RE: data buffer access confusion
- From: Hemant K Chitale <hkchital@xxxxxxxxxxxxxx>
- To: Mark.Bobak@xxxxxxxxxxxx, "amansharma1981@xxxxxxxxx" <amansharma1981@xxxxxxxxx>, "Dunbar, Norman (Capgemini)" <norman.dunbar.capgemini@xxxxxxxxxxxxxxxxxxxxxxxxx>
- Date: Mon, 23 Aug 2010 23:01:59 +0800
With 11g, not just PARALLEL but also SERIAL Direct Path Read operations.
See
<http://oracledoug.com/serendipity/index.php?/archives/1321-11g-and-direct-path-reads.html>http://oracledoug.com/serendipity/index.php?/archives/1321-11g-and-direct-path-reads.html
http://afatkulin.blogspot.com/2009/01/11g-adaptive-direct-path-reads-what-is.html
and
<http://dioncho.wordpress.com/2009/07/21/disabling-direct-path-read-for-the-serial-full-table-scan-11g/>http://dioncho.wordpress.com/2009/07/21/disabling-direct-path-read-for-the-serial-full-table-scan-11g/
I do agree with Mark that "I think it's safe to say that Oracle will
*never* read data from disk into PGA, and then move from PGA to
SGA" (I think !)
The Buffer Cache is for sharing buffers. Blocks read into the PGA
are not shared with other sessions.
Hemant K Chitale
At 03:29 PM Monday, Bobak, Mark wrote:
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,
-
Hemant K Chitale
http://hemantoracledba.blogspot.com
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