. 
   */ 
  public void close () throws IOException; 
}

If you implement this interface with your own class, your application code must instantiate your class and then use the setResultSetCache() method of an OracleStatement, OraclePreparedStatement, or OracleCallableStatement object to set the caching mechanism to use your implementation. Following is the method signature:

Call this method prior to executing a query. The result set produced by the query will then use your specified caching mechanism.

Creating Scrollable or Updatable Result Sets

Under JDBC 1.0, no special attention is required in creating and using a result set. A result set is produced automatically to store the results of a query, and no result set types or categories must be specified, because there is only one kind of result set available--forward-only/read-only. For example (given a connection object conn):

Statement stmt = conn.createStatement();
ResultSet rs = stmt.executeQuery("SELECT empno, sal FROM emp");

In using JDBC 2.0 result set enhancements, however, you may specify the result set type (for scrollability and sensitivity) and the concurrency type (for updatability) when you create a generic statement or prepare a prepared statement or callable statement that will execute a query.

(Note, however, that callable statements are intended to execute stored procedures and functions and rarely return a result set. Still, the callable statement class is a subclass of the prepared statement class and so inherits this functionality.)

This section discusses the creation of result sets to use JDBC 2.0 enhancements.

Specifying Result Set Scrollability and Updatability

Under JDBC 2.0, Connection classes have createStatement(), prepareStatement(), and prepareCall() method signatures that take a result set type and a concurrency type as input:

The statement objects created will have the intelligence to produce the appropriate kind of result sets.

You can specify one of the following static constant values for result set type:

And you can specify one of the following static constant values for concurrency type:

After creating a Statement, PreparedStatement, or CallableStatement object, you can verify its result set type and concurrency type by calling the following methods on the statement object:

Example

Following is an example of a prepared statement object that specifies a scroll-sensitive and updatable result set for queries executed through that statement (where conn is a connection object):

...
PreparedStatement pstmt = conn.prepareStatement
         ("SELECT empno, sal FROM emp WHERE empno = ?", 
         ResultSet.TYPE_SCROLL_SENSITIVE, ResultSet.CONCUR_UPDATABLE);

pstmt.setString(1, "28959");
ResultSet rs = pstmt.executeQuery();
...

Result Set Limitations and Downgrade Rules

Some types of result sets are not feasible for certain kinds of queries. If you specify an unfeasible result set type or concurrency type for the query you execute, the JDBC driver follows a set of rules to determine the best feasible types to use instead.

The actual result set type and concurrency type are determined when the statement is executed, with the driver issuing a SQLWarning on the statement object if the desired result set type or concurrency type is not feasible. The SQLWarning object will contain the reason why the requested type was not feasible. Check for warnings to verify whether you received the type of result set that you requested, or call the methods described in "Verifying Result Set Type and Concurrency Type".

FOR UPDATE Clause Limitation in an Updatable Result Set

A query cannot have the FOR UPDATE clause in the SELECT statement if you are using an updatable result set. If you use the FOR UPDATE clause and try to update a result set, an SQLException will be thrown.

Result Set Limitations

The following limitations are placed on queries for enhanced result sets. Failure to follow these guidelines will result in the JDBC driver choosing an alternative result set type or concurrency type.

To produce an updatable result set:

To produce a scroll-sensitive result set:

(See "Summary of New Methods for Result Set Enhancements" for general information about refetching.)

Workaround

As a workaround for the "SELECT *" limitation, you can use table aliases as in the following example:

SELECT t.* FROM TABLE t ...
Hint:

There is a simple way to determine if your query will probably produce a scroll-sensitive or updatable result set: If you can legally add a ROWID column to the query list, then the query is probably suitable for either a scroll-sensitive or an updatable result set. (You can try this out using SQL*Plus, for example.)

Result Set Downgrade Rules

If the specified result set type or concurrency type is not feasible, the Oracle JDBC driver uses the following rules in choosing alternate types:

Furthermore:

Verifying Result Set Type and Concurrency Type

After a query has been executed, you can verify the result set type and concurrency type that the JDBC driver actually used, by calling methods on the result set object.

Positioning and Processing in Scrollable Result Sets

Scrollable result sets (result set type TYPE_SCROLL_SENSITIVE or TYPE_SCROLL_INSENSITIVE) allow you to iterate through, them either forward or backward, and to position the result set to any desired row.

