Re: Shared row locking

Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
> You mean all empty/zero rows can be removed? Can we guarantee that on
> commit we can clean up the bitmap? If not the idea doesn't work.

For whatever data structure we use, we may reset the structure to empty
during backend-crash recovery. So your objection boils down to "what if
a backend exits normally but forgets to clean up its locks?" Assuming
that doesn't happen isn't any worse than assuming a backend will clean
up its shared memory state on non-crash exit, so I don't think it's a
serious concern.

That brings another thought: really what this is all about is working
around the fact that the standard lock manager can only cope with a
finite number of coexisting locks, because it's working in a fixed-size
shared memory arena. Maybe we should instead think about ways to allow
the existing lock table to spill to disk when it gets too big. That
would eliminate max_locks_per_transaction as a source of hard failures,
which would be a nice benefit.

regards, tom lane