Re: Performance improvements for src/port/snprintf.c

On Thu, Sep 27, 2018 at 3:55 PM Andrew Gierth
<andrew(at)tao11(dot)riddles(dot)org(dot)uk> wrote:
> >>>>> "Andres" == Andres Freund <andres(at)anarazel(dot)de> writes:
> Andres> Hm, stb's results just for floating point isn't bad. The above
> Andres> numbers were for %f %f. But as the minimal usage would be about
> Andres> the internal usage of dopr(), here's comparing %.*f:
>
> Andres> snprintf time = 1324.87 ms total, 0.000264975 ms per iteration
> Andres> pg time = 1434.57 ms total, 0.000286915 ms per iteration
> Andres> stbsp time = 552.14 ms total, 0.000110428 ms per iteration
>
> Hmm. We had a case recently on IRC where the performance of float8out
> turned out to be the major bottleneck: a table of about 2.7 million rows
> and ~70 float columns showed an overhead of ~66 seconds for doing COPY
> as opposed to COPY BINARY (the actual problem report was that doing
> "select * from table" from R was taking a minute+ longer than expected,
> we got comparative timings for COPY just to narrow down causes).
>
> That translates to approx. 0.00035 ms overhead (i.e. time(float8out) -
> time(float8send)) per conversion (Linux server, hardware unknown).
>
> That 66 seconds was the difference between 18s and 1m24s, so it wasn't a
> small factor but totally dominated the query time.

For perfect and cheap round trip to ASCII, not for human consumption,
I wonder about the hexadecimal binary float literal format from C99
(and showing up in other places too).

--
Thomas Munro
http://www.enterprisedb.com