This section discusses positioning within a scrollable result set and how to process a scrollable result set backward, instead of forward.

Positioning in a Scrollable Result Set

In a scrollable result set, you can use several result set methods to move to a desired position and to check the current position.

Methods for Moving to a New Position

The following result set methods are available for moving to a new position in a scrollable result set:

beforeFirst() Method

Positions to before the first row of the result set, or has no effect if there are no rows in the result set.

This is where you would typically start iterating through a result set to process it going forward, and is the default initial position for any kind of result set.

You are outside the result set bounds after a beforeFirst() call. There is no valid current row, and you cannot position relatively from this point.

afterLast() Method

Positions to after the last row of the result set, or has no effect if there are no rows in the result set.

This is where you would typically start iterating through a result set to process it going backward.

You are outside the result set bounds after an afterLast() call. There is no valid current row, and you cannot position relatively from this point.

first() Method

Positions to the first row of the result set, or returns false if there are no rows in the result set.

last() Method

Positions to the last row of the result set, or returns false if there are no rows in the result set.

absolute() Method

Positions to an absolute row from either the beginning or end of the result set. If you input a positive number, it positions from the beginning; if you input a negative number, it positions from the end. This method returns false if there are no rows in the result set.

Attempting to move forward beyond the last row, such as an absolute(11) call if there are 10 rows, will position to after the last row, having the same effect as an afterLast() call.

Attempting to move backward beyond the first row, such as an absolute(-11) call if there are 10 rows, will position to before the first row, having the same effect as a beforeFirst() call.

Note:

Calling absolute(1) is equivalent to calling first(); calling absolute(-1) is equivalent to calling last().
relative() Method

Moves to a position relative to the current row, either forward if you input a positive number or backward if you input a negative number, or returns false if there are no rows in the result set.

The result set must be at a valid current row for use of the relative() method.

Attempting to move forward beyond the last row will position to after the last row, having the same effect as an afterLast() call.

Attempting to move backward beyond the first row will position to before the first row, having the same effect as a beforeFirst() call.

A relative(0) call is valid but has no effect.

Important:

You cannot position relatively from before the first row (which is the default initial position) or after the last row. Attempting relative positioning from either of these positions would result in a SQL exception.

Methods for Checking the Current Position

The following result set methods are available for checking the current position in a scrollable result set:

Processing a Scrollable Result Set

In a scrollable result set you can iterate backward instead of forward as you process the result set. The following methods are available:

The previous() method works similarly to the next() method, in that it returns true as long as the new current row is valid, and false as soon as it runs out of rows (has passed the first row).

Backward versus Forward Processing

You can process the entire result set going forward, using the next() method as in JDBC 1.0. This is documented in "Process the Result Set". The default initial position in the result set is before the first row, appropriately, but you can call the beforeFirst() method if you have moved elsewhere since the result set was created.

To process the entire result set going backward, call afterLast(), then use the previous() method. For example (where conn is a connection object):

...
/* NOTE: The specified concurrency type, CONCUR_UPDATABLE, is not relevant to 
this example. */

Statement stmt = conn.createStatement 
         (ResultSet.TYPE_SCROLL_SENSITIVE, ResultSet.CONCUR_UPDATABLE);

ResultSet rs = stmt.executeQuery("SELECT empno, sal FROM emp");

rs.afterLast();
while (rs.previous())
{
   System.out.println(rs.getString("empno") + " " + rs.getFloat("sal"));
}
...

Unlike relative positioning, you can (and typically do) use next() from before the first row and previous() from after the last row. You do not have to be at a valid current row to use these methods.

Note:

In a non-scrollable result set, you can process only with the next() method. Attempting to use the previous() method will cause a SQL exception.

Presetting the Fetch Direction

The JDBC 2.0 standard allows the ability to pre-specify the direction, known as the fetch direction, for use in processing a result set. This allows the JDBC driver to optimize its processing. The following result set methods are specified:

The Oracle JDBC drivers support only the forward preset value, which you can specify by inputting the ResultSet.FETCH_FORWARD static constant value.

The values ResultSet.FETCH_REVERSE and ResultSet.FETCH_UNKNOWN are not supported--attempting to specify them causes a SQL warning, and the settings are ignored.

