Re: Tracking last scan time

On Thu, 1 Sept 2022 at 13:04, Bruce Momjian <bruce@momjian.us> wrote:

> On Thu, Sep  1, 2022 at 09:46:59AM +0100, Dave Page wrote:
> > On Wed, 31 Aug 2022 at 17:13, Bruce Momjian <bruce@momjian.us> wrote:
> >     Wow.  I was just thinking you need second-level accuracy, which must
> be
> >     cheap somewhere.
> >
> >
> > Second-level accuracy would indeed be fine for this. Frankly, for my use
> case
> > just the date would be enough, but I can imagine people wanting greater
> > accuracy than that.
> >
> > And yes, I was very surprised by the timing results I got as well. I
> guess it's
> > a quirk of macOS - on a Linux box I get ~4s for gettimeofday() and ~1s
> for time
> > ().
>
> i think we lose 95% of our users if we require it to be enabled so let's
> work to find a way it can be always enabled.
>

So based on Andres' suggestion, something like this seems like it might
work:

if (pgstat_track_scan_timestamps && lstats->t_counts.t_numscans)
{
    TimestampTz t = GetCurrentTransactionStopTimestamp();
    if (t > tabentry->lastscan)
        tabentry->lastscan = t;
}

If that seems like a good option, I can run some more benchmarks (and then
remove the GUC if it looks good).

-- 
Dave Page
Blog: https://pgsnake.blogspot.com
Twitter: @pgsnake

EDB: https://www.enterprisedb.com