14 Advanced Design Considerations

There are design considerations for more advanced application development issues.

Topicss:

14.1 Opening Persistent LOBs with the OPEN and CLOSE Interfaces

The OPEN and CLOSE interfaces enable you to explicitly open a persistent LOB instance.

When you open a LOB instance with the OPEN interface, the instance remains open until you explicitly close the LOB using the CLOSE interface. The ISOPEN interface enables you to determine whether a persistent LOB is open.

Note that the open state of a LOB is associated with the LOB instance, not the LOB locator. The locator does not save any information indicating whether the LOB instance that it points to is open.

Topics:

14.1.1 Index Performance Benefits of Explicitly Opening a LOB

Explicitly opening a LOB instance can benefit performance of a persistent LOB in an indexed column.

If you do not explicitly open the LOB instance, then every modification to the LOB implicitly opens and closes the LOB instance. Any triggers on a domain index are fired each time the LOB is closed. Note that in this case, any domain indexes on the LOB are updated as soon as any modification to the LOB instance is made; the domain index is always valid and can be used at any time.

When you explicitly open a LOB instance, index triggers do not fire until you explicitly close the LOB. Using this technique can increase performance on index columns by eliminating unneeded indexing events until you explicitly close the LOB. Note that any index on the LOB column is not valid until you explicitly close the LOB.

14.1.2 Closing Explicitly Open LOB Instances

If you explicitly open a LOB instance, then you must close the LOB before you commit the transaction.

Committing a transaction on the open LOB instance causes an error. When this error occurs, the LOB instance is closed implicitly, any modifications to the LOB instance are saved, and the transaction is committed, but any indexes on the LOB column are not updated. In this situation, you must rebuild your indexes on the LOB column.

If you subsequently rollback the transaction, then the LOB instance is rolled back to its previous state, but the LOB instance is no longer explicitly open.

You must close any LOB instance that you explicitly open:

  • Between DML statements that start a transaction, including SELECT ... FOR UPDATE and COMMIT

  • Within an autonomous transaction block

  • Before the end of a session (when there is no transaction involved)

    If you do not explicitly close the LOB instance, then it is implicitly closed at the end of the session and no index triggers are fired.

Keep track of the open or closed state of LOBs that you explicitly open. The following actions cause an error:

  • Explicitly opening a LOB instance that has been explicitly open earlier.

  • Explicitly closing a LOB instance that is has been explicitly closed earlier.

This occurs whether you access the LOB instance using the same locator or different locators.

14.2 Read-Consistent Locators

Oracle Database provides the same read consistency mechanisms for LOBs as for all other database reads and updates of scalar quantities.

Read consistency has some special applications to LOB locators that you must understand. The following sections discuss read consistency and include examples which should be looked at in relationship to each other.

See Also:

Topics:

14.2.1 A Selected Locator Becomes a Read-Consistent Locator

A selected locator, regardless of the existence of the FOR UPDATE clause, becomes a read-consistent locator, and remains a read-consistent locator until the LOB value is updated through that locator.

A read-consistent locator contains the snapshot environment as of the point in time of the SELECT operation.

This has some complex implications. Suppose you have created a read-consistent locator (L1) by way of a SELECT operation. In reading the value of the persistent LOB through L1, note the following:

  • The LOB is read as of the point in time of the SELECT statement even if the SELECT statement includes a FOR UPDATE.

  • If the LOB value is updated through a different locator (L2) in the same transaction, then L1 does not see the L2 updates.

  • L1 does not see committed updates made to the LOB through another transaction.

  • If the read-consistent locator L1 is copied to another locator L2 (for example, by a PL/SQL assignment of two locator variables — L2:= L1), then L2 becomes a read-consistent locator along with L1 and any data read is read as of the point in time of the SELECT for L1.

You can use the existence of multiple locators to access different transformations of the LOB value. However, in doing so, you must keep track of the different values accessed by different locators.

14.2.2 Example of Updating LOBs and Read-Consistency

Read-consistent locators provide the same LOB value regardless of when the SELECT occurs.

The following example demonstrates the relationship between read-consistency and updating in a simple example. Using the print_media table described in "Table for LOB Examples: The PM Schema print_media Table" and PL/SQL, three CLOB instances are created as potential locators: clob_selected, clob_update, and clob_copied.

Observe these progressions in the code, from times t1 through t6:

  • At the time of the first SELECT INTO (at t1), the value in ad_sourcetext is associated with the locator clob_selected.

