SCN and undo

hi,
say we start a transaction, and then we want to read a value, and that value has not been changed since our transaction started, but it is in a data block where another row in it has been changed since our transaction started.
So Oracle will see that that block's SCN is later than our transactions, and so it will go to the undo segment. This makes no sense to me.
Please help.
thanks

OracleGuy777 wrote:
hi,
say we start a transaction, and then we want to read a value, and that value has not been changed since our transaction started,This means that the block's transaction header has not recorded any active transaction and there is no Transaction SCN assigned within it. Whatever is there in the block, it can be read as it is without any kind of modifications done over it.
but it is in a data block where another row in it has been changed since our transaction started. That value change should not effect you. I believe what you are trying to say that there in the block, you got two values, Aman and employee code 1234 and you want to read the value Aman which is unchanged. The second value is getting updated ,may be to 5678 right?
So Oracle will see that that block's SCN is later than our transactions, and so it will go to the undo segment. This makes no sense to me.Okay, this certainly makes no sense. If you are not querying what's not changed, there is no point for Oracle to read the data from Undo for it. Oracle would do a current read for the data and would give the same content to you.
>
Please help.HTH
Aman....
thanks

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SCN and undo

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