RE: AW: Plans for solving the VACUUM problem

> > 1. Compact log files after checkpoint (save records of uncommitted
> >    transactions and remove/archive others).
> 
> On the grounds that undo is not guaranteed anyway (concurrent
> heap access), why not simply forget it,

We can set flag in ItemData and register callback function in
buffer header regardless concurrent heap/index access. So buffer
will be cleaned before throwing it out from buffer pool
(little optimization: if at the time when pin drops to 0 buffer
is undirty then we shouldn't really clean it up to avoid unnecessary
write - we can save info in FSM that space is available and clean it
up on first pin/read).
So, only ability of *immediate reusing* is not guaranteed. But this is
general problem of space reusing till we assume that buffer pin is
enough to access data.

> since above sounds rather expensive ?

I'm not sure. For non-overwriting smgr required UNDO info is pretty
small because of we're not required to keep tuple data, unlike
Oracle & Co. We can even store UNDO info in non-WAL format to avoid
log record header overhead. UNDO files would be kind of Oracle rollback
segments but muuuuch smaller. But yeh - additional log reads.

> The downside would only be, that long running txn's cannot
> [easily] rollback to savepoint.

We should implement savepoints for all or none transactions, no?

> > 2. Abort long running transactions.
> 
> This is imho "the" big downside of UNDO, and should not
> simply be put on the TODO without thorow research. I think it
> would be better to forget UNDO for long running transactions
> before aborting them.

Abort could be configurable.

Vadim