  • In the second operation (at t2), the value in ad_sourcetext is associated with the locator clob_updated. Because there has been no change in the value of ad_sourcetext between t1 and t2, both clob_selected and clob_updated are read-consistent locators that effectively have the same value even though they reflect snapshots taken at different moments in time.

  • The third operation (at t3) copies the value in clob_selected to clob_copied. At this juncture, all three locators see the same value. The example demonstrates this with a series of DBMS_LOB.READ() calls.

  • At time t4, the program uses DBMS_LOB.WRITE() to alter the value in clob_updated, and a DBMS_LOB.READ() reveals a new value.

  • However, a DBMS_LOB.READ() of the value through clob_selected (at t5) reveals that it is a read-consistent locator, continuing to refer to the same value as of the time of its SELECT.

  • Likewise, a DBMS_LOB.READ() of the value through clob_copied (at t6) reveals that it is a read-consistent locator, continuing to refer to the same value as clob_selected.

Example 14-1

INSERT INTO PRINT_MEDIA VALUES (2056, 20020, EMPTY_BLOB(), 
    'abcd', EMPTY_CLOB(), EMPTY_CLOB(), NULL, NULL, NULL, NULL);

COMMIT;

DECLARE
  num_var           INTEGER;
  clob_selected     CLOB;
  clob_updated      CLOB;
  clob_copied       CLOB;
  read_amount       INTEGER;
  read_offset       INTEGER;
  write_amount      INTEGER;
  write_offset      INTEGER;
  buffer            VARCHAR2(20);

BEGIN
  -- At time t1:
  SELECT ad_sourcetext INTO clob_selected
     FROM Print_media
     WHERE ad_id = 20020;

  -- At time t2:
  SELECT ad_sourcetext INTO clob_updated
     FROM Print_media
     WHERE ad_id = 20020
     FOR UPDATE;

  -- At time t3:
  clob_copied := clob_selected;
  -- After the assignment, both the clob_copied and the
  -- clob_selected have the same snapshot as of the point in time
  -- of the SELECT into clob_selected

  -- Reading from the clob_selected and the clob_copied does
  -- return the same LOB value. clob_updated also sees the same
  -- LOB value as of its select:
  read_amount := 10;
  read_offset := 1;
  dbms_lob.read(clob_selected, read_amount, read_offset, buffer);
  dbms_output.put_line('clob_selected value: ' || buffer);
  -- Produces the output 'abcd'

  read_amount := 10;
  dbms_lob.read(clob_copied, read_amount, read_offset, buffer);
  dbms_output.put_line('clob_copied value: ' || buffer);
  -- Produces the output 'abcd'

  read_amount := 10;
  dbms_lob.read(clob_updated, read_amount, read_offset, buffer);
  dbms_output.put_line('clob_updated value: ' || buffer);
  -- Produces the output 'abcd'

  -- At time t4:
  write_amount := 3;
  write_offset := 5;
  buffer := 'efg';
  dbms_lob.write(clob_updated, write_amount, write_offset, buffer);

  read_amount := 10;
  dbms_lob.read(clob_updated, read_amount, read_offset, buffer);
  dbms_output.put_line('clob_updated value: ' || buffer);
  -- Produces the output 'abcdefg'

  -- At time t5:
  read_amount := 10;
  dbms_lob.read(clob_selected, read_amount, read_offset, buffer);
  dbms_output.put_line('clob_selected value: ' || buffer);
  -- Produces the output 'abcd'

  -- At time t6:
  read_amount := 10;
  dbms_lob.read(clob_copied, read_amount, read_offset, buffer);
  dbms_output.put_line('clob_copied value: ' || buffer);
  -- Produces the output 'abcd'
END;
/

14.2.3 Example of Updating LOBs Through Updated Locators

When you update the value of the persistent LOB through the LOB locator (L1), L1 is updated to contain the current snapshot environment.

This snapshot is as of the time after the operation was completed on the LOB value through locator L1. L1 is then termed an updated locator. This operation enables you to see your own changes to the LOB value on the next read through the same locator, L1.

Note:

The snapshot environment in the locator is not updated if the locator is used to merely read the LOB value. It is only updated when you modify the LOB value through the locator using the PL/SQL DBMS_LOB package or the OCI LOB APIs.

Any committed updates made by a different transaction are seen by L1 only if your transaction is a read-committed transaction and if you use L1 to update the LOB value after the other transaction committed.

Note:

When you update a persistent LOB value, the modification is always made to the most current LOB value.

