| query in concept of undo data [message #152807] |
Tue, 27 December 2005 01:42  |
kinjal
Messages: 124
Registered: June 2001
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Senior Member |
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Hi,
I have a conceptual question regarding undo tablespace.
suppose one user A is fetching some set of rows to update it.
First time the blocks are fetched from datafile to database buffer cache.
that user A is doing some updates for that particular set of rows but not commiting.
So before image of that rows are copied to undo segments.
Now User B fetches same set of rows. so obviously he will be viewing before image of that rows because userA has not yet commited the records.
So my question is from which location he will be accessing the rows? from datafile or from undo segments?
If he is fetching from undo then why not from datafile?
and if from undo then why not datafiles?
I am confused at this stage..
Can anyone please tell me exact concept?
Thanking you,
Kinjal
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| Re: query in concept of undo data [message #152874 is a reply to message #152807] |
Tue, 27 December 2005 06:53  |
sunil_v_mishra
Messages: 506
Registered: March 2005
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Senior Member |
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hi kinjal,
As per your question user B will fetch record from the undo segment because user A already using the row for update of same table which B is trying to access so that row is locked by user A so the old image of Row which is used by user A is in undosegment and B can use that Image for select purpose from there.
Regards
Sunilkumar
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