Re: Shared row locking

BTom Lane wrote:
> 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.

Agreed. Once concern I have about allowing the lock table to spill to
disk is that a large number of FOR UPDATE locks could push out lock
entries used by other backends, causing very poor performance.

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