Why does RAC have two different SCN? [message #677109] |
Mon, 19 August 2019 22:59 ![Go to next message Go to next message](/forum/theme/orafaq/images/down.png) |
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kalenko
Messages: 40 Registered: January 2018
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Hi,
As my understanding, in Rac environment, there are two or more instances and there is only one database. But why does Rac environment have multiple SCN, each instance has one SCN. Why does one database have multiple SCN. Please explain to me why?
Thanks.
[Updated on: Mon, 19 August 2019 23:28] Report message to a moderator
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Re: Why does RAC have two different SCN? [message #677112 is a reply to message #677111] |
Tue, 20 August 2019 01:38 ![Go to previous message Go to previous message](/forum/theme/orafaq/images/up.png) ![Go to next message Go to next message](/forum/theme/orafaq/images/down.png) |
John Watson
Messages: 8944 Registered: January 2010 Location: Global Village
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No. Each instance is reporting the current scn, as of the time of the query as seen by each instance. GV$ views are populated by a union all of queries run in each instance. The content is not protected by undo, hence no read consistency.
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Re: Why does RAC have two different SCN? [message #677116 is a reply to message #677115] |
Tue, 20 August 2019 01:53 ![Go to previous message Go to previous message](/forum/theme/orafaq/images/up.png) ![Go to next message Go to next message](/forum/theme/orafaq/images/down.png) |
John Watson
Messages: 8944 Registered: January 2010 Location: Global Village
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I'll try to explain again, in even simpler terms.
Log on to one instance, and query v$database. Then log on to the other instance, and query v$database. The results will not be the same. That is all a GV$ view is: a UNION ALL of queries run in each instance. There is no reason why the results would be the same.
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Re: Why does RAC have two different SCN? [message #677117 is a reply to message #677116] |
Tue, 20 August 2019 02:11 ![Go to previous message Go to previous message](/forum/theme/orafaq/images/up.png) |
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kalenko
Messages: 40 Registered: January 2018
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John Watson wrote on Tue, 20 August 2019 01:53I'll try to explain again, in even simpler terms.
Log on to one instance, and query v$database. Then log on to the other instance, and query v$database. The results will not be the same. That is all a GV$ view is: a UNION ALL of queries run in each instance. There is no reason why the results would be the same.
OK, I've got the answer for my question. I will query the v$database for the SCN and continue to work in the current session or work in the session which connects to the same instance. Is this correct?
[Updated on: Tue, 20 August 2019 02:13] Report message to a moderator
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