Updating the value of the persistent LOB through any of the available methods, such as OCI LOB APIs or PL/SQL DBMS_LOB package, updates the LOB value and then reselects the locator that refers to the new LOB value.

Note:

Once you have selected out a LOB locator by whatever means, you can read from the locator but not write into it.

Note that updating the LOB value through SQL is merely an UPDATE statement. It is up to you to do the reselect of the LOB locator or use the RETURNING clause in the UPDATE statement so that the locator can see the changes made by the UPDATE statement. Unless you reselect the LOB locator or use the RETURNING clause, you may think you are reading the latest value when this is not the case. For this reason you should avoid mixing SQL DML with OCI and DBMS_LOB piecewise operations.

14.2.4 Example of Updating a LOB Using SQL DML and DBMS_LOB

Using the Print_media table in the following example, a CLOB locator is created as clob_selected. Note the following progressions in the example, from times t1 through t3:

  • At the time of the first SELECT INTO (at t1), the value in ad_sourcetext is associated with the locator clob_selected.

  • In the second operation (at t2), the value in ad_sourcetext is modified through the SQL UPDATE statement, without affecting the clob_selected locator. The locator still sees the value of the LOB as of the point in time of the original SELECT. In other words, the locator does not see the update made using the SQL UPDATE statement. This is illustrated by the subsequent DBMS_LOB.READ() call.

  • The third operation (at t3) re-selects the LOB value into the locator clob_selected. The locator is thus updated with the latest snapshot environment which allows the locator to see the change made by the previous SQL UPDATE statement. Therefore, in the next DBMS_LOB.READ(), an error is returned because the LOB value is empty, that is, it does not contain any data.

INSERT INTO Print_media VALUES (3247, 20010, EMPTY_BLOB(), 
    'abcd', EMPTY_CLOB(), EMPTY_CLOB(), NULL, NULL, NULL, NULL);

COMMIT;

DECLARE
  num_var           INTEGER;
  clob_selected     CLOB;
  read_amount       INTEGER;
  read_offset       INTEGER;
  buffer            VARCHAR2(20);

BEGIN

  -- At time t1:
  SELECT ad_sourcetext INTO clob_selected
  FROM Print_media
  WHERE ad_id = 20010;

  read_amount := 10;
  read_offset := 1;
  dbms_lob.read(clob_selected, read_amount, read_offset, buffer);
  dbms_output.put_line('clob_selected value: ' || buffer);
  -- Produces the output 'abcd'

  -- At time t2:
  UPDATE Print_media SET ad_sourcetext = empty_clob()
      WHERE ad_id = 20010;
  -- although the most current LOB value is now empty,
  -- clob_selected still sees the LOB value as of the point
  -- in time of the SELECT

  read_amount := 10;
  dbms_lob.read(clob_selected, read_amount, read_offset, buffer);
  dbms_output.put_line('clob_selected value: ' || buffer);
  -- Produces the output 'abcd'

  -- At time t3:
  SELECT ad_sourcetext INTO clob_selected FROM Print_media WHERE
       ad_id = 20010;
  -- the SELECT allows clob_selected to see the most current
  -- LOB value

  read_amount := 10;
  dbms_lob.read(clob_selected, read_amount, read_offset, buffer);
  -- ERROR: ORA-01403: no data found
END;
/

14.2.5 Example of Using One Locator to Update the Same LOB Value

Note:

Avoid updating the same LOB with different locators. You may avoid many pitfalls if you use only one locator to update a given LOB value.

In the following example, using table Print_media, two CLOBs are created as potential locators: clob_updated and clob_copied.

Note these progressions in the example at times t1 through t5:

  • At the time of the first SELECT INTO (at t1), the value in ad_sourcetext is associated with the locator clob_updated.

  • The second operation (at time t2) copies the value in clob_updated to clob_copied. At this time, both locators see the same value. The example demonstrates this with a series of DBMS_LOB.READ() calls.

  • At time t3, the program uses DBMS_LOB.WRITE() to alter the value in clob_updated, and a DBMS_LOB.READ() reveals a new value.

  • However, a DBMS_LOB.READ() of the value through clob_copied (at time t4) reveals that it still sees the value of the LOB as of the point in time of the assignment from clob_updated (at t2).

  • It is not until clob_updated is assigned to clob_copied (t5) that clob_copied sees the modification made by clob_updated.

