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Petre Radut

When you change the UNDO_RETENTION parameter, the LOB segment’s retention value is not modified

Below, I will try to explain, a particular case for the general error : ORA-01555 snapshot too old error..

Normally, when we have this error, we are trying to adapt the retention parameters or to tune our queries.

SQL> show parameter undo;

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
temp_undo_enabled                    boolean     FALSE
undo_management                      string      AUTO
undo_retention                       integer     3600 --extended from 900,
undo_tablespace                      string      UNDOTBS1

But, are some scenarios where the above rule is not working .

We got from the alert log file of the DB the sql id which caused the issue : pmrbk5fdfd665

But, when you want to search for it, in V$SQL/V$SQLAREA  is not there

SQL> select sql_fulltext from v$sql where sql_id like '%pmrbk5fdfd665%';

no rows selected


Seems that, sql_id is present in V$OPEN_CURSOR, with an input into the sqltext column.
The issue is coming from the fact that the statement is accessing a LOB column, which causes to Oracle to generate a new sql id.
The execution part related to the LOBs will not appear into V$SQL/V$SQLAREA and is not captured into the AWR reports.

SQL>  select distinct * from v$open_cursor
  2     where rownum < 25
  3     and sql_id like '%pmrbk5fdfd665%';

SADDR                   SID USER_NAME                      ADDRESS          HASH_VALUE SQL_ID        SQL_TEXT                                                     LAST_SQL SQL_EXEC_ID CURSOR_TYPE
---------------- ---------- ------------------------------ ---------------- ---------- ------------- ------------------------------------------------------------ -------- ----------- ---------------
0000000670A19780         74 my_user                   00000002EB91F1F0 3831220380 pmrbk5fdfd665 table_104_11_XYZT_0_0_0
00000006747F0478        131 my_user                   00000002EB91F1F0 3831220380 pmrbk5fdfd665 table_104_11_XYZT_0_0_0

Apparently, the string into the sql_text column is  a  HEX representation of the object_id that is being accessed.
In our case is :XYZT

SQL>    select owner, object_name, object_type
  2    from dba_objects
  3    where object_id = (select to_number('&hex_value','XXXXXX') from dual);
Enter value for hex_value: XYZT
old   3:   where object_id = (select to_number('&hex_value','XXXXXX') from dual)
new   3:   where object_id = (select to_number('XYZT','XXXXXX') from dual)

OWNER                  OBJECT_TYPE                                               OBJECT_NAME
---------------------- --------------------------------------------------------------------------
my_user                TABLE                                                     my_table

SQL> desc my_user.my_table;
 Name                  Type
 -------------------   ----------------
 DOCUMENT               BLOB

If we are looking for the retention on the ” DOCUMENT ” column, we will see :

SQL> select table_name, pctversion, retention,segment_name from dba_lobs where table_name in ('my_table');

                                                  PCTVERSION  RETENTION                  SEGMENT_NAME
---------------------------------------------------------------------------------------- ------------------------------------
my_table                                                       900                       SYS_LOB0000027039C00002$$

In order to fix it , try to run this column to adapt the retention of the blob column, related to the new value of the UNDO_RETENTION parameter,


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Petre Radut
Petre Radut