Updating Result Sets

A concurrency type of CONCUR_UPDATABLE allows you to update rows in the result set, delete rows from the result set, or insert rows into the result set.

After you perform an UPDATE or INSERT operation in a result set, you propagate the changes to the database in a separate step that you can skip if you want to cancel the changes.

A DELETE operation in a result set, however, is immediately executed (but not necessarily committed) in the database as well.

Note:

When using an updatable result set, it is typical to also make it scrollable. This allows you to position to any row that you want to change. With a forward-only updatable result set, you can change rows only as you iterate through them with the next() method.

Performing a DELETE Operation in a Result Set

The result set deleteRow() method will delete the current row. Following is the method signature:

Presuming the result set is also scrollable, you can position to a row using any of the available positioning methods (except beforeFirst() and afterLast(), which do not go to a valid current row), and then delete that row, as in the following example (presuming a result set rs):

...
rs.absolute(5);
rs.deleteRow();
...

See "Positioning in a Scrollable Result Set" for information about the positioning methods.

Important:

The deleted row remains in the result set object even after it has been deleted from the database.

In a scrollable result set, by contrast, a DELETE operation is evident in the local result set object--the row would no longer be in the result set after the DELETE. The row preceding the deleted row becomes the current row, and row numbers of subsequent rows are changed accordingly.

Refer to "Seeing Internal Changes" for more information.

Performing an UPDATE Operation in a Result Set

Performing a result set UPDATE operation requires two separate steps to first update the data in the result set and then copy the changes to the database.

Presuming the result set is also scrollable, you can position to a row using any of the available positioning methods (except beforeFirst() and afterLast(), which do not go to a valid current row), and then update that row as desired.

See "Positioning in a Scrollable Result Set" for information about the positioning methods.

Here are the steps for updating a row in the result set and database:

  1. Call the appropriate updateXXX() methods to update the data in the columns you want to change.

    With JDBC 2.0, a result set object has an updateXXX() method for each datatype, as with the setXXX() methods previously available for updating the database directly.

    Each of these methods takes an int for the column number or a string for the column name and then an item of the appropriate datatype to set the new value. Following are a couple of examples for a result set rs:

    rs.updateString(1, "mystring");
rs.updateFloat(2, 10000.0f);
  2. Call the updateRow() method to copy the changes to the database (or the cancelRowUpdates() method to cancel the changes).

    Once you call updateRow(), the changes are executed and will be made permanent with the next transaction COMMIT operation. Be aware that by default, the auto-commit flag is set to true so that any executed operation is committed immediately.

    If you choose to cancel the changes before copying them to the database, call the cancelRowUpdates() method instead. This will also revert to the original values for that row in the local result set object. Note that once you call the updateRow() method, the changes are written to the transaction and cannot be canceled unless you roll back the transaction (auto-commit must be disabled to allow a ROLLBACK operation).

    Positioning to a different row before calling updateRow() also cancels the changes and reverts to the original values in the result set.

    Before calling updateRow(), you can call the usual getXXX() methods to verify that the values have been updated correctly. These methods take an int column index or string column name as input. For example:

    float myfloat = rs.getFloat(2);
...process myfloat to see if it's appropriate...
    Note:

    Result set UPDATE operations are visible in the local result set object for all result set types (forward-only, scroll-sensitive, and scroll-insensitive).

    Refer to "Seeing Internal Changes" for more information.
Example

Following is an example of a result set UPDATE operation that is also copied to the database. The tenth row is updated. (The column number is used to specify column 1, and the column name--sal-- is used to specify column 2.)

...
Statement stmt = conn.createStatement
         (ResultSet.TYPE_SCROLL_SENSITIVE, ResultSet.CONCUR_UPDATABLE);

ResultSet rs = stmt.executeQuery("SELECT empno, sal FROM emp");

if (rs.absolute(10))        // (returns false if row does not exist)
{
   rs.updateString(1, "28959");
   rs.updateFloat("sal", 100000.0f);
   rs.updateRow();
}
// Changes will be made permanent with the next COMMIT operation.
...

Performing an INSERT Operation in a Result Set

Result set INSERT operations use what is called the result set insert-row, which is a staging area that holds the data for the inserted row until it is copied to the database. You must explicitly move to this row to write the data that will be inserted.

