Re: Should I Defragment on RAID?

Hi David,

Thanks for your reply. Unfortunately I'm a bit confused now! What's bothering me is:

1. How can indexing speed up full table scans? I thought that Oracle would only use the index to access data if accessing a small % of rows are to be fetched.
2. In the Storage Management document it says that the performance is not sensitive to the number of extents. If I have 3 tables A, B, C which all have 3 extents organised as ABCABCABC. Surely there will be more (expensive) disk head movement to read A than if it were a sequential read of a single extent ie. AAABBBCCC. Are the blocks not organised on disk as it appears logically in the tablespace - in this case three "groups" of blocks to be read?

Sorry if I'm misunderstanding!

Thanks,

• Bill.

David Sisk <davesisk_at_ipass.net> wrote in message news:UQqs3.801$tp2.630_at_news.ipass.net...
> Hi Bill:
>
> Contrary to some beliefs, the number of extents in a segment is not
> necessarily equivalent to the level of fragmentation of the segment. See
> the article on the site below under Performance -> Storage.
>
> If your application is doing a large percentage of full table scans, you
> should be able to get quite a considerable performance boost by doing some
> indexing. (We're talking 5, 10, 20, sometimes even 100-200 times faster
for
> individual queries.) AND access by index if immune to table
fragmentation
> (if you define "fragmentation" as scattered empty space within a segment,
> which, IMHO, is the correct definition).
>
> Good luck,
>
> --
> David C. Sisk
> Need tech info on Oracle? Visit The Unofficial ORACLE on NT site at
> http://www.ipass.net/~davesisk/oont.htm
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>
>
> Bill Buchan wrote in message <7osb59$ss0$1_at_phys-ma.sol.co.uk>...
> >Hi,
> >
> >I have an Oracle 8.0.4 database on WinNT. The datafiles are all on
RAID-5.
> >Some of my segments have got seriously fragmented with dozens of extents
> all
> >over their tablespace. Is it still worth defragmenting these given that
> the
> >hardware will stripe this data all over the place anyway?
> >
> >I have a lot of queries that do full scans of the fragmented segments.
> >
> >Any advice would be much appreciated.
> >
> >Thanks,
> >
> >- Bill.
> >
> >
> >
>
>
Received on Fri Aug 13 1999 - 03:38:03 CDT