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From Mike Matrigali <mikem_...@sbcglobal.net>
Subject Re: RowLocation validation, for holdable SUR
Date Tue, 21 Feb 2006 17:48:11 GMT
The SUR should not know anything about the underlying implementation
of the access method getting the row, so having it "read a timestamp"
from page does not work. If the timestamp is not in the rowlocation,
we could add a get a timestamp for row at this rowlocation, but forcing
two trips to the store for every row is a overhead.  Rather than discuss
implementation it would be nice to understand the minimum necessary
services needed to be provided by the access method.  Do the same 
interfaces need to be provided by VTI's?  At least
for your use I think the timestamp need only guarantee to be different
after a truncate from previous version on page.

Since you are ok with invalidating the SUR in the case of offline 
compress, what about invalidating the SUR in the case of online
compress also?  One way to do this is for the system catalogs to
maintain a table version number, which would be guaranteed to not
change while any sort of table intent lock was present.  Any operation
which either copied rows to another container or truncated the
table would bump the version number.  And holdable cursors would need
to recheck the version number after losing the lock at commit time.

The downside is that some SUR's are invalidated that didn't need to be,
but compress kicking in, in a holdable cursor in the time between a 
commit and then next operation in the cursor is going to be a
rare event.  The upside is that there is no extra per row overhead in
the system for the normal case.

There already exists a ddl invalidation scheme for invalidating query
plans, maybe this existing structure could be used to invalidate
SUR's after the commit?

Andreas Korneliussen wrote:
> I will modify the suggestion somewhat. I think first, that offline 
> compress is not a problem, even for the holdable SUR. Since offline 
> compress moves the records to another container, the SUR cursors should 
>  detect that container they use is no longer valid, when renavigating to 
> the row.
> If a client of store moves a row by deleting and inserting it somewhere 
> else, the SUR should not find the row when trying to do renavigate to it 
> for update or delete, and can give an error.
> What our problem is, is the case where a row is inserted into the 
> container, and it gets the same RowLocation as a row which we have read 
> into the SUR. The row which we had previously read into the SUR, must 
> have been deleted and purged for this to happen.
> In addition, as far as I can see, for a new row to get the same 
> RowLocation as a row previously deleted and purged, the page for the 
> row, must have been truncated, and recreated.
> So then how can we detect that a page has been recreated ? We could i.e 
> use a timestamp on the create/recreate time of the page. This timestamp 
> could be read by the SUR as it reads the RowLocation (so we do not need 
> to change the impl. of RowLocation), and again, we would probably need 
> to change the header for the page, so that we can store the timestamp.
> Andreas
> Mike Matrigali wrote:
>> Some questions:
>> o row locations are stored in every index row.  Are you proposing a 
>> data level upgrade of every row in all databases?
>> o What is your proposal in the case of soft upgrade (note I believe not
>>   supporting "holdable" SUR in soft upgrade is an option).
>> o The hard case is the compress case that removes pages from a file, in
>>   this case there is no place to store the version number that you
>>   are relying on (the same problem in the current system why truncte 
>> can't support non-reusable rowlocations).
>> o Is it worth the on disk and in memory overhead to every row location 
>> to support holdable SUR?
>> I believe one of the operations you are trying to address is when a 
>> client of store moves a record by deleting and inserting it.  This is
>> what compress does today.  So if we start with row loc A pointing at
>> row A, and compress deletes row A and inserts it at row loc B.  In both
>> the current and new system access to A will return an error, but neither
>> will "know" that the row has been moved to a new ID.  Is this ok?
>> If the current system always supported non-reusable row id's, even in
>> the truncate case do you have what you need?  Again this will not 
>> prevent clients of store from moving a row by inserting and deleting
>> it somewhere else.
>> Andreas Korneliussen wrote:
>>> Following is a proposal to ensure that a client of store can verify 
>>> the validity of a RowLocation.  A RowLocation has become invalid if a 
>>> store operation has caused it to point to another row or to a 
>>> non-existent position (deleted row or non-existing page/record-id).
>>> I think we need a mechanism to detect that a RowLocation has become 
>>> invalid in order to implement *holdable* SUR.
>>> To do this, I would propose:
>>> - The RowLocation object should contain a version number for the page.
>>> - A version number should be stored in the header for a Page
>>> - Whenever an operation which may invalidate row-locations is 
>>> executed, the version number for the page is updated. These 
>>> operations include online/offline compress.
>>> - When navigating to a RowLocation which has invalid version number, 
>>> the store may fail (i.e return false)
>>> The page header for a stored page, currently has a number of fields 
>>> which are intended for future use, and it seems that it is possible 
>>> to use these fields without breaking backward compatibility.
>>> I noticed one of the fields in the header is named "generation" (from 
>>> StoredPage.java):
>>>      *  4 bytes integer    generation      generation number of this 
>>> page(FUTURE USE)
>>>      *  4 bytes integer    prevGeneration  previous generation of 
>>> page (FUTURE USE)
>>> Could I use the generation field for this, or has it been reserved 
>>> for something else ? Alternatively, I could use one of the other long 
>>> fields reserved for future use.
>>> Any comments ?
>>> Thanks
>>> --Andreas

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