RE: Plans for solving the VACUUM problem

> > We could keep share buffer lock (or add some other kind of lock)
> > untill tuple projected - after projection we need not to read data
> > for fetched tuple from shared buffer and time between fetching
> > tuple and projection is very short, so keeping lock on buffer will
> > not impact concurrency significantly.
> 
> Or drop the pin on the buffer to show we no longer have a pointer to it.

This is not good for seqscans which will return to that buffer anyway.

> > Or we could register callback cleanup function with buffer so bufmgr
> > would call it when refcnt drops to 0.
> 
> Hmm ... might work.  There's no guarantee that the refcnt
> would drop to zero before the current backend exits, however.
> Perhaps set a flag in the shared buffer header, and the last guy
> to drop his pin is supposed to do the cleanup?

This is what I've meant - set (register) some pointer in buffer header
to cleanup function.

> But then you'd be pushing VACUUM's work into productive transactions,
> which is probably not the way to go.

Not big work - I wouldn't worry about it.

> > Two ways: hold index page lock untill heap tuple is checked
> > or (rough schema) store info in shmem (just IndexTupleData.t_tid
> > and flag) that an index tuple is used by some scan so cleaner could
> > change stored TID (get one from prev index tuple) and set flag to
> > help scan restore its current position on return.
> 
> Another way is to mark the index tuple "gone but not forgotten", so to
> speak --- mark it dead without removing it. (We could know that we need
> to do that if we see someone else has a buffer pin on the index page.)

Register cleanup function just like with heap above.

> None of these seem real clean though.  Needs more thought.

Vadim