RE: data buffer access confusion
- From: "Bobak, Mark" <Mark.Bobak@xxxxxxxxxxxx>
- To: "amansharma1981@xxxxxxxxx" <amansharma1981@xxxxxxxxx>, "Dunbar, Norman (Capgemini)" <norman.dunbar.capgemini@xxxxxxxxxxxxxxxxxxxxxxxxx>
- Date: Mon, 23 Aug 2010 03:29:25 -0400
Aman,
Actually, it depends. Physical reads can be into cache, or directly into PGA.
Examples of reads directly to PGA and bypassing buffer cache would be read of
temp data from TEMP tablespace or parallel direct reads when parallel query is
involved.
However, I think it's safe to say that Oracle will *never* read data from disk
into PGA, and then move from PGA to SGA. What would be the point? So, reads
can go from disk to PGA or to SGA, but should never "hop" from disk to PGA to
SGA. That just doesn't make sense, in any scenario I can imagine.
Hope that helps,
-Mark
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx] On
Behalf Of Aman Sharma
Sent: Monday, August 23, 2010 3:16 AM
To: Dunbar, Norman (Capgemini)
Cc: oracle-l@xxxxxxxxxxxxx
Subject: Re: data buffer access confusion
Very good morning Norman,
Thanks for the reply first of all! I asked this question because this came up
at two different places, one in a conversation with an oracle professional and
the 2nd, in a book which I have just purchased. Before replying to the author,
I wanted to be sure that I am not missing anything but despite checking at
various places, I didn't find any explanation for this term that data buffers
can be stored in the PGA. Yes, there is a change from 10g(10.2 I guess) that
the data buffers can be stored in the shared pool but that's all what I at
least know. I never heard/read that PGA can be used for the same.
Thank you so much for the reply and clarification once again Norman!
Regards
Aman....
On Mon, Aug 23, 2010 at 12:30 PM, Dunbar, Norman (Capgemini)
<norman.dunbar.capgemini@xxxxxxxxxxxxxxxxxxxxxxxxx<mailto:norman.dunbar.capgemini@xxxxxxxxxxxxxxxxxxxxxxxxx>>
wrote:
Morning Aman,
>> Is it a correct thing to say that when the data buffer is
>> being read from the disk, its first kept in the PGA and then
>> later on, it would be copied from the PGA memory to the
>> standard buffer cache?
Not quite.
>> If no, then its alright but if yes, why such behaviour is there?
It isn't - unless of course Oracle changed things recently!
<SNIP>
>> If anyone can explain and
>> clear this confusion, it would be just great!
The disc buffer(s) are read from disc - assuming that they are not
already in the buffer cache - and placed into the buffer cache directly.
Your server process then returns the block or blocks you requested to
your user process.
Now, I have to say that the last time I dealt with this was way back at
Oracle 8.0 (!!!) so I'm now looking forward to being (a) corrected and
(b) educated by those who know better than me!
Cheers,
Norman.
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