Re: Why is vacuum_defer_cleanup_age PGC_USERSET?

On Sat, 2010-07-03 at 13:07 -0400, Tom Lane wrote:
> It seems to me it ought to be PGC_SIGHUP.  In the first place, the
> actually important value under normal circumstances is going to be
> that seen by autovac workers.  There is also some impact on HOT
> cleanup, but I can't see any sane argument why you don't want all
> backends to be applying the same value for that, since it's difficult
> or impossible to predict which session is actually going to HOT-clean
> any page.  In the second place, it seems completely foolish to let
> ordinary unprivileged users mess with the value.  Arguably an individual
> user could produce a denial of service on HS slaves by cutting his local
> value of vacuum_defer_cleanup_age and then vacuuming tables that queries
> on the slaves will look at.  To the extent that the variable does
> anything useful at all, it is the system-wide behavior that is
> important, so I see no use-case for changing it in individual sessions
> anyway.

Happy with that argument, so agreed.

> It also appears to me to be misclassified.  WAL_STANDBY_SERVERS should
> be the category for variables that you'd adjust on an HS slave, no?
> But this is something that has to be set on the master.  Possibly the
> best place for it is WAL_REPLICATION.

Those didn't exist when it was originally classified, thats all.

Yes, those categories sounds good.

-- Simon Riggs           www.2ndQuadrant.com