Re: Vacuum: allow usage of more than 1GB of work mem

On 6 September 2016 at 19:00, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> Yeah, but I've seen actual breakage from exactly this issue on
>> customer systems even with the 1GB limit, and when we start allowing
>> 100GB it's going to get a whole lot worse.
>
> While it's not necessarily a bad idea to consider these things,
> I think people are greatly overestimating the consequences of the
> patch-as-proposed.  AFAICS, it does *not* let you tell VACUUM to
> eat 100GB of workspace.  Note the line right in front of the one
> being changed:
>
>          maxtuples = (vac_work_mem * 1024L) / sizeof(ItemPointerData);
>          maxtuples = Min(maxtuples, INT_MAX);
> -        maxtuples = Min(maxtuples, MaxAllocSize / sizeof(ItemPointerData));
> +        maxtuples = Min(maxtuples, MaxAllocHugeSize / sizeof(ItemPointerData));
>
> Regardless of what vac_work_mem is, we aren't gonna let you have more
> than INT_MAX ItemPointers, hence 12GB at the most.  So the worst-case
> increase from the patch as given is 12X.  Maybe that's enough to cause
> bad consequences on some systems, but it's not the sort of disaster
> Robert posits above.

Is there a reason we can't use repalloc here?

-- 
Simon Riggs                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services