Re: wal stats questions

Hi,

On 2021-04-23 09:26:17 +0900, Masahiro Ikeda wrote:
> On 2021/04/23 0:36, Andres Freund wrote:
> > On Thu, Apr 22, 2021, at 06:42, Fujii Masao wrote:
> >> On 2021/04/21 18:31, Masahiro Ikeda wrote:
> >>>> BTW, is it better to change PgStat_Counter from int64 to uint64 because> there aren't any counters with negative number?
> >> On second thought, it's ok even if the counters like wal_records get overflowed.
> >> Because they are always used to calculate the diff between the previous and
> >> current counters. Even when the current counters are overflowed and
> >> the previous ones are not, WalUsageAccumDiff() seems to return the right
> >> diff of them. If this understanding is right, I'd withdraw my comment and
> >> it's ok to use "long" type for those counters. Thought?
> >
> > Why long? It's of a platform dependent size, which doesn't seem useful here?
>
> I think it's ok to unify uint64. Although it's better to use small size for
> cache, the idea works well for only some platform which use 4bytes for "long".
>
>
> (Although I'm not familiar with the standardization...)
> It seems that we need to adopt unsinged long if use "long", which may be 4bytes.
>
> I though WalUsageAccumDiff() seems to return the right value too. But, I
> researched deeply and found that ISO/IEC 9899:1999 defines unsinged integer
> never overflow(2.6.5 Types 9th section) although it doesn't define overflow of
> signed integer type.
>
> If my understanding is right, the definition only guarantee
> WalUsageAccumDiff() returns the right value only if the type is unsigned
> integer. So, it's safe to change "long" to "unsigned long".

I think this should just use 64bit counters. Emitting more than 4
billion records in one transaction isn't actually particularly rare. And
obviously WalUsageAccumDiff() can't do anything about that, once
overflowed it overflowed.

Greetings,

Andres Freund