Re: Sequences in OPS/RAC

In the "Oracle9i Real Application Clusters Administration" manual, there is a chapter about sequence numbers generator. Before believing to the snake oil sellers, read the fine manual. Sequence numbers are guaranteed to be unique PER DATABASE. What they're not guaranteed is to come in ordered fashion. When sequence number are cached (that is the default), they're cached separately, for each instance. Each instance returns the contents of its cache, so it is possible for the smaller number being returned after a larger one. There is "ORDERED" flag to deal with that, but that can be extremely expensive and impose significant overhead on your cluster.

On 11/03/2003 10:39:26 AM, Hemant K Chitale wrote:
>
>
> I have always been comfortable with the idea that Sequences continue to
> guarantee
> uniqueness even in OPS / RAC environments.
>
> However, a recent Builder.Com article by Scott Stephens on the SYS_GUID
> function has these lines :
> "Sequence generator numbers are guaranteed to be unique only for a single
> instance, which is unsuitable for use as a primary key in parallel or
> remote environments, where a sequence in each environment might generate
> the same number and result in conflicts. An identifier created by SYS_GUID
> is guaranteed to be unique for each database."
>
> Huh ?! Do the lines mean that a single sequence can have duplicate values
> in the two instances of an RAC cluster ?
>
>
> Hemant K Chitale
> Oracle 9i Database Administrator Certified Professional
> My personal web site is : http://hkchital.tripod.com
>
>
> --
> Please see the official ORACLE-L FAQ: http://www.orafaq.net
> --
> Author: Hemant K Chitale
> INET: hkchital_at_singnet.com.sg
>
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Mladen Gogala
Oracle DBA

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Author: Mladen Gogala
  INET: mladen_at_wangtrading.com

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