Re: db file sequential/scattered read

Regarding your words toward the end, I'm curious why Oracle reads db files into scattered memory locations instead of contiguous blocks. Does it always do so? Is it due to operating system memory fragmentation?

I always thought the apparent "flip" of scattered read vs sequential read wait events was due to an Oracle documentation error that has been around for a long time.

Yong Huang
yong321_at_yahoo.com

In article <979151756.2709.0.nnrp-09.9e984b29_at_news.demon.co.uk>,   "Jonathan Lewis" <jonathan_at_jlcomp.demon.co.uk> wrote:
>
> There are two different contexts here -
> Steve is talking about the disk being read
> sequentially - which usually occur because
> Oracle is doing a table scan or index fast full scan
>
> Anjo is talking about a wait event called
> 'db file scattered read' - which usually means
> that Oracle is doing a tablescan or index fast
> full scan (or as Kevin Brand mentioned, 8.1.6
> read-ahead). It is unfortunate that a disk sequential
> read happens to be displayed as an Oracle scattered
> read. The reason is that the read is into scattered
> memory locations, rather than a single sequential
> memory block.
>
> --
> Jonathan Lewis
> Yet another Oracle-related web site: http://www.jlcomp.demon.co.uk
>
> Practical Oracle 8i: Building Efficient Databases
>
> Publishers: Addison-Wesley
> See a first review at:
> http://www.ixora.com.au/resources/index.htm#practical_8i
> More reviews at: http://www.jlcomp.demon.co.uk/book_rev.html
>
> Ivan Bajon wrote in message <93h9gi$eof$1_at_news.inet.tele.dk>...
> >Could someone please clarify whether full table scans cause waits
 for db
> >file sequential read or scattered reads. I've seen claims on both.
> >
> >Steve Adams says:
> >"Sequential reads occur mainly in connection with full table scans
 and fast
> >full index scans." (Taken from Tips->Disk Configuration for
 Sequential
 Scans
> >on Ixora)
> >
> >In "Description of Oracle7 Wait Events and Enqueues" (2.36 db file
 scattered
> >read) Anjo Kolk says:
> >"This event is normally an indicator that a Full Table Scan is
 happening on
> >the object identified before."
> >
> >This leaves me confused. Which is it? Both?
> >
> >Regards,
> >Ivan Bajon, ocp

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http://www.deja.com/ Received on Wed Jan 10 2001 - 17:58:46 CST