Re: Unhappy about API changes in the no-fsm-for-small-rels patch

Andres Freund <andres(at)anarazel(dot)de> writes:
> On 2019-05-07 12:04:11 -0400, Tom Lane wrote:
>> I do not think sinval messaging is going to be sufficient to avoid that
>> problem. sinval is only useful to tell you about changes if you first
>> take a lock strong enough to guarantee that no interesting change is
>> happening while you hold the lock. We are certainly not going to let
>> writes take an exclusive lock, so I don't see how we could be certain
>> that we've seen an sinval message telling us about FSM status change.

> Sure, but it'll be pretty darn close, rather than there basically not
> being any limit except backend lifetime to how long we might not notice
> that we'd need to switch to the on-disk FSM.

Why do you think there's no limit? We ordinarily do
RelationGetNumberOfBlocks at least once per query on a table, and
I should think we could fix things so that a "free" side-effect of
that is to get the relcache entry updated with whether an FSM
ought to exist or not. So I think at worst we'd be one query behind.

regards, tom lane