Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc?

Hi,

On 2023-01-17 10:50:53 -0800, Andres Freund wrote:
> On 2023-01-17 12:26:57 -0500, Tom Lane wrote:
> > > 0001) Add INSTR_TIME_SET_ZERO() calls where otherwise 0002 causes gcc to
> > > warn
> > > Alternatively we can decide to deprecate INSTR_TIME_SET_ZERO() and
> > > just allow to assign 0.
> >
> > I think it's probably wise to keep the macro. If we ever rethink this
> > again, we'll be glad we kept it. Similarly, IS_ZERO is a good idea
> > even if it would work with just compare-to-zero.
>
> Perhaps an INSTR_TIME_ZERO() that could be assigned in variable definitions
> could give us the best of both worlds?

I tried that in the attached 0005. I found that it reads better if I also add
INSTR_TIME_CURRENT(). If we decide to go for this, I'd roll it into 0001
instead, but I wanted to get agreement on it first.

Comments?

> > I'm almost tempted to suggest you define instr_time as a struct with a
> > uint64 field, just to help keep us honest about that.
>
> I can see that making sense. Unless somebody pipes up with opposition to that
> plan soon, I'll see how it goes.

Done in the attached. I think it looks good. Actually found a type confusion
buglet in 0004, so the type safety benefit is noticable.

It does require a new INSTR_TIME_IS_LT() for the loop exit condition in 0004,
but that seems fine.

Besides cosmetic stuff I also added back the cast to double in window's
INSTR_TIME_GET_NANOSEC() - I think there's an overflow danger without it.

We should make this faster by pre-computing
(double) NS_PER_S / GetTimerFrequency()
once, as that'd avoid doing the the slow division on every conversion. But
that's an old issue and thus better tackled separately.

Greetings,

Andres Freund