Re: Tracking last scan time
| От | Dave Page |
|---|---|
| Тема | Re: Tracking last scan time |
| Дата | |
| Msg-id | CA+OCxoxkuMF+hkVWDkiQWnjNpjeap=mYMkHzsdSvFxjjBocrZg@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: Tracking last scan time (Andres Freund <andres@anarazel.de>) |
| Ответы |
Re: Tracking last scan time
|
| Список | pgsql-hackers |
Hi
On Thu, 1 Sept 2022 at 19:35, Andres Freund <andres@anarazel.de> wrote:
Hi,
On 2022-09-01 14:18:42 +0200, Matthias van de Meent wrote:
> On Wed, 31 Aug 2022 at 20:56, Andres Freund <andres@anarazel.de> wrote:
> > But given this is done when stats are flushed, which only happens after the
> > transaction ended, we can just use GetCurrentTransactionStopTimestamp() - if
> > we got to flushing the transaction stats we'll already have computed that.
>
> I'm not entirely happy with that, as that would still add function
> call overhead, and potentially still call GetCurrentTimestamp() in
> this somewhat hot loop.
We already used GetCurrentTransactionStopTimestamp() (as you reference below)
before we get to this point, so I doubt that we'll ever call
GetCurrentTimestamp(). And it's hard to imagine that the function call
overhead of GetCurrentTransactionStopTimestamp() matters compared to acquiring
locks etc.
Vik and I looked at this a little, and found that we actually don't have generally have GetCurrentTransactionStopTimestamp() at this point - a simple 'select * from pg_class' will result in 9 passes of this code, none of which have xactStopTimestamp != 0.
After discussing it a little, we came to the conclusion that for the stated use case, xactStartTimestamp is actually accurate enough, provided that we only ever update it with a newer value. It would only likely be in extreme edge-cases where the difference between start and end transaction time would have any bearing on whether or not one might drop a table/index for lack of use.
Doing it this way also means we no longer need the GUC to enable the feature, which as Bruce notes, is likely to lose 95% of users.
Updated patch attached:
- GUC removed.
- The timestamp recorded is xactStartTimestamp.
- Docs updated to make it clear we're recording transaction start time.
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