Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc?

Hi,

On 2023-01-20 07:43:00 +0100, David Geier wrote:
> On 1/18/23 13:52, David Geier wrote:
> > On 1/16/23 21:39, Pavel Stehule wrote:
> > >
> > > po 16. 1. 2023 v 21:34 odesílatel Tomas Vondra
> > > <tomas(dot)vondra(at)enterprisedb(dot)com> napsal:
> > >
> > >     Hi,
> > >
> > >     there's minor bitrot in the Mkvcbuild.pm change, making cfbot
> > > unhappy.
> > >
> > >     As for the patch, I don't have much comments. I'm wondering if
> > > it'd be
> > >     useful to indicate which timing source was actually used for EXPLAIN
> > >     ANALYZE, say something like:
> > >
> > >      Planning time: 0.197 ms
> > >      Execution time: 0.225 ms
> > >      Timing source: clock_gettime (or tsc)
> > >
> > > +1
> >
> > I like the idea of exposing the timing source in the EXPLAIN ANALYZE
> > output.
> > It's a good tradeoff between inspectability and effort, given that RDTSC
> > should always be better to use.
> > If there are no objections I go this way.
> Thinking about this a little more made me realize that this will cause
> different pg_regress output depending on the platform. So if we go this
> route we would at least need an option for EXPLAIN ANALYZE to disable it. Or
> rather have it disabled by default and allow for enabling it. Thoughts?

The elapsed time is already inherently unstable, so we shouldn't have any test
output showing the time.

But I doubt showing it in every explain is a good idea - we use instr_time in
plenty of other places. Why show it in explain, but not in all those other
places?

Greetings,

Andres Freund