RE: db file sequential read [WAS:wait event puzzler]

From: Gogala, Mladen <MGogala_at_oxhp.com>
Date: Thu, 12 Jun 2003 08:31:00 -0700
Message-ID: <F001.005B01BA.20030612065946@fatcity.com>

I must confess that I checked by using "strace" on Linux and, true enough, for index read, I've got "read" calls and for full table scan, I've got "readv"
(scatter/gather IO). Obviously, I'll have to re-learn some stuff from version 7.
I successfully forgot versions 4, 5.0, 5.1.22, 6.0.36, 7.0.16, 7.1.16 and 7.2.
I'll try forgetting 7.3.4 as well although it's the last really stable version
of oracle.

Mladen Gogala
Oracle DBA
Phone:(203) 459-6855
Email:[EMAIL PROTECTED]

-----Original Message-----
Sent: Thursday, June 12, 2003 4:24 AM
To: Multiple recipients of list ORACLE-L

Mladen:

Prior to 8i (or 8) you see the multiblock sequential reads while doing the reads from the sort segments or temporary segments. But in 8i sort IO is done as direct IO and we see some other waits.

Starting from 8i the db file sequential read is always a single block read which TYPICALLY happens during index scans..

Are you convinced?

Best Regards,
K Gopalakrishnan

• "Gogala, Mladen" <[EMAIL PROTECTED]> wrote:
  > A SINGLE block? I was convinced that it was <=
  > DB_FILE_MULTIBLOCK_READ_COUNT
  > blocks.
  > Are you sure?
  >
  > Mladen Gogala
  > Oracle DBA
  > Phone:(203) 459-6855
  > Email:[EMAIL PROTECTED]
  >
  >
  > -----Original Message-----
  > Sent: Wednesday, June 11, 2003 2:19 PM
  > To: Multiple recipients of list ORACLE-L
  >
  >
  > Dennis:
  >
  > THe event 'db file sequential read' is NOT always an INDEX Scan. It
  > is
  > just a single block read which TYPICALLY happens during an INDEX
  > scan.
  > The word 'TYPICALLY' is the key here.
  >
  >
  >
  > Best Regards,
  > K Gopalakrishnan
  >
  >
  >
  >
  > -----Original Message-----
  > Meng, Dennis
  > Sent: Wednesday, June 11, 2003 11:02 AM
  > To: Multiple recipients of list ORACLE-L
  >
  >
  > Good day all,
  > We have a bunch of queries in our data warehouse environment that are
  > taking
  > a long time to complete.
  > I did a select on v$session_wait on one of the queries when it is
  > running
  > and it shows the wait event to be 'db file sequential read', which
  > means it
  > is scanning index blocks, correct? But when I put the p1 p2 p3
  > values into
  > another query to see which segment it belongs to, it shows me a
  > table, not
  > an index. How can this be?
  >
  >
  -- Please see the official ORACLE-L FAQ: http://www.orafaq.net -- Author: K Gopalakrishnan INET: [EMAIL PROTECTED]

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-- 
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-- 
Author: Gogala, Mladen
  INET: [EMAIL PROTECTED]

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Received on Thu Jun 12 2003 - 10:31:00 CDT