Hi Tony, As the referenced documentation states, the PostgreSQL SERIALIZABLE transaction isolation level complies with the ANSI/ISO requirements, but not with a mathematically pure interpretation of the term. (The only quibble I have with that documentation is that you have to be averting your eyes to not find several commercial products which do enforce the stricter interpretation.)
As far as I can see, the difference is only significant if you need to have two concurrent transactions where one transaction is selecting from a set of data A to modify something within a set of data B at the same time that another transaction is selecting from B to modify something within A -- without any overlap between the rows updated by the transactions. In practice, this seems unlikely to be meaningful outside of some theoretical science; you don't normally want recursive redundancies in your database. So to address the original concern -- PostgreSQL absolutely gives you a stable view of the data during a SERIALIZABLE transaction. The only thing it doesn't give you is a guarantee that some other transaction hasn't made modifications which would change what the same SELECTs would show if you were to start a NEW transaction. -Kevin >>> Tony Caduto <[EMAIL PROTECTED]> >>> Tom Lane wrote: >> http://candle.pha.pa.us/main/writings/pgsql/sgml/sql-set-transaction.html >> http://candle.pha.pa.us/main/writings/pgsql/sgml/transaction-iso.html >> > > It's a bit amusing that this person is dissing us for not having > REPEATABLE READ, when what he actually seems to want is SERIALIZABLE > (which we've had since 1999). Certainly REPEATABLE READ does *not* > guarantee a "stable view of data during one transaction" --- see the > discussion of phantom reads in the second link given above. > > regards, tom lane > > Tom, This is what the firebird guy said: > Serializable is stricter and somehwat unusable in a multi-user, loaded > database, because only one transaction can run at any time. Let's say you > would have one long running serializable transaction encapsulating a > reporting query, this will cause other transactions to wait. > > There is a pretty good paper on discussing why it was a somewhat bad idea to > describe transaction isolation levels in terms of phenomena in the SQL > standard. This paper also describes transaction isolation levels for MVCC > databases. The paper is from 1995. http://www.cs.duke.edu/~junyang/courses/cps216-2003-spring/papers/berenson-etal-1995.pdf > > SNAPSHOT in Firebird isn't a SQL standard compliant REPEATBLE READ either. > SNAPSHOT in Firebird is between REPEATABLE READ and SERIALIZABLE, but > without blocking other transactions. Is this true? will SERIALIZABLE block all transactions on the whole server, or just on that one connection? Thanks, Tony ---------------------------(end of broadcast)--------------------------- TIP 4: Have you searched our list archives? http://archives.postgresql.org
