隔离级别“ READ COMMITED”在Oracle DB中如何工作？

Question

i am interesting to know know how the isolation level"READ COMMITTED" is provided in Oracle DB implementation. I already know that DB makes records in REDO log, but for now i think that that REDO log is only used to repeat the transaction in case when some unpredictable crash will happen during the transaction. Also i know that DBWR writes the "dirty blocks" every time the REDO log file is filled. But my question is: if DBWR writes "dirty"(changed blocks) to the disk, how isolation level"READ COMMITTED" is provided. I mean during writing DBWR writes data directly to data files or in some special "place" on disk that is visible from current transaction and invisible from other transaction? So after the COMMIT this "place" becomes visible and that's all ? How this works in reality? Sorry for bad English.

Answer 1

In addition to the REDO log, you also have the UNDO tablespace.

When updating data, the old value is stored in the UNDO tablespace. When Oracle sees that you would be reading uncommitted data for a record, it reconstructs the old value from there.

UNDO is also used during database recovery: In addition to re-applying writes that have been committed but not made it to the database files before the crash, the opposite can also take place: rolling back uncommitted changes to database files that happened before the crash.