Size your Undo tablespace

Size your Undo tablespace

Here is the step by step approach:

Step 1: Longest running query.

SQL> select max(maxquerylen) from v$undostat;

MAX(MAXQUERYLEN)

----------------

1793

This gives you ideal value for UNDO_RETENTION. To be on the safer size you should add few more seconds to get the right value. So in my case, the size of undo retention should be say 2000 secs.

Step 2: Size of UNDO tablespace.

Size of UNDO needed = UNDO_RETENTIONx[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