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Re: Re: oraperf comment

From: Stephane Faroult <>
Date: Tue, 22 Oct 2002 14:14:24 -0800
Message-ID: <>

> Yechiel,
> You had mentioned only one possible scenario (i.e. "user A accesses table while user B simultaneously
> accesses index") where there are several other possible, equally-likely scenarios (i.e. "user A accesses
> table while user B simultaneously accesses table", "user A accesses index while user B simultaneously
> accesses index", etc). Separating tables and indexes to separate devices does nothing for those other,
> equally-likely scenarios, does it? That's the reason for the question "why?" in the beginning of my last
> reply...
> At issue here is not the concept of parallelism in I/O. At issue (at least for me) is the "conventional
> wisdom" that states/implies that there is some performance benefit of separating tables and indexes to
> separate devices. My assertion is that this is irrelevant for two reasons: a) within a single process the
> accessing of table blocks and index blocks are purely sequential and b) tables and indexes have different
> I/O characteristics which make it less likely that they will conflict with each other. In fact, in most
> situations datafiles/tablespaces containing indexes generate far fewer physical I/Os than
> datafiles/tablespaces containing tables. From an I/O perspective, the key is not to focus on whether the
> datafile/tablespace contains tables or indexes but rather to focus on the volume and type of physical I/O
> they generate.
> By focusing on the I/O statistics rather than whether they are tables or indexes, one can make better
> determinations on how to distribute I/O across non-RAID devices.
> Hope this helps...
> -Tim


  I fully subscribe to your conclusion but I wouldn't be that harsh about conventional wisdom, which once had some ring of truth to it and still has it on rustic configurations. Granted, for a given user parallelizing his or her table and index accesses doesn't make much sense. But when you have a lot of happy users merrily issuing their queries, you can hope that at some point in time some will be hitting indexes while others will be hitting tables - and when dbwr and its gang will join the party, both indexes and tables will be hit too. This is probably what Yechiel meant. I see conventional wisdom as a rough-and-ready rule-of-thumb to make people spread their I/Os. And at least the benefit of having separate tablespaces is that you have separate files which are easier to move around when you have a finer appreciation of what is going on.


Stephane Faroult
Oriole Software
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Author: Stephane Faroult

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Received on Tue Oct 22 2002 - 17:14:24 CDT

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