Re: Phantom Inserts

In article <pwongD4C773.F15@netcom.com>, Philip S. Wong wrote: > I was wondering how different databases/vendors implement protection > against phantom inserts. In particular, how can they achieve phantom > protection while maintaining a good degree of concurrency? The ORACLE7 Concepts Manual (10-6) mentions something called a SCN (system change number) which is used to ensure that transaction A never sees data committed by transaction B after transaction A started. ORACLE calls this "statement-level read consistency". When providing read-consistency, the manual says, "blocks with changed data (more recent SCNs) are reconstructured using data in the rollback segments...". I don't doubt that it is necessary to provide some kind of read-consistency, but doesn't this approach cause an enormous performance degradation? Especially for long-running transactions the same data could have been changed a number of times. zeppelin -- ____________ ___ ____ _ _ _ _ |_ | _/| _ | _ | _/| | | | \\| | zeppelin@login.dknet.dk / /| _>| _| _| _>| |_| | | /___|___\\|_| |_| |___\\|___|_|_|\\_|