Re: db file sequential/scattered read

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

>

>