On Saturday, May 23, 2015, Ravi Krishna <
sravikrishna3@gmail.com> wrote:
undo means that reading the WAL logs and able to rollback a row back
to its original state before the update. Typically it is used to
rollback a long running transaction which got aborted due to a crash.
Here is an example:
Not an expert here but...
It doesn't matter why the newly written data was deemed invalid, MVCC ensures that absent a successful commit all of that data is considered invisible to anything other than the session writing the data. Eventually vacuum is needed to cleanup the dead data.
One possible implementation is that uncommitted data simply is never written to the data files...only the buffers see the changes until commit when the WAL sees it. This seems likely but I do not know for sure.