INSERT INTO PRINT_MEDIA VALUES (2049, 20030, EMPTY_BLOB(), 
    'abcd', EMPTY_CLOB(), EMPTY_CLOB(), NULL, NULL, NULL, NULL);

COMMIT;

DECLARE
  num_var          INTEGER;
  clob_updated     CLOB;
  clob_copied      CLOB;
  read_amount      INTEGER;
  read_offset      INTEGER;
  write_amount     INTEGER;
  write_offset     INTEGER;
  buffer           VARCHAR2(20);
BEGIN

-- At time t1:
  SELECT ad_sourcetext INTO clob_updated FROM PRINT_MEDIA
      WHERE ad_id = 20030
      FOR UPDATE;

  -- At time t2:
  clob_copied := clob_updated;
  -- after the assign, clob_copied and clob_updated see the same
  -- LOB value

  read_amount := 10;
  read_offset := 1;
  dbms_lob.read(clob_updated, read_amount, read_offset, buffer);
  dbms_output.put_line('clob_updated value: ' || buffer);
  -- Produces the output 'abcd'

  read_amount := 10;
  dbms_lob.read(clob_copied, read_amount, read_offset, buffer);
  dbms_output.put_line('clob_copied value: ' || buffer);
  -- Produces the output 'abcd'

  -- At time t3:
  write_amount := 3;
  write_offset := 5;
  buffer := 'efg';
  dbms_lob.write(clob_updated, write_amount, write_offset,
        buffer);

  read_amount := 10;
  dbms_lob.read(clob_updated, read_amount, read_offset, buffer);
  dbms_output.put_line('clob_updated value: ' || buffer);
  -- Produces the output 'abcdefg'


  -- At time t4:
  read_amount := 10;
  dbms_lob.read(clob_copied, read_amount, read_offset, buffer);
  dbms_output.put_line('clob_copied value: ' || buffer);
  -- Produces the output 'abcd'

  -- At time t5:
  clob_copied := clob_updated;

  read_amount := 10;
  dbms_lob.read(clob_copied, read_amount, read_offset, buffer);
  dbms_output.put_line('clob_copied value: ' || buffer);
  -- Produces the output 'abcdefg'
END;
/

14.2.6 Example of Updating a LOB with a PL/SQL (DBMS_LOB) Bind Variable

When a LOB locator is used as the source to update another persistent LOB (as in a SQL INSERT or UPDATE statement, the DBMS_LOB.COPY routine, and so on), the snapshot environment in the source LOB locator determines the LOB value that is used as the source.

If the source locator (for example L1) is a read-consistent locator, then the LOB value as of the time of the SELECT of L1 is used. If the source locator (for example L2) is an updated locator, then the LOB value associated with the L2 snapshot environment at the time of the operation is used.

In the following example, three CLOBs are created as potential locators: clob_selected, clob_updated, and clob_copied.

Note these progressions in the example at times t1 through t5:

  • At the time of the first SELECT INTO (at t1), the value in ad_sourcetext is associated with the locator clob_updated.

  • The second operation (at t2) copies the value in clob_updated to clob_copied. At this juncture, both locators see the same value.

  • Then (at t3), the program uses DBMS_LOB.WRITE() to alter the value in clob_updated, and a DBMS_LOB.READ() reveals a new value.

  • However, a DBMS_LOB.READ() of the value through clob_copied (at t4) reveals that clob_copied does not see the change made by clob_updated.

  • Therefore (at t5), when clob_copied is used as the source for the value of the INSERT statement, the value associated with clob_copied (for example, without the new changes made by clob_updated) is inserted. This is demonstrated by the subsequent DBMS_LOB.READ() of the value just inserted.

INSERT INTO PRINT_MEDIA VALUES (2056, 20020, EMPTY_BLOB(), 
    'abcd', EMPTY_CLOB(), EMPTY_CLOB(), NULL, NULL, NULL, NULL);

COMMIT;

DECLARE
  num_var           INTEGER;
  clob_selected     CLOB;
  clob_updated      CLOB;
  clob_copied       CLOB;
  read_amount       INTEGER;
  read_offset       INTEGER;
  write_amount      INTEGER;
  write_offset      INTEGER;
  buffer            VARCHAR2(20);
BEGIN

  -- At time t1:
  SELECT ad_sourcetext INTO clob_updated FROM PRINT_MEDIA
      WHERE ad_id = 20020
      FOR UPDATE;

  read_amount := 10;
  read_offset := 1;
  dbms_lob.read(clob_updated, read_amount, read_offset, buffer);
  dbms_output.put_line('clob_updated value: ' || buffer);
  -- Produces the output 'abcd'

  -- At time t2:
  clob_copied := clob_updated;


