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Re: oracle - mysql comparison

From: VC <>
Date: Thu, 15 Jul 2004 22:01:59 GMT
Message-ID: <l9DJc.98700$XM6.25133@attbi_s53>

"Alex Filonov" <> wrote in message

> According to some information M$ is going to implement read-only
> consistency (Oracle style) in the next version.

Yes, MS SQL Server is going to have multiversion concurrency control (the latest Yukon beta actually already has it). It'll be exposed to the user as SNAPSHOT isolation level.

>As for correct concurrency
> model,

A correct concurrency control/model ensures that all the permitted transactions are serializable. In this sense, databases like DB2 or MSSQL implement a correct concurrency model albeit at the expense of lower concurrency in some circumstances. Funnily enough, none of the Oracle isolation levels can make the same promise, i.e. ensure serializable transaction histories, in any of its isolation levels. Usually, it's quite easy to obtain correct results by augmenting an isolation level with something like 'select for update', though..

>I remember one project, when data warehouse was build using MSSQL,
> everything was almost OK, management was pleased immensely. Until they
> started running reports. When management found out that reports
> stopped online work, mood changed.

Well, there are several possible responses to this anecdote:

  1. If the described system was a true data warehouse/DSS, then there should not have been a problem at all since there should have been no OLTP activity by definition.
  2. On the other hand, if the implementation was a mixed OLTP/DSS project, then judging by your story, the database architect was not qualified to design a system like that. His/her being surprised that a locking scheduler's reads block writes, and vice versa, is akin to a swimmer's being amazed that water is wet. There are several well-known approaches to the reporting problem such as replicating the main database to a reporting database, transaction log shipping, scheduling reports off-hours, etc.

VC Received on Thu Jul 15 2004 - 17:01:59 CDT

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