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Find the exact data block causing a hung session

Oracle Tips by Burleson Consulting

May 18, 2003, Updated 18 May 2015

It is not uncommon for an end-user session to ?hang? when a user is trying to access a shared data resource that is being held by another end-user.  When a session is waiting on a resource, information can be found in the v$session view in the row_wait_file# and row_wait_block# .  These values will help you identify the source of the contention. 

Also see how to find blocking sessions and how to find Oracle RAC blocking sessions

If we know the data file and block number, we can zoom-in and see exactly what resource is causing the contention.  The file number and block number can then be cross-referenced into the dba_extents view to see the name of the table where the session is waiting on a block.

Here is the script that I use to find the source of hung sessions.  Note how the v$session column row_wait_file# is joined into the dba_extents view.

 
column host     format a6
column username format a10
column os_user  format a8
column program  format a30
column tsname   format a12
 
select
   b.machine         host,
   b.username        username,
   b.server,
   b.osuser          os_user,
   b.program         program,
   a.tablespace_name ts_name,
   row_wait_file#    file_nbr,
   row_wait_block#   block_nbr,
   c.owner,
   c.segment_name,
   c.segment_type
from
   dba_data_files a,
   v$session      b,
   dba_extents    c
See code depot for full script
where
   b.row_wait_file# = a.file_id
and
   c.file_id = row_wait_file#
and
  row_wait_block#  between c.block_id and c.block_id + c.blocks - 1
and
  row_wait_file# <> 0
and
  type='USER'
;

Here is the output from this script:

BOX    USERNAME   SERVER    OS_USER  PROGRAM
------ ---------- --------- -------- ------------------------------
TS_NAME        FILE_NBR  BLOCK_NBR OWNER
------------ ---------- ---------- ------------------------------
SEGMENT_NAME
---------------------------------------------------------------------------
SEGMENT_TYPE
-----------------
avmak1 JONES        DEDICATED server      ?  @avmak1 (TNS interface)
TS_ITEM                9      55342 SYSADM
ITEM
TABLE

The result shows that a session owned by user ?JONES? is waiting for a resource in the TS_ITEM tablespace at data block number 70945. The ITEM table is at data block 55432.

Now that the source of the contention has been identified, you can locate other users who may be holding locks on this table.  This is done by changing the WHERE clause of the above query to include the clause

   WHERE row_wait_block# = 5532;

By the way, this type of wait can commonly be issued when a large update task is holding locks on the table, or when an individual task has placed an exclusive lock on specific rows in the table.

If you like DBA internal tricks, check-out my new book Oracle Tuning: The Definitive Reference by Rampant TechPress. 

[http://dba-oracle.com/include_tuning_book3.htm]

 


 

 

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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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