How are rollback segments assigned?

From: <Rajesh.Rao_at_jpmchase.com>
Date: Mon, 25 Nov 2002 07:39:13 -0800
Message-ID: <F001.0050AC32.20021125073913@fatcity.com>

Hello Friends,

Oracle Parallel Server 8.0.6.2.0 on Solaris 2.6 Nodes

We were having some locking and rollback issues, and I set up some scripts to alert me in case there are more than 4 transactions in the rollback segments, and more than 1 in any of the rollback segments.

Select sum(xacts) from v$rollstat --- Alert if more than 4, condition set to exclude system rollback
where usn != 0

Select count(*) from v$rollstat --- Alert if more than 0, where condition to exclude system rollback
where xacts >=2
and usn != 0

We have 20 rollback segments, and there is very minimal DML activity on this database. I happened to see a scenario where all rollback segments had 0 transactions, except for one which had 2 in them. Under what scenario, can this happen?

I thought the criteria for assigning rollback segments to transactions was

1. If object in system tablespace, use system rolback segment
2. Use the one with the least number of active transactions
3. If 2 or more rollback segments fit the second criteria, use the LRU

algorithm.

Thanks
Raj

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Author:
  INET: Rajesh.Rao_at_jpmchase.com

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