My 1/2 cents on this...
Another way of thinking of scattered -vs- sequential
reads is that a scattered read is used when you know
up front that you want to read several blocks and know
which blocks they are. You can then use a vectored
read to do this or farm them all out in parallel using
asynchronous read. Sequential reads implies that you
need the result of each block before you can determine
the next block to read. Therefore you do single block
I/O.
Bill
- Jonathan Lewis <jonathan_at_jlcomp.demon.co.uk>
wrote:
>
> Well, Tim did say he was making a long story short.
>
> scattered reads = multiblock reads which are
> typically
> associated with tablescans, but can be index
> fast full
> scans.
>
> sequential reads = single block reads which are
> typically
> associated with index block reads, table blocks
> accessed
> as a consequence of index block reads, but can
> be
> one-off blocks in table scans and index fast
> full scans
> due to the existence of previously buffered
> blocks or
> tail-end blocks on extents.
>
> And then there are cached LOB reads and ....
>
> Jonathan Lewis
> http://www.jlcomp.demon.co.uk
>
> Author of:
> Practical Oracle 8i: Building Efficient Databases
>
> Next Seminar - Australia - July/August
> http://www.jlcomp.demon.co.uk/seminar.html
>
> Host to The Co-Operative Oracle Users' FAQ
> http://www.jlcomp.demon.co.uk/faq/ind_faq.html
>
>
>
> -----Original Message-----
> To: Multiple recipients of list ORACLE-L
> <ORACLE-L_at_fatcity.com>
> Date: 25 April 2002 00:21
>
>
> |> "db file scattered read" (associated with FULL
> |> table scans) and "db file sequential
> |> read" (associated with indexed scans)
> |
> |But see, Tuning 101, p. 35, where a "db file
> sequential read" is
> |investigated and the waits are found to be reads
> from a *table*.
> |
> |And other examples elsewhere, where "db file
> scattered read" is found
> to be
> |associated with reads from an index.
> |
>
>
> --
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> Author: Jonathan Lewis
> INET: jonathan_at_jlcomp.demon.co.uk
>
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Received on Thu Apr 25 2002 - 09:03:26 CDT
