Re: Fix floating-point noise in pg_stat_us_to_ms()

On Mon, Jun 29, 2026 at 2:02 PM Bertrand Drouvot
<bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
>
> Hi hackers,
>
> while reviewing [1], I noticed that the IO timings displayed in pg_stat_io can
> produce floating-point noise like:
>
> postgres=# select read_time from pg_stat_io where read_time > 0;
> read_time
> ---------------------
> 2.2640000000000002
> 0.08700000000000001
>
> That's because 0.001 cannot be represented exactly in binary floating
> point. I think this output looks weird, even if understandable. Note that with
> extra_float_digits set to 0 you don't see the noise (but 1 is the default).

Yes, multiplying by that constant also multiplies the rounding error.

> Given that / 1000.0 is the most common way to do this kind of computation in the
> code tree, I think that it makes sense to update pg_stat_us_to_ms() to do so.

+1, I'll take care of this.

--
John Naylor
Amazon Web Services