why does undo segment wrap around?
822756Dec 9 2010 — edited Dec 9 2010hi,buddy,now i am reading tom's book, Expert Oracle Database,2005.
when i read the chapter 9( redo and undo), there is a problem puzzled me.
we all know undo segment will wrap around in case of it filled up,
so if the transaction is too long we probably meet the ORA-01555 error.
my questions as follows:
why doesn't oracle database refresh and write undo segment to disk(undo tablespace) to avoid ORA-01555?
what data the undo tablespace store(disk data file)?
thanks!
Edited by: user13449793 on 2010-12-9 下午6:42