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From Andreas Korneliussen <Andreas.Kornelius...@Sun.COM>
Subject Re: RowLocation validation, for holdable SUR
Date Mon, 27 Feb 2006 14:50:03 GMT
Mike Matrigali wrote:
> 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.
> 

I think I could go for the following solution to invalidate the SUR in 
case of online compress:
- A sequence number is associated with each Container
- The sequence number is updated when doing truncate

A holdable cursor will need to reopen the controller after a commit, 
since the controllers get closed at the end of the transaction (in 
closeForEndTransaction(..)).

When reopening a controller, one may check that the sequence number has 
not been changed since it was initially opened. If it has changed, one 
can conclude that there has been a online compress, and updates cannot 
be safely executed, and we may reject the reopen.

Any attempt to do update on a non-reopened controller, will fail, and a 
warning given (cursor operation conflict).

This solution does not have the downside of requiring any changes to the 
page layout, or RowLocation. It also does not have a cost per row. The 
downside, is that a online compress will invalidate the cursor from 
doing any update, even for rows which are unaffected of the truncate.

Note: the ScrollInsensitiveResultSet does not need to know anything 
about the sequence number.

Andreas

> 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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