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

On Tue, Sep 6, 2016 at 2:51 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> On 6 September 2016 at 19:23, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> On Tue, Sep 6, 2016 at 2:16 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
>>> What occurs to me is that we can exactly predict how many tuples we
>>> are going to get when we autovacuum, since we measure that and we know
>>> what the number is when we trigger it.
>>>
>>> So there doesn't need to be any guessing going on at all, nor do we
>>> need it to be flexible.
>>
>> No, that's not really true. A lot can change between the time it's
>> triggered and the time it happens, or even while it's happening.
>> Somebody can run a gigantic bulk delete just after we start the
>> VACUUM.
>
> Which wouldn't be removed by the VACUUM, so can be ignored.

OK, true. But I still think it's very unlikely that we can calculate
an exact count of how many dead tuples we might run into. I think we
shouldn't rely on the stats collector to be perfectly correct anyway -
for one thing, you can turn it off - and instead cope with the
uncertainty.

--
Robert Haas
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