Oracle Concepts - Oracle undo tips Oracle Tips by Burleson Consulting

Administering Oracle UNDO

Oracle allows you to make changes to the database in kind of a “try it before you buy it mode”. What this means is that the changes you make in the database can only be seen by you, at first. Hence, you can make changes, check those changes and make sure they are ok, before you let anyone else see them. Once you are sure you like your changes, you issue the commit command, and everyone else can then see them.

This type of functionality gives us database consistency. This means that the data in the database, when you query it, is always consistent to the same point in time. For example, if you query two tables at the same time, say tables named PARENT and CHILD, you will see record from both tables as they looked at the same point in time. This means that no one can slip a record into the CHILD table, while you are reading the PARENT table, or vice versa. The assurance that the records you will read will be consistent to the same point in time, is known as read consistency.  Read consistency gives your query’s results as they were the moment you pressed Enter.

Oracle Undo segments support these read consistent operations (and other database operations such as flashback operations which we will discuss later in this book). Oracle creates an undo segment in the SYSTEM tablespace when the database is created. This undo segment only supports operations in the SYSTEM tablespace. Hence, if you are going to use other tablespaces (and you should!), then you will need other undo segments.

select  
   substr(a.os_user_name,1,15)   "OS User",
   substr(a.oracle_username,1,8) "DB User",
   substr(b.owner,1,8)           "Schema",
   substr(b.object_name,1,20)    "Object Name",
   substr(b.object_type,1,10)    "Type",
   substr(c.segment_name,1,15)   "RBS",
   substr(d.used_urec,1,12)      "# of Records",
   e.sid,
   e.serial#
from v$locked_object    a,
     dba_objects        b,
     dba_rollback_segs  c,
     v$transaction      d
     v$session e
see code depot for full script
where  
   a.object_id =  b.object_id
 and
   a.xidusn    =  c.segment_id
and
   a.xidusn    =  d.xidusn
and
   a.xidslot   =  d.xidslot
and
   d.addr      =  e.taddr;

So, where do we get these other undo segments? We create an undo tablespace with the create undo tablespace command (we will talk about tablespaces in more detail next) and Oracle Database 10g will create the undo segments and manage the for us automatically.

When an Oracle Database is first created, an undo tablespace can be created at that time, and we recommend this approach. Before you can use the undo tablespace, we need to set a few parameters (see, there are those parameters again, they show up everywhere!)

You can use the alter system command to change any of the parameters listed above at any time.

However, please note that if you change the Undo Tablespace by changing the undo_tablespace parameter, there still may be active transactions against the old tablespace.  Keep that in mind before you drop it, it could result in one of the DBA’s most common errors: ORA-01555, snapshot too old, rollback segment too small!