V$SESSION’s ROW_WAIT_OBJ# Column Shows the DATA_OBJECT_ID – Verify the Documentation Challenge

June 7, 2011

While reading through the alpha copy of a performance tuning book, I noticed a set of SQL statements that were identical to those found in the Oracle Database 11.2 Performance Tuning Guide.  The set of SQL statements from the Performance Tuning Guide follow:

10.3.1.1 Causes
To determine the possible causes, first query V$SESSION to identify the value of ROW_WAIT_OBJ# when the session waits for buffer busy waits. For example:

SELECT row_wait_obj#
  FROM V$SESSION
 WHERE EVENT = 'buffer busy waits';

To identify the object and object type contended for, query DBA_OBJECTS using the value for ROW_WAIT_OBJ# that is returned from V$SESSION. For example:

SELECT owner, object_name, subobject_name, object_type
  FROM DBA_OBJECTS
 WHERE data_object_id = &row_wait_obj;

In an earlier article we found that V$SESSION’s ROW_WAIT_OBJ# column indicated the OBJECT_ID, not the DATA_OBJECT_ID of the object (as found in DBA_OBJECTS) and the same was true for the obj# entry found in a 10046 trace file.  When you first create an index the OBJECT_ID and the DATA_OBJECT_ID values will be the same for the index, as displayed in DBA_OBJECTS.  However, when you rebuild the index the DATA_OBJECT_ID value for the index will change while the OBJECT_ID value remains unchanged.  Likewise, when you first create a table the OBJECT_ID and the DATA_OBJECT_ID values will be the same, as displayed in DBA_OBJECTS.  If you TRUNCATE a table, the OBJECT_ID value will remain the same while the DATA_OBJECT_ID value for the table will change.

The challenge is to build a test case that either confirms or refutes the claim made by the Oracle Database Performance Tuning Guide.  Possible references:

• AskTom thread
• Metalink (MOS) ID 15476.1 “FAQ about Detecting and Resolving Locking Conflicts”
• Metalink (MOS) Doc ID 603661.1 “ORA-01410: Invalid Rowid When Using rowid Generated from V$Session and Dba_objects”
• Another article found on this blog (take a look at the Other Resources links)