Re: db file sequential/scattered read

my mnemonic for this is

    db file sequential read - is a read from a sequence #, the rowid usually gotten from an index. We read the block by its address. Just one block.

    db file scattered read - a real misnomer, apparently from a scattered multi block read across memory, but in my nmemonic I think of the fact that a full table scan could be accessing segments scattered at various disk locations. To be scattered it is forcibly a mulit block read - makes remembering that its probably a full table scan alot easier.

In article <93isvi$q3t$1_at_nnrp1.deja.com>,   yong321_at_yahoo.com wrote:
> Regarding your words toward the end, I'm curious why Oracle reads db
> files into scattered memory locations instead of contiguous blocks.
> Does it always do so? Is it due to operating system memory
> fragmentation?
>
> I always thought the apparent "flip" of scattered read vs sequential
> read wait events was due to an Oracle documentation error that has
 been
> around for a long time.
>
> Yong Huang
> yong321_at_yahoo.com
>
> In article <979151756.2709.0.nnrp-09.9e984b29_at_news.demon.co.uk>,
> "Jonathan Lewis" <jonathan_at_jlcomp.demon.co.uk> wrote:
> >
> > There are two different contexts here -
> > Steve is talking about the disk being read
> > sequentially - which usually occur because
> > Oracle is doing a table scan or index fast full scan
> >
> > Anjo is talking about a wait event called
> > 'db file scattered read' - which usually means
> > that Oracle is doing a tablescan or index fast
> > full scan (or as Kevin Brand mentioned, 8.1.6
> > read-ahead). It is unfortunate that a disk sequential
> > read happens to be displayed as an Oracle scattered
> > read. The reason is that the read is into scattered
> > memory locations, rather than a single sequential
> > memory block.
> >
> > --
> > Jonathan Lewis
> > Yet another Oracle-related web site: http://www.jlcomp.demon.co.uk
> >
> > Practical Oracle 8i: Building Efficient Databases
> >
> > Publishers: Addison-Wesley
> > See a first review at:
> > http://www.ixora.com.au/resources/index.htm#practical_8i
> > More reviews at: http://www.jlcomp.demon.co.uk/book_rev.html
> >
> > Ivan Bajon wrote in message <93h9gi$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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>

Sent via Deja.com
http://www.deja.com/ Received on Wed Jan 10 2001 - 20:02:00 CST