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From: mark.powell@eds.com
Newsgroups: comp.databases.oracle.server
Subject: Re: rollback segment usage
Date: Tue, 02 Feb 1999 18:51:45 GMT
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Answers intermixed with original post

In article <7978ia$9vc$1@nnrp1.dejanews.com>,
  nguffey@jdriscoll.com wrote:
> Several Questions: 1.  When Oracle writes a before image to a rollback
> segment, is it writing only the row being modified, or is it writing the
> entire datablock?

The first time a user session modifies a block the entire block has to be
copied to the rbs segment.  The next time a block is copied only vector
information has to be copied to the rbs segment.

> 2.  Does a SCN apply only to the row on a data block being
> modified, or to the entire block?

I read, but I do not remember in what book, that the answer depends on it you
are accessing the table via full table scan or index read.  When the block is
accessed via a full table scan the scn applies to the block, but if the
access is via an index, the scn is at the row level.

>3.  Multiple transactions can use the same
> rollback segment, but under what conditions, if any, can multiple
> transactions use the same extent in a particular rollback segment?

The rbs segment and extents are always shared.	It is at the block level that
multiple transactions do not share rbs space.


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Mark D. Powell  -- The only advice that counts is the advice that
 you follow so follow your own advice --

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