  -- At time t3:
  write_amount := 3;
  write_offset := 5;
  buffer := 'efg';
  dbms_lob.write(clob_updated, write_amount, write_offset, buffer);

  read_amount := 10;
  dbms_lob.read(clob_updated, read_amount, read_offset, buffer);
  dbms_output.put_line('clob_updated value: ' || buffer);
  -- Produces the output 'abcdefg'
  -- note that clob_copied does not see the write made before
  -- clob_updated


  -- At time t4:
  read_amount := 10;
  dbms_lob.read(clob_copied, read_amount, read_offset, buffer);
  dbms_output.put_line('clob_copied value: ' || buffer);
  -- Produces the output 'abcd'

  -- At time t5:
  -- the insert uses clob_copied view of the LOB value which does
  -- not include clob_updated changes
  INSERT INTO PRINT_MEDIA VALUES (2056, 20022, EMPTY_BLOB(), 
    clob_copied, EMPTY_CLOB(), EMPTY_CLOB(), NULL, NULL, NULL, NULL)
    RETURNING ad_sourcetext INTO clob_selected;

  read_amount := 10;
  dbms_lob.read(clob_selected, read_amount, read_offset, buffer);
  dbms_output.put_line('clob_selected value: ' || buffer);
  -- Produces the output 'abcd'
END;
/

14.3 LOB Locators and Transaction Boundaries

LOB locators can be used in transactions and transaction IDs.

See Also:

LOB Locators and BFILE Locators for more information about LOB locators

Topics:

14.3.1 About LOB Locators and Transaction Boundaries

Note the following regarding LOB locators and transactions:

  • Locators contain transaction IDs when:

    You Begin the Transaction, Then Select Locator: If you begin a transaction and subsequently select a locator, then the locator contains the transaction ID. Note that you can implicitly be in a transaction without explicitly beginning one. For example, SELECT... FOR UPDATE implicitly begins a transaction. In such a case, the locator contains a transaction ID.

  • Locators Do Not Contain Transaction IDs When...

    • You are Outside the Transaction, Then Select Locator: By contrast, if you select a locator outside of a transaction, then the locator does not contain a transaction ID.

    • When Selected Prior to DML Statement Execution: A transaction ID is not assigned until the first DML statement executes. Therefore, locators that are selected prior to such a DML statement do not contain a transaction ID.

14.3.2 Read and Write Operations on a LOB Using Locators

You can always read LOB data using the locator irrespective of whether or not the locator contains a transaction ID.

  • Cannot Write Using Locator:

    If the locator contains a transaction ID, then you cannot write to the LOB outside of that particular transaction.

  • Can Write Using Locator:

    If the locator does not contain a transaction ID, then you can write to the LOB after beginning a transaction either explicitly or implicitly.

  • Cannot Read or Write Using Locator With Serializable Transactions:

    If the locator contains a transaction ID of an older transaction, and the current transaction is serializable, then you cannot read or write using that locator.

  • Can Read, Not Write Using Locator With Non-Serializable Transactions:

    If the transaction is non-serializable, then you can read, but not write outside of that transaction.

The examples Selecting the Locator Outside of the Transaction Boundary, Selecting the Locator Within a Transaction Boundary, LOB Locators Cannot Span Transactions, and Example of Locator Not Spanning a Transaction show the relationship between locators and non-serializable transactions

14.3.3 Selecting the Locator Outside of the Transaction Boundary

Two scenarios describe techniques for using locators in non-serializable transactions when the locator is selected outside of a transaction.

First Scenario:

  1. Select the locator with no current transaction. At this point, the locator does not contain a transaction id.

  2. Begin the transaction.

  3. Use the locator to read data from the LOB.

  4. Commit or rollback the transaction.

  5. Use the locator to read data from the LOB.

  6. Begin a transaction. The locator does not contain a transaction id.

  7. Use the locator to write data to the LOB. This operation is valid because the locator did not contain a transaction id prior to the write. After this call, the locator contains a transaction id.

Second Scenario:

  1. Select the locator with no current transaction. At this point, the locator does not contain a transaction id.
  2. Begin the transaction. The locator does not contain a transaction id.
  3. Use the locator to read data from the LOB. The locator does not contain a transaction id.
  4. Use the locator to write data to the LOB. This operation is valid because the locator did not contain a transaction id prior to the write. After this call, the locator contains a transaction id. You can continue to read from or write to the LOB.
  5. Commit or rollback the transaction. The locator continues to contain the transaction id.
  6. Use the locator to read data from the LOB. This is a valid operation.
  7. Begin a transaction. The locator contains the previous transaction id.
  8. Use the locator to write data to the LOB. This write operation fails because the locator does not contain the transaction id that matches the current transaction.

