Re: Sequences in OPS/RAC

Sequences are mastered by the single SYS.SEQ$ table in each database. Cached or uncached, RAC or non-RAC, OPS or non-OPS, sequence numbers generated by this mechanism are unique across a database, not by instance. Each instance updates SEQ$ as individual numbers (noncached) or ranges of numbers (cached) are "reserved", and those updates are controlled by the same synchronization mechanisms used by OPS/RAC for all UPDATE statements. This is precisely the reason that setting CACHE on sequence numbers help performance, as the number of updates to SEQ$ are reduced, minimizing the bottleneck.

However, because of the simplicity of this caching mechanism, sequence numbers are not guaranteed to be in order (i.e. sequentially ascending) across multiple instances.

Perhaps Mr. Stephens mis-spoke, confusing uniqueness for ordering? Or perhaps he is confusing "distributed databases" for "clustered databases"?

>
>
> 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
>
>
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