How oracle creates CR copies of a block

From: stan mowers <stan_mowers_jr_at_yahoo.com>
Date: 19 Aug 2001 16:14:30 -0700
Message-ID: <b37781b8.0108191514.45ad19e4@posting.google.com>

Hi,

I was interested in knowing how Oracle creates a Consistent Read version
of a block in cases where the ITL entry in the block itself has been over written by subsequent transactions. In case there are live transactions affecting a block, it is relatively easy to create a CR copy by linking the affected rows on the block to the ITL and then linking back to the Undo block from there on. This is only possible as the ITL would always contain valid information for live ( uncommitted) transactions. However, if this ITL entry itself has been overwritten by subsequent transactions affecting the same block but different rows, how does Oracle know which Undo block to read for creating CR copies of the block as they were during the previous transactions. Any information would be appreciated

Stan Mowers
Sr. Oracle DBA
New York Received on Sun Aug 19 2001 - 18:14:30 CDT