As with UPDATE operations, result set INSERT operations require separate steps to first write the data to the insert-row and then copy it to the database .

Following are the steps in executing a result set INSERT operation.

  1. Move to the insert-row by calling the result set moveToInsertRow() method.

    Note:

    The result set will remember the current position prior to the moveToInsertRow() call. Afterward, you can go back to it with a moveToCurrentRow() call.
  2. As with UPDATE operations, use the appropriate updateXXX() methods to write data to the columns. For example: 
    rs.updateString(1, "mystring");
rs.updateFloat(2, 10000.0f);

    (Note that you can specify a string for column name, instead of an integer for column number.)

    Important:

    Each column value in the insert-row is undefined until you call the updateXXX() method for that column. You must call this method and specify a non-null value for all non-nullable columns, or else attempting to copy the row into the database will result in a SQL exception.

    It is permissible, however, to not call updateXXX() for a nullable column. This will result in a value of null.
  3. Copy the changes to the database by calling the result set insertRow() method.

    Once you call insertRow(), the insert is executed and will be made permanent with the next transaction COMMIT operation.

    Positioning to a different row before calling insertRow() cancels the insert and clears the insert-row.

    Before calling insertRow() you can call the usual getXXX() methods to verify that the values have been set correctly in the insert-row. These methods take an int column index or string column name as input. For example:

    float myfloat = rs.getFloat(2);
...process myfloat to see if it's appropriate...
    Note:

    No result set type (neither scroll-sensitive, scroll-insensitive, nor forward-only) can see a row inserted by a result set INSERT operation.

    Refer to "Seeing Internal Changes" for more information.
Example

The following example performs a result set INSERT operation, moving to the insert-row, writing the data, copying the data into the database, and then returning to what was the current row prior to going to the insert-row. (The column number is used to specify column 1, and the column name--sal-- is used to specify column 2.)

...
Statement stmt = conn.createStatement
         (ResultSet.TYPE_SCROLL_SENSITIVE, ResultSet.CONCUR_UPDATABLE);

ResultSet rs = stmt.executeQuery("SELECT empno, sal FROM emp");

rs.moveToInsertRow();
rs.updateString(1, "28959");
rs.updateFloat("sal", 100000.0f);
rs.insertRow();
// Changes will be made permanent with the next COMMIT operation.
rs.moveToCurrentRow();  // Go back to where we came from...
...

Update Conflicts

It is important to be aware of the following facts regarding updatable result sets with the JDBC drivers:

A conflict will occur if you try to perform a DELETE or UPDATE operation on a row updated by another committed transaction.

The Oracle JDBC drivers use the ROWID to uniquely identify a row in a database table. As long as the ROWID is still valid when a driver tries to send an UPDATE or DELETE operation to the database, the operation will be executed.

The driver will not report any changes made by another committed transaction. Any conflicts are silently ignored and your changes will overwrite the previous changes.

To avoid such conflicts, use the Oracle FOR UPDATE feature when executing the query that produces the result set. This will avoid conflicts, but will also prevent simultaneous access to the data. Only a single write lock can be held concurrently on a data item.

Fetch Size

By default, when Oracle JDBC executes a query, it receives the result set 10 rows at a time from the database cursor. This is the default Oracle row-prefetch value. You can change the number of rows retrieved with each trip to the database cursor by changing the row-prefetch value (see "Oracle Row Prefetching" for more information).

JDBC 2.0 also allows you to specify the number of rows fetched with each database round trip for a query, and this number is referred to as the fetch size. In Oracle JDBC, the row-prefetch value is used as the default fetch size in a statement object. Setting the fetch size overrides the row-prefetch setting and affects subsequent queries executed through that statement object.

Fetch size is also used in a result set. When the statement object executes a query, the fetch size of the statement object is passed to the result set object produced by the query. However, you can also set the fetch size in the result set object to override the statement fetch size that was passed to it. (Also note that changes made to a statement object's fetch size after a result set is produced will have no affect on that result set.)

The result set fetch size, either set explicitly, or by default equal to the statement fetch size that was passed to it, determines the number of rows that are retrieved in any subsequent trips to the database for that result set. This includes any trips that are still required to complete the original query, as well as any refetching of data into the result set. (Data can be refetched, either explicitly or implicitly, to update a scroll-sensitive or scroll-insensitive/updatable result set. See "Refetching Rows".)