14.3.4 Selecting the Locator Within a Transaction Boundary

Two scenarios describe techniques for using locators in non-serializable transactions when the locator is selected within a transaction.

First Scenario:

  1. Select the locator within a transaction. At this point, the locator contains the transaction id.

  2. Begin the transaction. The locator contains the previous transaction id.

  3. Use the locator to read data from the LOB. This operation is valid even though the transaction id in the locator does not match the current transaction.

    See Also:

    "Read-Consistent Locators" for more information about using the locator to read LOB data.

  4. Use the locator to write data to the LOB. This operation fails because the transaction id in the locator does not match the current transaction.

Second Scenario:

  1. Begin a transaction.
  2. Select the locator. The locator contains the transaction id because it was selected within a transaction.
  3. Use the locator to read from or write to the LOB. These operations are valid.
  4. Commit or rollback the transaction. The locator continues to contain the transaction id.
  5. Use the locator to read data from the LOB. This operation is valid even though there is a transaction id in the locator and the transaction was previously committed or rolled back.
  6. Use the locator to write data to the LOB. This operation fails because the transaction id in the locator is for a transaction that was previously committed or rolled back.

14.3.5 LOB Locators Cannot Span Transactions

Modifying a persistent LOB value through the LOB locator using DBMS_LOB, OCI, or SQL INSERT or UPDATE statements changes the locator from a read-consistent locator to an updated locator.

The INSERT or UPDATE statement automatically starts a transaction and locks the row. Once this has occurred, the locator cannot be used outside the current transaction to modify the LOB value. In other words, LOB locators that are used to write data cannot span transactions. However, the locator can be used to read the LOB value unless you are in a serializable transaction.

See Also:

"LOB Locators and Transaction Boundaries ", for more information about the relationship between LOBs and transaction boundaries.

In Example of Locator Not Spanning a Transaction , a CLOB locator is created: clob_updated

  • At the time of the first SELECT INTO (at t1), the value in ad_sourcetext is associated with the locator clob_updated.

  • The second operation (at t2), uses the DBMS_LOB.WRITE function to alter the value in clob_updated, and a DBMS_LOB.READ reveals a new value.

  • The commit statement (at t3) ends the current transaction.

  • Therefore (at t4), the subsequent DBMS_LOB.WRITE operation fails because the clob_updated locator refers to a different (already committed) transaction. This is noted by the error returned. You must re-select the LOB locator before using it in further DBMS_LOB (and OCI) modify operations.

14.3.6 Example of Locator Not Spanning a Transaction

The example uses the print_media table described in "Table for LOB Examples: The PM Schema print_media Table"

INSERT INTO PRINT_MEDIA VALUES (2056, 20010, EMPTY_BLOB(), 
    'abcd', EMPTY_CLOB(), EMPTY_CLOB(), NULL, NULL, NULL, NULL);

COMMIT;

DECLARE
  num_var          INTEGER;
  clob_updated     CLOB;
  read_amount      INTEGER;
  read_offset      INTEGER;
  write_amount     INTEGER;
  write_offset     INTEGER;
  buffer           VARCHAR2(20);

BEGIN
          -- At time t1:
     SELECT      ad_sourcetext
     INTO        clob_updated
     FROM        PRINT_MEDIA
     WHERE       ad_id = 20010
     FOR UPDATE;
     read_amount := 10;
     read_offset := 1;
     dbms_lob.read(clob_updated, read_amount, read_offset, buffer);
     dbms_output.put_line('clob_updated value: ' || buffer);
     -- This produces the output 'abcd'

     -- At time t2:
     write_amount := 3;
     write_offset := 5;
     buffer := 'efg';
     dbms_lob.write(clob_updated, write_amount, write_offset, buffer);
     read_amount := 10;
     dbms_lob.read(clob_updated, read_amount, read_offset, buffer);
     dbms_output.put_line('clob_updated value: ' || buffer);
     -- This produces the output 'abcdefg'

    -- At time t3:
    COMMIT;

     -- At time t4:
    dbms_lob.write(clob_updated , write_amount, write_offset, buffer);
    -- ERROR: ORA-22990: LOB locators cannot span transactions
END;
/

14.4 LOBs in the Object Cache

When you copy one object to another in the object cache with a LOB locator attribute, only the LOB locator is copied.

