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Data accessed from Undo Segment OR Buffer Cache [message #260499]

Mon, 20 August 2007 02:57

naveenkumar001
Messages: 62
Registered: July 2007
Location: Bangalore

Member

Hi,

I have some doubts related to Undo Segment please help me.
I have created a table called Toy and If a user named Raj has increased the toy price from 30$ to 50$ but not commited then the old image wil be stored in undo segment.

Question:-
1) Whether all the users except Raj will be accessing toy price from undo segment directly or that undo segment data is brought to Bufer Cache from Server Process so that all users can access that data from Buffer Cache?

2) Is there any way to know whether data is being accessed from Undo Segment or Buffer Cache?

Regards,
Naveen

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Re: Data accessed from Undo Segment OR Buffer Cache [message #260512 is a reply to message #260499]

Mon, 20 August 2007 03:28

Michel Cadot
Messages: 68718
Registered: March 2007
Location: Saint-Maur, France, https...

Senior Member
Account Moderator

Undo blocks are data blocks for Oracle.
There is no difference between accessing your data block or undo block.
So blocks go to buffer cache before reading them into user process.

Regards
Michel

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Re: Data accessed from Undo Segment OR Buffer Cache [message #260553 is a reply to message #260512]

Mon, 20 August 2007 05:14

naveenkumar001
Messages: 62
Registered: July 2007
Location: Bangalore

Member

Hi Michel,

Thank you for your valuable answer it has helped me a lot.

When i was studying Tuning Buffer Cache i came across V$BUFFER_POOL_STATISTICS that view contained two fields,
1)DB_BLOCK_GETS :- Number of blocks that were present in the memory that were read from normal segment.
2)CONSISTENT_GETS :- Number of blocks that were present in the memory that were read from a rollback segment.

Now my doubts got cleared, that undo segments blocks come to Buffer Cache, that is useful for calculating hit ratio of buffer cache.

Regards,
Naveen

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Re: Data accessed from Undo Segment OR Buffer Cache [message #260579 is a reply to message #260553]

Mon, 20 August 2007 06:24

Michel Cadot
Messages: 68718
Registered: March 2007
Location: Saint-Maur, France, https...

Senior Member
Account Moderator

Quote:

calculating hit ratio of buffer cache

Take care: buffer cache hit ratio is good for only one thing:
detecting there is a change in your workload.
Nothing more.

Regards
Michel

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