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

Andres Freund <andres@anarazel.de> writes:
> On 2018-10-02 17:54:31 -0400, Tom Lane wrote:
>> Here's a version of this patch rebased over commit 625b38ea0.

> Cool.  Let's get that in...

Cool, I'll push it shortly.

>> While there might be value in implementing our own float printing code,
>> I have a pretty hard time getting excited about the cost/benefit ratio
>> of that.  I think that what we probably really ought to do here is hack
>> float4out/float8out to bypass the extra overhead, as in the 0002 patch
>> below.

> I'm thinking we should do a bit more than just that hack. I'm thinking
> of something (barely tested) like

Meh.  The trouble with that is that it relies on the platform's snprintf,
not sprintf, and that brings us right back into a world of portability
hurt.  I don't feel that the move to C99 gets us out of worrying about
noncompliant snprintfs --- we're only requiring a C99 *compiler*, not
libc.  See buildfarm member gharial for a counterexample.

I'm happy to look into whether using strfromd when available buys us
anything over using sprintf.  I'm not entirely convinced that it will,
because of the need to ASCII-ize and de-ASCII-ize the precision, but
it's worth checking.

> FWIW, I think there's still a significant argument to be made that we
> should work on our floating point IO performance. Both on the input and
> output side. It's a significant practical problem. But both a fix like
> you describe, and my proposal, should bring us to at least the previous
> level of performance for the hot paths. So that'd then just be an
> independent consideration.

Well, an independent project anyway.  I concur that it would have value;
but whether it's worth the effort, and the possible behavioral changes,
is not very clear to me.

            regards, tom lane