Oracle-L: RE: Index move

From: DENNIS WILLIAMS <DWILLIAMS_at_LIFETOUCH.COM>
Date: Wed, 21 Aug 2002 12:52:04 -0800
Message-ID: <F001.004BBCA4.20020821125204@fatcity.com>

Seema - My interpretation of your question is that you are asking whether having the indexes for a table on a separate physical would increase performance.

Since this was a standard recommendation for many years, I feel it merits a more detailed reply.

If you have an OLTP benchmarking situation with a single table that is being read randomly using the index, then I would expect that moving the index to a separate physical drive would increase performance significantly. I have not tested this directly that I can recall, but I believe this was based on benchmarks. The reason related to the idea that this arrangement would minimize disk head movement. If the index and table are on the same disk, likely they will reside on different positions on the disk. The concern is that random accesses would be slowed by the time it took to reposition the disk drive head from one location to another. This is a mechanical device, so movement inevitably takes time. So, why isn't this practice emphasized today? Here are the reasons I can think of:

Benchmarking is not production. A production OLTP system has many simultaneous users accessing many tables. This tends to obscure the benefits of table/index placement.
Full table scans don't use the index (by definition), so don't benefit from this placement strategy.
Modern systems usually use RAID drives which are more complex, obscuring the benefits of this strategy.
Caching inside Oracle (block buffers) and in the I/O system itself also obscure the benefits of this simplistic strategy.

Personally I perform this placement whenever it is convenient but I don't make a fetish of it. One practical reason for doing this is when you rebuild an index, the performance benefits are noticeable. But how often do you rebuild indexes?

Hi
Is any performance gain/impact if I move unique indexes from one tablespace to another tablespace if both tablespaces are on diffrent physical disk? Thanks
-Seema
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Author: DENNIS WILLIAMS
INET: DWILLIAMS_at_LIFETOUCH.COM

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