Setting the Fetch Size

The following methods are available in all Statement, PreparedStatement, CallableStatement, and ResultSet objects for setting and getting the fetch size:

To set the fetch size for a query, call setFetchSize() on the statement object prior to executing the query. If you set the fetch size to N, then N rows are fetched with each trip to the database.

After you have executed the query, you can call setFetchSize() on the result set object to override the statement object fetch size that was passed to it. This will affect any subsequent trips to the database to get more rows for the original query, as well as affecting any later refetching of rows. (See "Refetching Rows".)

Use of Standard Fetch Size versus Oracle Row-Prefetch Setting

Using the JDBC 2.0 fetch size is fundamentally similar to using the Oracle row-prefetch value, except that with the row-prefetch value you do not have the flexibility of distinct values in the statement object and result set object. The row prefetch value would be used everywhere.

Furthermore, JDBC 2.0 fetch size usage is portable and can be used with other JDBC drivers. Oracle row-prefetch usage is vendor-specific.

See "Oracle Row Prefetching" for a general discussion of this Oracle feature.

Note:

Do not mix the JDBC 2.0 fetch size API and the Oracle row prefetching API in your application. You can use one or the other, but not both.

Refetching Rows

The result set refreshRow() method is supported for some types of result sets for refetching data. This consists of going back to the database to re-obtain the database rows that correspond to N rows in the result set, starting with the current row, where N is the fetch size (described above in "Fetch Size"). This lets you see the latest updates to the database that were made outside of your result set, subject to the isolation level of the enclosing transaction.

Because refetching re-obtains only rows that correspond to rows already in your result set, it does nothing about rows that have been inserted or deleted in the database since the original query. It ignores rows that have been inserted, and rows will remain in your result set even after the corresponding rows have been deleted from the database. When there is an attempt to refetch a row that has been deleted in the database, the corresponding row in the result set will maintain its original values.

Following is the refreshRow() method signature:

You must be at a valid current row when you call this method, not outside the row bounds and not at the insert-row.

With the 8.1.6 release, the refreshRow() method is supported for the following result set categories:

Oracle JDBC might support additional result set categories in future releases.

Note:

Scroll-sensitive result set functionality is implemented through implicit calls to refreshRow(). See "Oracle Implementation of Scroll-Sensitive Result Sets" for details.

Seeing Database Changes Made Internally and Externally

This section discusses the ability of a result set to see the following:

Near the end of the section is a summary table.

Note:

External changes are referred to as "other's changes" in the Sun Microsystems JDBC 2.0 specification.

Seeing Internal Changes

The ability of an updatable result set to see its own changes depends on both the result set type and the kind of change (UPDATE, DELETE, or INSERT). This is discussed at various points throughout the "Updating Result Sets" section beginning on , and is summarized as follows:

An internal change being "visible" essentially means that a subsequent getXXX() call will see the data changed by a preceding updateXXX() call on the same data item.

JDBC 2.0 DatabaseMetaData objects include the following methods to verify this. Each takes a result set type as input (ResultSet.TYPE_FORWARD_ONLY, ResultSet.TYPE_SCROLL_SENSITIVE, or ResultSet.TYPE_SCROLL_INSENSITIVE).

Seeing External Changes

Only a scroll-sensitive result set can see external changes to the underlying database, and it can only see the changes from external UPDATE operations. Changes from external DELETE or INSERT operations are never visible.

Note:

Any discussion of seeing changes from outside the enclosing transaction presumes the transaction itself has an isolation level setting that allows the changes to be visible.

For implementation details of scroll-sensitive result sets, including exactly how and how soon external updates become visible, see "Oracle Implementation of Scroll-Sensitive Result Sets".

JDBC 2.0 DatabaseMetaData objects include the following methods to verify this. Each takes a result set type as input (ResultSet.TYPE_FORWARD_ONLY, ResultSet.TYPE_SCROLL_SENSITIVE, or ResultSet.TYPE_SCROLL_INSENSITIVE).

Visibility versus Detection of External Changes

Regarding changes made to the underlying database by external sources, there are two similar but distinct concepts with respect to visibility of the changes from your local result set:

A change being "visible" means that when you look at a row in the result set, you can see new data values from changes made by external sources to the corresponding row in the database.

