Re: Phantom Inserts

In article <finkel.212.0036CAE6@links.com>, Richard Finkelstein wrote: > Read-only transactions can use MVRC effectively. The whole transaction can be > set to read-only and then the transaction would only see a consistent version > of the result set at a point in time. Your point concerning the construction > of the MVRC result set is a good one. There is assuredly some overhead > associated with reading rollback segments (especially if they are the long > rollback segments) when try to find the correct version of a record. This > problem would be most acute when accessing a database that is being heavilty > updated. Another side-effect is that it is possible that Oracle cannot > construct the consistent result set - i.e., the correct version of the record > is no longer on the rollback segment because it has been overwritten. This is > most likely to occur when a MVRC transaction has been executing for a long > time and there is a high volume of update transaction. In this case the MVRC > will abort. > > Hope this helps a bit. This was pretty much what the Concepts Manual said. I especially don't like the idea of the possibility of getting an ORA-1555 message: Snapshot too old (rollback segment too small). I am planning to make my very simple database package transactional and perhaps put SQL on top of it. I used to think that designing and implementing the actual parsing, optimization and execution of SQL would be a difficult task, but now that all seem like nothing compared to the problems involved in providing ACID properties. I have read "Transaction Processing: Concepts and Techniques" by Andreas Reuter and Jim Gray, which I can highly recommend to anyone interested in the subject. But I am now looking for a similiar book about implementing SQL which hopefully addresses some of these problems. Thomas Pedersen -- ____________ ___ ____ _ _ _ _ |_ | _/| _ | _ | _/| | | | \\| | zeppelin@login.dknet.dk / /| _>| _| _| _>| |_| | | /___|___\\|_| |_| |___\\|___|_|_|\\_|