I'm no DBA, I just want to learn about Oracle's Multi-Version Concurrency model.
When launching a DML operation, the first step in the MVCC protocol is to bind a undo segment. The question is why one undo segment can only serve for one active transaction?
thank you for your time~~
Multi-Version Concurrency is probably the most important concept to grasp when it comes to Oracle. It is good for programmers to understand it even if they don't want to become DBAs.
There are a few aspects but to this, but they all come down to efficiency: undo management is overhead, so minimizing the number of cycles devoted to it contributes to the overall performance of the database.
- A transaction can consist of many statements and generate a lot of undo: it might insert a single row, it might delete thirty thousands. It is better to assign one empty UNDO block at the start rather than continually scouting around for partially filled blocks with enough space.
- Following one from that, sharing undo blocks would require the kernel to track of usage at a much finer granularity, which is just added complexity.
- When the transaction completes the undo is released (unless, see next point). The fewer blocks the transaction has used the fewer latches have to be reset. Plus, if the blocks are shared we would have to free shards of a block, which is just more effort.
- The key thing about MVCC is read consistency. This means that all the records returned by a longer running query will appear in the state they had when the query started. So if I issue a SELECT on the EMP table which takes fifteen minutes to run and halfway through you commit an update of all the salaries I won't see your change, The database does this by retrieving the undo data from the blocks your transaction used. Again, this is a lot easier when all the undo data is collocated in a one or two blocks.