Re: data buffer access confusion
- From: Aman Sharma <amansharma1981@xxxxxxxxx>
- To: Hemant K Chitale <hkchital@xxxxxxxxxxxxxx>
- Date: Mon, 23 Aug 2010 21:58:14 +0530
Thanks Mark and Hemant so much for the reply. Yes, I am aware about the
direct path read operations going into the PGA. As I mentioned, this topic
did arise because of a statement that I have read in a book which I have
just purchased. Before contacting with the author and mentioning that he has
put it wrong, I wanted to be sure that I am not missing anything. I guess, I
can now safely write to him pointing to this statement of his and hopefully
would get a positive reply that I shall share with the list.
Thanks once again for an excellent reply as like always!
Regards
Aman
On Mon, Aug 23, 2010 at 8:31 PM, Hemant K Chitale
<hkchital@xxxxxxxxxxxxxx>wrote:
>
> 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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