A change being "detected", however, means that the result set is aware that this is a new value since the result set was first populated.

Even when an Oracle result set sees new data (as with an external UPDATE in a scroll-sensitive result set), it has no awareness that this data has changed since the result set was populated. Such changes are not "detected".

JDBC 2.0 DatabaseMetaData objects include the following methods to verify this. Each takes a result set type as input (ResultSet.TYPE_FORWARD_ONLY, ResultSet.TYPE_SCROLL_SENSITIVE, or ResultSet.TYPE_SCROLL_INSENSITIVE).

It follows, then, that result set methods specified by JDBC 2.0 to detect changes--rowDeleted(), rowUpdated(), and rowInserted()--will always return false with the 8.1.6 Oracle JDBC drivers. There is no use in calling them.

Summary of Visibility of Internal and External Changes

Table 13-1 summarizes the discussion in the preceding sections regarding whether a result set object in the Oracle JDBC implementation can see changes made internally through the result set itself, and changes made externally to the underlying database from elsewhere in your transaction or from other committed transactions.

Table 13-1 Visibility of Internal and External Changes for Oracle JDBC 
Result Set Type Can See Internal DELETE? Can See Internal UPDATE? Can See Internal INSERT? Can See External DELETE? Can See External UPDATE? Can See External INSERT?

forward-only

no

yes

no

no

no

no

scroll-sensitive

yes

yes

no

no

yes

no

scroll-insensitive

yes

yes

no

no

no

no

For implementation details of scroll-sensitive result sets, including exactly how and how soon external updates become visible, see "Oracle Implementation of Scroll-Sensitive Result Sets".

Notes:
  • Remember that explicit use of the refreshRow() method, described in "Refetching Rows", is distinct from the concept of "visibility" of external changes. This is discussed in "Seeing External Changes".
  • Remember that even when external changes are "visible", as with UPDATE operations underlying a scroll-sensitive result set, they are not "detected". The result set rowDeleted(), rowUpdated(), and rowInserted() methods always return false. This is further discussed in "Visibility versus Detection of External Changes".

Oracle Implementation of Scroll-Sensitive Result Sets

The Oracle implementation of scroll-sensitive result sets involves the concept of a window, with a window size that is based on the fetch size. The window size affects how often rows are updated in the result set.

Once you establish a current row by moving to a specified row (as described in "Positioning in a Scrollable Result Set"), the window consists of the N rows in the result set starting with that row, where N is the fetch size being used by the result set (see "Fetch Size"). Note that there is no current row, and therefore no window, when a result set is first created. The default position is before the first row, which is not a valid current row.

As you move from row to row, the window remains unchanged as long as the current row stays within that window. However, once you move to a new current row outside the window, you redefine the window to be the N rows starting with the new current row.

Whenever the window is redefined, the N rows in the database corresponding to the rows in the new window are automatically refetched through an implicit call to the refreshRow() method (described in "Refetching Rows"), thereby updating the data throughout the new window.

So external updates are not instantaneously visible in a scroll-sensitive result set; they are only visible after the automatic refetches just described.

Note:

Because this kind of refetching is not a highly efficient or optimized methodology, there are significant performance concerns. Consider carefully before using scroll-sensitive result sets as currently implemented. There is also a significant tradeoff between sensitivity and performance. The most sensitive result set is one with a fetch size of 1, which would result in the new current row being refetched every time you move between rows. However, this would have a significant impact on the performance of your application.

Summary of New Methods for Result Set Enhancements

This section summarizes all the new connection, result set, statement, and database meta data methods added for JDBC 2.0 result set enhancements. These methods are more fully discussed throughout this chapter.

Modified Connection Methods

Following is an alphabetical summary of modified connection methods that allow you to specify result set and concurrency types when you create statement objects.

New Result Set Methods

Following is an alphabetical summary of new result set methods for JDBC 2.0 result set enhancements.

Statement Methods

Following is an alphabetical summary of statement methods for JDBC 2.0 result set enhancements. These methods are available in generic statement, prepared statement, and callable statement objects.

Database Meta Data Methods

Following is an alphabetical summary of database meta data methods for JDBC 2.0 result set enhancements.

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