Re: Db2, Oracle, SQL Server

From: VC <boston103_at_hotmail.com>
Date: Wed, 9 Feb 2005 19:12:46 -0500
Message-ID: <K6CdnRszsaicOpffRVn-uA@comcast.com>

Hi:

"Serge Rielau" <srielau_at_ca.ibm.com> wrote in message news:36ucs9F575gaaU1_at_individual.net...

> VC wrote:

>> "Serge Rielau" <srielau_at_ca.ibm.com> wrote in message
>> news:36qshqF564is0U1_at_individual.net...
>> ...
>>
>>>Isn't it ironic that if SQL Server 2005 ever ships it will be the only
>>>mainstream DBMS supporting all major isolation levels?
>>
>>
>> ...
>>
>> What exactly do you mean by the above ? What are "major isolation
>> levels" ? IBMS's DB2, among others, supports all the *standard*
>> isolation levels while Oracle does not.
> If I'm not mistaken SQL Server falls under "among others". In addition SQL > Server 2005 allegedly supports multi versioning.

You are confusing isolation levels with concurrency control. There are, roughly speaking, two approaches to concurrency control: locking (DB2/SQL Server/Sybase) and multiversioning (Oracle/Firebird/Postgress). Each of the two can implement various isolation levels that improve concurrency at the expense of creating various anomalies. E.g. Sybase (as well as DB2 or SQL Server) implements SERIALIZABLE, REPEATABLE READ, READ COMMITTED, READ UNCOMMITTED with SERIALIZABLE ensuring correct behaviour but allowing the least concurrency. Oracle has SNAPSHOT (mis-named SERIALIZABLE) and READ COMMITTED. Oracle also has the READ ONLY isolation level , but it's really just a subset of SNAPSHOT.

There are hybrid schedulers (locking+MVCC), such as MySQL InnoDB and SQL Server 2005. The latter implements MVCC and, thanks to that, has Oracle style SNAPSHOT isolation level.

VC Received on Wed Feb 09 2005 - 18:12:46 CST