This means that the LOB attribute in these two different objects contain exactly the same locator which refers to one and the same LOB value. Only when the target object is flushed is a separate, physical copy of the LOB value made, which is distinct from the source LOB value.

See Also:

"Example of Updating LOBs and Read-Consistency" for a description of what version of the LOB value is seen by each object if a write is performed through one of the locators.

Therefore, in cases where you want to modify the LOB that was the target of the copy, you must flush the target object, refresh the target object, and then write to the LOB through the locator attribute.

Consider these object cache issues for internal and external LOB attributes:

  • Persistent LOB attributes: Creating an object in object cache, sets the LOB attribute to empty.

    When you create an object in the object cache that contains a persistent LOB attribute, the LOB attribute is implicitly set to empty. You may not use this empty LOB locator to write data to the LOB. You must first flush the object, thereby inserting a row into the table and creating an empty LOB — that is, a LOB with 0 length. Once the object is refreshed in the object cache (use OCI_PIN_LATEST), the real LOB locator is read into the attribute, and you can then call the OCI LOB API to write data to the LOB.

  • External LOB (BFILE) attributes: Creating an object in object cache, sets the BFILE attribute to NULL.

    When creating an object with an external LOB (BFILE) attribute, the BFILE is set to NULL. It must be updated with a valid directory object name and file name before reading from the BFILE.

14.5 Terabyte-Size LOB Support

Terabyte-size LOBs are LOBs that are up to a maximum size of 8 to 128 terabytes depending on database block size.

Topics:

14.5.1 About Terabyte-Size LOB Support

Terabyte-size LOBs are supported by the following APIs:

  • Java using JDBC (Java Database Connectivity)

  • PL/SQL using the DBMS_LOB Package

  • C using OCI (Oracle Call Interface)

You cannot create and use LOB instances of size greater than 4 gigabytes "terabyte-size LOBs"— in the following programmatic environments:

  • COBOL using the Pro*COBOL Precompiler

  • C or C++ using the Pro*C/C++ Precompiler

Note:

Oracle Database does not support BFILEs larger than 2^64-1 bytes (UB8MAXVAL in OCI) in any programmatic environment. Any additional file size limit imposed by your operating system also applies to BFILEs.

14.5.2 Maximum Storage Limit for Terabyte-Size LOBs

In supported environments, you can create and manipulate LOBs that are up to the maximum storage size limit for your database configuration.

Oracle Database lets you create tablespaces with block sizes different from the database block size, and the maximum size of a LOB depends on the size of the tablespace blocks. CHUNK is a parameter of LOB storage whose value is controlled by the block size of the tablespace in which the LOB is stored.

Note:

The CHUNK parameter does not apply to SecureFiles. It is only used for BasicFiles LOBs.

When you create a LOB column, you can specify a value for CHUNK, which is the number of bytes to be allocated for LOB manipulation. The value must be a multiple of the tablespace block size, or Oracle Database rounds up to the next multiple. (If the tablespace block size is the same as the database block size, then CHUNK is also a multiple of the database block size.)

The maximum allowable storage limit for your configuration depends on the tablespace block size setting, and is calculated as (4 gigabytes - 1) times the value obtained from DBMS_LOB.GETCHUNKSIZE or OCILobGetChunkSize(). This value, in number of bytes for BLOBs or number of characters for CLOBs, is actually less than the size of the CHUNK parameter due to internal storage overhead. With the current allowable range for the tablespace block size from 2K to 32K, the storage limit ranges from 8 terabytes to 128 terabytes.

For example, suppose your database block size is 32K bytes and you create a tablespace with a nonstandard block size of 8K. Further suppose that you create a table with a LOB column and specify a CHUNK size of 16K (which is a multiple of the 8K tablespace block size). Then the maximum size of a LOB in this column is (4 gigabytes - 1) * 16K.

See Also:

This storage limit applies to all LOB types in environments that support terabyte-size LOBs. However, note that CLOB and NCLOB types are sized in characters, while the BLOB type is sized in bytes.

14.5.3 Using Terabyte-Size LOBs with JDBC

You can use the LOB APIs included in the Oracle JDBC classes to access terabyte-size LOBs.

14.5.4 Using Terabyte-Size LOBs with the DBMS_LOB Package

You can access terabyte-size LOBs with all APIs in the DBMS_LOB PL/SQL package.

