Mike Matrigali wrote:
Andreas Korneliussen wrote:
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
This sounds like a good direction.
I was suggesting that the sequence number be maintained in the system
catalogs and owned by upper layer of the system. It seems like you are
proposing the sequence number be owned by store. If owned by store
I think I would describe the sequence number something like:
An implementation specific long which will be changed to a never
previously used number if the table undergoes a change which
results in the possibility of a RowLocation which was previously
allocated being reused in a container which was built requesting
no RowLocation reuse.
Can you explain at what point, and in which part of the code does the
system check that the sequence number has changed and then fail the
SUR? If only for SUR then there will be some querying from SUR to
store after every commit. If only in store then the closing will affect
existing holdable cursors.
When the GenericScanController class (or one of its subclasses) calls
OpenConglom.reopen(), it can read the timestamp from the container, and
based on its own scan_state, and previous timestamp read, it can set a
flag (oldRowLocationsInvalid).
The SUR uses a method currently called "setRowLocation(..)" which it
uses to renavigate the controller. This method could check the
oldRowLocationsInvalid flag and return false if the old row locations
have become invalidated.
So, the setting of the flag, could happen for all holdable cursors,
however the call to setRowLocation(..), which is only used by SUR and
requires the RowLocation parameter to be a valid row location, is the
only call which need to check on that flag, and have logic to fail the
operation.
If the setRowLocation(..) call fails, the CurrentOfResultSet's will get
a null reference to the RowLocation it is going to update. This will
cause a positioned update / delete / updateRow() / deleteRow() to fail,
and give a warning (Cursor operation conflict)
Andreas
