Data Base Buffer Cache Ful

Question

Hi,
what if, an update statement causes the existing db cache ful with the updated information(in this case this is uncommited transaction). in this case, if another user run the update statement where that update information stored before written to the datafile.

Answer 1

No this is not correct. A block/buffer , when undergoes any kind of dml, is marked as dirty. Once marked dirty , there are various events which push it to the dirty list , from where it has to be flushed out. So the buffer , if it is marked as dirty, it will be sent to the datafile for sure, without waiting for it to be committed or not. That's why , because of this operation only , we need to rollback the changes with the roll backward process , to make sure that the only committed buffers stay in the datafile not the uncommitted ones.
The only buffers which are not flushed to the dataifle are the buffers which are used for the read consistacny mechanism by oracle, snapshot buffers. The snapshot buffers are created in the same hash bucket of the dirty buffers but they are not flushed out to the datafile. They are simply thrown away.
ok... the dirty buffers are witten to datafiles irrespective of the type of transaction(committed or un committed)..
if this is true, if you roll back the transaction, from where oracle get the previous data(the previous data must be stored some where in order to replace), in this case the previous data will store in UNDO tablespace, this will also provide read consistency. if this is not true how the read consistency provided by snapshot buffers.