RE: Transportable Tablespace, SCNs and ORA-1555s.

From: "Steve Adams" <steve.adams@xxxxxxxxxxxx>
To: "'Daniel Fink'" <Daniel.Fink@xxxxxxx>, <oracle-l@xxxxxxxxxxxxx>
Date: Thu, 18 Mar 2004 16:39:27 +1100

Hi Dan,

The problem you are contemplating cannot happen, because when you plug in a
set of tablespaces, the SCN is advanced if necessary to ensure that it is
higher than that of the source database. So the SCN of your database2 would
be bumped up to 100001.

Oracle also records the SCN at the time of the plug in operation in SPARE1
and SPARE2 of TS$. Queries with an older snapshot SCN will not see the new
segments. Queries with a later snapshot SCN do not attempt to determine the
status of uncommitted transactions unless the block SCN is higher than the
plug-in SCN, which it will never be unless the tablespace has been
read/write in the new database and the block has been changed, in which case
the block will have been completely cleaned out.

There is a similar shortcut for consistent reads against read only
tablespaces. The read-only SCN is kept in SCNWRP and SCNBAS of TS$. If the
snapshot SCN for a consistent read is more recent than that, nothing needs
to be done because all transactions are known to have been committed at that
point. Queries with an older snapshot SCN do consistent reads in the normal
way. 

@   Regards,
@   Steve Adams
@   http://www.ixora.com.au/         - For DBAs
@   http://www.christianity.net.au/  - For all 

-----Original Message-----
From: Daniel Fink
Sent: Thursday, 18 March 2004 9:38 AM
To: oracle-l@xxxxxxxxxxxxx
Subject: Transportable Tablespace, SCNs and ORA-1555s.


This is not in response to an actual problem, it is just the
result of a brain that goes off on tangents way to often...

How does a transaction know NOT to build a consistent version of
data that has been transported?

Sequence of events
1) Insert records into table1. A Commit SCN of 100000 is
assigned. Due to the size of the insert, there are blocks in the
table that still have an Uncommitted state.
2) Alter tablespace1 (containing table1) to read only and
transport it to database2.
3) Start a query in database2 (with an SCN of 1000) that
accesses the newly transported table1.

Will the query get a 1555?

Yes - It cannot create a read consistent view to SCN 1000
No - It knows that this data came from another database, so it
does not attempt to build a read-consistent version

Thoughts on which is correct? I know, I know...build a damn
testcase and see for myself. I'm working on it, but I wanted to
see what you all thought first.

Daniel

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References:
- Transportable Tablespace, SCNs and ORA-1555s.
  - From: Daniel Fink

RE: Transportable Tablespace, SCNs and ORA-1555s.

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