a question about oracle undo segment binding

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开发者_运维技巧 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.

1. 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.
2. 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.
3. 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.
4. 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.

"why one undo segment can only serve for one active transaction?"

It is simply a design decision. That is how undo segments are designed to work. I guess that it was done to address some of the issues that could occur with the previous rollback mechanism.

Rollback (which is still available but deprecated in favor of undo) included explicit creation of rollback segments by the DBA, and multiple transactions could be assigned to a single rollback segment. This had some drawbacks, most obviously that if one transaction assigned to a given segment generated enough rollback data that the segment was full (and could no longer extend), then other transactions using the same segment would be unable to perform any operation that would generate rollback data.

I'm surmising that one design goal of the new undo feature was to prevent this sort of inter-transaction dependency. Therefore, they designed the mechanism so that the DBA sizes and creates the undo tablespace, but the management of segments within it is done internally by Oracle. This allows the use of dedicated segments by each transaction. They can still cause problems for each other if the tablespace fills up (and cannot autoextend), but at the segment level there is no possibility of one transaction causing problems for another.