Hi, Mike> It is true that the current access methods don't reuse row locations Mike> until a table level lock is granted. But your project would be the Mike> first dependency on this outside of the access method implementations Mike> themselves. It is very clear the contract that the access methods Mike> have with their clients while locks are held on the data they are Mike> looking at, what you are proposing is a contract on unlocked data.
So, is the right approach to always lock all rows seen as part of an updatable scrollable insensitive result set? Currently, this would seem the only way to know whether an update/delete of a row can safely be performed when navigating (back) to a previously seen row, since non-locked rows are legal prey for compress, and there is no way(?) to tell if a row on a page has changed its "identity" since the previous read. Some kind of notification from store before compress starts, as proposed by Andreas, would allow invalidation of the result set to take place, offering higher concurrency, at the price of a new contract between store and the access layer. It would solve rowlocation validity problem for both the "in-place" compress (which apparently can happen before the transaction commits) and the off-line compress (which is an issue when trying to realize holdability for insensitive scrollable updatable result sets). The standard doesn't really offer much help as to what should and should not work here, since it requires updatable result sets to be sensitive. For sensitive cursors, one would navigate back and forth and there would be no (or less of a) need to remember and (in)validate row locations. But we are building insensitive.. ;-) For the non-holdable case, I guess it boils down to a trade-off between the desire to avoid locking all result set rows and the desire to avoid new contracts between store and access...? For the holdable case, I am not sure I see how we can implement that without some kind of support from store.. Thanks, Dag Mike> Mike> Note that the current "in-place" compress will MOVE rows from one Mike> row location to another if one does not have a row lock on the row. Mike> This is done in the 2nd phase and only holds an intent lock, and Mike> exclusive row locks on the rows being moved. Mike> The off-line compress only does work under an X table lock. Mike> So the row that you are updating actually will exist in the table, Mike> but currently you will request the old location and will get back Mike> a delete row indicator. I think because of this option 1 does not Mike> work. Mike> Mike> The state of held cursors across commits is very murky in the standards. Mike> We looked very carefully at forward only held cursors, and the standards Mike> there are carefully worded to basically not promise anything about the Mike> rows that were viewed that preceded the commit (clearly since the Mike> standard says the only thing you can do after the commit is a next to Mike> get a new row or close - never can access rows looked at before the Mike> commit). What options are legal Mike> implementations of updatable scrollable result sets for held cursors Mike> across commits? Do the standards guarantee anything about data in the Mike> cursor looked at before the commit? Mike> Mike> Mike> Mike> Andreas Korneliussen wrote: Mike> > Mike Matrigali wrote: Mike> > .. Mike> > Mike> >> If possible I would like to see a solution that does not require special Mike> >> messages sent back and forth between modules about state. Mike> >> Mike> > Mike> > I am not entirely sure what restrictions you want to put on the design, Mike> > it is a bit unclear to me. Mike> > Mike> > I have considered some other solutions: Mike> > Mike> > 1. Change the locking behaviour, so that a table intent lock which is Mike> > set by an updatable cursor, is kept as long as the cursor is open - this Mike> > will ensure that the RowLocations are valid. Mike> > Mike> > 2. After a commit, we could clear all data in the internal table in the Mike> > SUR. The problem with this approach is that the resultset would not Mike> > necessarily be repopulated with the same data - it would be sensitive Mike> > for changes across its own transactions commits, it would be highly Mike> > ineffecient. Mike> > Mike> > 3. Let the cursors notify the OnlineCompress module that it should fail Mike> > any attempt to compress/defragment or purge the table. Mike> > Mike> > More details on what I suggested yesterday: Mike> > Mike> > The OnlineCompress class could provide an event mechanism, where Mike> > subscribers (OnlineCompressListener) register themselves to listen to Mike> > OnlineCompressEvents. The ScrollInsensitiveResultSet class could then Mike> > implement the OnlineCompressListener interface, and register itself once Mike> > it starts populating the table with RowLocations. The OnlineCompress Mike> > class then simply notifies all listeners once it is doing defragment / Mike> > compress. Mike> > The listeners should unregister themselves (i.e Mike> > ScrollInsensitiveResultSet class could do it once it closes). The Mike> > OnlineCompress class could use a WeakHashMap to put the listeners into, Mike> > in case they are not well-behaved. I have not checked if derby already Mike> > has event manager type of modules, if it does, I would attempt to reuse Mike> > them. Mike> > Mike> > Please also let me know if any of the other alternatives seems better. Mike> > Mike> > Mike> > Andreas Mike> > Mike> > Mike> >> Andreas Korneliussen wrote: Mike> >> Mike> >> Mike> >>> Some context: In scrollable updatable resultsets, we populate an Mike> >>> internal table with the following data: Mike> >>> Mike> >>> <Position> <RowLocation> <RowUpdated> <RowDeleted> [<DataColumn>]+ Mike> >>> Mike> >>> Example layeout: Mike> >>> Mike> >>> 1 <1,10> false false 1,"a",3 Mike> >>> 2 <1,11> false false 2,"b",2 Mike> >>> 3 <1,12> false false 3,"c",9 Mike> >>> Mike> >>> Mike> >>> When doing updateRow(), or deleteRow(), we use the RowLocation to Mike> >>> navigate to the row being updated. Mike> >>> Mike> >>> Problem: Mike> >>> For holdable cursors, we will release the table intent lock when doing Mike> >>> commit on the transaction for the cursor. Mike> >>> Mike> >>> The table intent lock, prevents the system from doing a compress of the Mike> >>> table, causing all RowLocations to be invalid. In addition, it prevents Mike> >>> reuse of RowLocation for deleted + purged rows. Mike> >>> Mike> >>> In order to support holdable scrollable updatable cursors, we consider Mike> >>> having a service which allows the system to notify subscribers (i.e Mike> >>> cursors) that it has executed i.e a compress. Mike> >>> Mike> >>> If the user then calls updateRow() or deleteRow(), we can then give an Mike> >>> exception like: Mike> >>> Mike> >>> "The row could not be updated, because its location has been updated by Mike> >>> the system" Mike> >>> Mike> >>> In addition, we consider having a reclaim of locks, so that immediatly Mike> >>> after a commit, the new transaction with the holdable cursor, may Mike> >>> reclaim the table intent lock. This will reduce the time period which Mike> >>> the system may compress the table, however not completely remove the Mike> >>> possibility of a compress. Mike> >>> Mike> >>> Any comments on implementing such strategy ? Mike> >>> Mike> >>> An alternative to this strategy, could be to go the other way: cursors Mike> >>> notify the system that it should not do compress. Mike> >>> Mike> >>> I would appreciate feedback on this topic, especially if you find any Mike> >>> pitfalls with the proposed strategies, or have better alternatives. Mike> >>> Mike> >>> Andreas Mike> >>> Mike> > Mike> > Mike> > Mike> -- Dag H. Wanvik Sun Microsystems, Database Technology Group (DBTG) Haakon VII gt. 7b, N-7485 Trondheim, Norway Tel: x43496/+47 73842196, Fax: +47 73842101
