Listing Chained Rows of Tables and Clusters

 

You can look at the chained and migrated rows of a table or cluster using the ANALYZE statement with the LIST CHAINED ROWS clause. The results of this statement are stored in a specified table created explicitly to accept the information returned by the LIST CHAINED ROWS clause. These results are useful in determining whether you have enough room for updates to rows.

Creating a CHAINED_ROWS Table

To create the table to accept data returned by an ANALYZE...LIST CHAINED ROWS statement, execute the UTLCHAIN.SQL or UTLCHN1.SQL script. These scripts are provided by the database. They create a table named CHAINED_ROWS in the schema of the user submitting the script.

Note:

Your choice of script to execute for creating the CHAINED_ROWS table is dependent upon the compatibility level of your database and the type of table you are analyzing. See the Oracle Database SQL Language Reference for more information.

After a CHAINED_ROWS table is created, you specify it in the INTO clause of the ANALYZE statement. For example, the following statement inserts rows containing information about the chained rows in the emp_dept cluster into the CHAINED_ROWS table:

ANALYZE CLUSTER emp_dept LIST CHAINED ROWS INTO CHAINED_ROWS;

See Also:

• Oracle Database Reference for a description of the CHAINED_ROWS table
• "Using the Segment Advisor" for information on how the Segment Advisor reports tables with excess row chaining.

Eliminating Migrated or Chained Rows in a Table

You can use the information in the CHAINED_ROWS table to reduce or eliminate migrated and chained rows in an existing table. Use the following procedure.

1. Use the ANALYZE statement to collect information about migrated and chained rows.

   ANALYZE TABLE order_hist LIST CHAINED ROWS;
   

2. Query the output table:

   SELECT *
   FROM CHAINED_ROWS
   WHERE TABLE_NAME = 'ORDER_HIST';
   
   OWNER_NAME  TABLE_NAME  CLUST... HEAD_ROWID          TIMESTAMP
   ----------  ----------  -----... ------------------  ---------
   SCOTT       ORDER_HIST       ... AAAAluAAHAAAAA1AAA  04-MAR-96
   SCOTT       ORDER_HIST       ... AAAAluAAHAAAAA1AAB  04-MAR-96
   SCOTT       ORDER_HIST       ... AAAAluAAHAAAAA1AAC  04-MAR-96
   

   The output lists all rows that are either migrated or chained.
3. If the output table shows that you have many migrated or chained rows, then you can eliminate migrated rows by continuing through the following steps:
4. Create an intermediate table with the same columns as the existing table to hold the migrated and chained rows:

   CREATE TABLE int_order_hist
      AS SELECT *
         FROM order_hist
         WHERE ROWID IN
            (SELECT HEAD_ROWID
               FROM CHAINED_ROWS
               WHERE TABLE_NAME = 'ORDER_HIST');
   

5. Delete the migrated and chained rows from the existing table:

   DELETE FROM order_hist
      WHERE ROWID IN
         (SELECT HEAD_ROWID
            FROM CHAINED_ROWS
            WHERE TABLE_NAME = 'ORDER_HIST');
   

6. Insert the rows of the intermediate table into the existing table:

   INSERT INTO order_hist
      SELECT *
      FROM int_order_hist;
   

7. Drop the intermediate table:

   DROP TABLE int_order_history;
   

8. Delete the information collected in step 1 from the output table:

   DELETE FROM CHAINED_ROWS
      WHERE TABLE_NAME = 'ORDER_HIST';
   

9. Use the ANALYZE statement again, and query the output table.

Any rows that appear in the output table are chained. You can eliminate chained rows only by increasing your data block size. It might not be possible to avoid chaining in all situations. Chaining is often unavoidable with tables that have a LONG column or large CHAR or VARCHAR2 columns.