Use DBMS_LOB.GETCHUNKSIZE to obtain the value to be used in reading and writing LOBs. The number of bytes stored in a chunk is actually less than the size of the CHUNK parameter due to internal storage overhead. The DBMS_LOB.GET_STORAGE_LIMIT function returns the storage limit for your database configuration. This is the maximum allowable size for LOBs. BLOBs are sized in bytes, while CLOBs and NCLOBs are sized in characters.

See Also:

Oracle Database PL/SQL Packages and Types Referencefor details on the initialization parameter setting for your database installation.

14.5.5 Using Terabyte-Size LOBs with OCI

The Oracle Call Interface API provides a set of functions for operations on LOBs of all sizes.

OCILobGetChunkSize() returns the value, in bytes for BLOBs, or in characters for CLOBs, to be used in reading and writing LOBs. For varying-width character sets, the value is the number of Unicode characters that fit. The number of bytes stored in a chunk is actually less than the size of the CHUNK parameter due to internal storage overhead. The function OCILobGetStorageLimit() returns the maximum allowable size, in bytes, of internal LOBs in the current database installation. If streaming mode is used, where the whole LOB is read, there is no requirement to get the chunk size.

See Also:

Oracle Call Interface Programmer's Guide for details about OCI functions that support LOBs

14.6 Guidelines for Creating Gigabyte LOBs

To create gigabyte LOBs in supported environments, use the following guidelines to make use of all available space in the tablespace for LOB storage:

  • Single Data File Size Restrictions:

    There are restrictions on the size of a single data file for each operating system. For example, Solaris 2.5 only allows operating system files of up to 2 gigabytes. Hence, add more data files to the tablespace when the LOB grows larger than the maximum allowed file size of the operating system on which your Oracle Database runs.

  • Set PCT INCREASE Parameter to Zero:

    PCTINCREASE parameter in the LOB storage clause specifies the percent growth of the new extent size. When a LOB is being filled up piece by piece in a tablespace, numerous new extents get created in the process. If the extent sizes keep increasing by the default value of 50 percent every time, then extents become unmanageable and eventually waste space in the tablespace. Therefore, the PCTINCREASE parameter should be set to zero or a small value.

  • Set MAXEXTENTS to a Suitable Value or UNLIMITED:

    The MAXEXTENTS parameter limits the number of extents allowed for the LOB column. A large number of extents are created incrementally as the LOB size grows. Therefore, the parameter should be set to a value that is large enough to hold all the LOBs for the column. Alternatively, you could set it to UNLIMITED.

  • Use a Large Extent Size:

    For every new extent created, Oracle generates undo information for the header and other metadata for the extent. If the number of extents is large, then the rollback segment can be saturated. To get around this, choose a large extent size, say 100 megabytes, to reduce the frequency of extent creation, or commit the transaction more often to reuse the space in the rollback segment.

14.6.1 Creating a Tablespace and Table to Store Gigabyte LOBs

The following example illustrates how to create a tablespace and table to store gigabyte LOBs.

CREATE TABLESPACE lobtbs1 DATAFILE '/your/own/data/directory/lobtbs_1.dat'
SIZE 2000M REUSE ONLINE NOLOGGING DEFAULT STORAGE (MAXEXTENTS UNLIMITED);
ALTER TABLESPACE lobtbs1 ADD DATAFILE
'/your/own/data/directory/lobtbs_2.dat' SIZE 2000M REUSE;

CREATE TABLE print_media_backup
  (product_id NUMBER(6), 
   ad_id NUMBER(6), 
   ad_composite BLOB, 
   ad_sourcetext CLOB, 
   ad_finaltext CLOB, 
   ad_fltextn NCLOB, 
   ad_textdocs_ntab textdoc_tab, 
   ad_photo BLOB, 
   ad_graphic BLOB, 
   ad_header adheader_typ)
   NESTED TABLE ad_textdocs_ntab STORE AS textdocs_nestedtab5 
   LOB(ad_sourcetext) STORE AS (TABLESPACE lobtbs1 CHUNK 32768 PCTVERSION 0 
                                NOCACHE NOLOGGING
                                STORAGE(INITIAL 100M NEXT 100M MAXEXTENTS 
                                UNLIMITED PCTINCREASE 0));

Note the following with respect to this example:

  • The storage clause in this example is specified in the CREATE TABLESPACE statement.

  • You can specify the storage clause in the CREATE TABLE statement as an alternative.

  • The storage clause is not allowed in the CREATE TEMPORARY TABLESPACE statement.

  • Setting the PCTINCREASE parameter to 0 is recommended for gigabyte LOBs. For small, or medium size lobs, the default PCTINCREASE value of 50 is recommended as it reduces the number of extent allocations.