Re: Teaching users how they can get the most out of HOT in Postgres 14

On Tue, May 11, 2021 at 04:42:27PM +0900, Michael Paquier wrote:
> Whatever the solution chosen, the thing I can see we agree on here is
> that we need to do something, at least in the shape of an on/off
> switch to have an escape path in case of problems.  Peter, could we
> get something by beta1 for that?  FWIW, I would use a float GUC to
> control that, and not a boolean switch, but I am just one voice here,
> and that's not a feature I worked on.

So, I have been thinking more about this item, and a boolean switch
still sounded weird to me, so attached is a patch to have two GUCs,
one for manual VACUUM and autovacuum like any other parameters, to 
control this behavior, with a default set at 2% of the number of
relation pages with dead items needed to do the index cleanup work.

Even if we switch the parameter type used here, the easiest and most
consistent way to pass down the parameter is just to use VacuumParams
set within ExecVacuum() and the autovacuum code path.  The docs need
more work, I guess.

Thoughts?
--
Michael