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From Mike Matrigali <mikem_...@sbcglobal.net>
Subject Re: conflict detection strategies
Date Fri, 24 Feb 2006 22:25:03 GMT
I was not aware of the alter table dependency check for offline
compress, this must be outside of store somehow  - maybe something
tied to all alter table statements.  It may make sense
to change online compress to hook up with whatever offline compress
is doing to make this happen.

Just testing the current system does not mean that future changes
won't break SUR.  Unless we agree to change the contract on unlocked
RowLocations, then it still isn't right for code to depend on
an unlocked RowLocation not ever pointing to the wrong row - because
of the issue with truncate.   Some possible issues with your test
in the online compress case:
1) online compress has 3 separate phases, all of which do different
    types of locking.  Some use internal transactions, which explain
    the conflict lock.  I would try the following test:
    o autocommit off
    o hold cursor as your example, with a next
    o commit transaction
    o execute in place compress now that hold cursor has released all 
it's locks.

Andreas Korneliussen wrote:
> Andreas Korneliussen wrote:
>> Daniel John Debrunner wrote:
>>> Andreas Korneliussen wrote:
>>>> Problem:
>>>> For holdable cursors, we will release the table intent lock when doing
>>>> commit on the transaction for the cursor.
>>>> The table intent lock, prevents the system from doing a compress of the
>>>> table, causing all RowLocations to be invalid. In addition, it prevents
>>>> reuse of RowLocation for deleted + purged rows.
>>> I think this last paragraph is an incorrect assuption. The table intent
>>> lock prevents other transactions from doing a compress, but not the
>>> transaction holding the lock.
> It seems to me that that online compress will not use the same transaction:
> ij> autocommit off;
> ij>  get cursor c1 as 'select * from t1 for update';
> ERROR 40XL1: A lock could not be obtained within the time requested
> ij> rollback;
> Offline compress is rejected if executed from the same connection:
> ij>   get cursor c1 as 'select * from t1 for update';
> ij> next c1;
> ID
> -----------
> 1
> ERROR 38000: The exception 'SQL Exception: Operation 'ALTER TABLE' 
> cannot be performed on object 'T1' because there is an open ResultSet 
> dependent on that object.' was thrown while evaluating an expression.
> ERROR X0X95: Operation 'ALTER TABLE' cannot be performed on object 'T1' 
> because there is an open ResultSet dependent on that object.
> ij>
> Are there other user-visible mechanisms to start online compress ?
> If not, I think we could conclude that there are no known issues with 
> the use of RowLocation in non-holdable SUR (given the discussions about 
> validity of RowLocation in separate threads)
> Andreas
>> That is a good point.
>> The main problem would be the system doing a compress, however we 
>> should take into account the fact that the user can run compress from 
>> the same transaction, and then maybe invalidate the resultset, or 
>> prevent the compress from running.
>>> I think there are other situations where the RowLocation will become
>>> invalid, such as the transaction deleteing the row.
>> Yes, however as far as I understood, the RowLocation would not be 
>> reused as long as at least some sort of table level intent lock is 
>> held, and the store will simply return false if one tries to do update 
>> / delete / fetch on a RowLocation which is deleted, or deleted+purged.
>> Andreas

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