UNDO tablespace resize/shrink.

Question

Hi All ,
is it possible to shrink/resize undo tablespace.

Answer 1

Hi Shobhit!
I is correct that you're unable to set this parameter with ALTER SYSTEM. Please never use this parameter unless you are told to do so from oracle support. Maybe you're playing with your support if you set it by your self. To tune the size of your undo tablespace set the parameter undo_retention. Whit this parameter you tell your DB in seconds how long undo records should be kept within your undo tablespace. Automatic undotuning will be automatically activated if this parameter has a value greater than 0. To determine the correct size you need for your undotablespace use the following example:
Size of UNDO needed = UNDO_RETENTION x [UNDO block Generation per sec x DB_BLOCK_SIZE] + Overhead(30xDB_BLOCK_SIZE)
Out of these we know UNDO_RETENTION and DB_BLOCK_SIZE
All we need is to find out “UNDO Blocks per second”
Which can be easily fetched from v$undostat
SQL> SELECT (SUM(undoblks))/ SUM ((end_time - begin_time) * 24*60*60) "UPS"
2 FROM v$undostat;
UPS
8.11985583
V$undostat
stores data for every 10 mins and begin/end times are start/end time of
those intervals. We multiplied it with 24*60*60 because the difference
between two dates will be in days and to get to seconds, we need it to
multiply with 24hrs*60mins*60secs
So now we have all the values needed.
Undo size needed = [8.12 x 2000 x 8192] + [30 x 8192] = 133283840 bytes = 127.11 MB
Hope this helps!
yours sincerely