Subject: Re: micro-optimizing json.c

From: Jeff Davis <[email-masked]>
Date: Thu, 07 Dec 2023 15:43:47 -0800

On Thu, 2023-12-07 at 17:12 -0600, Nathan Bossart wrote:
> Here's a patch that removes a couple of strlen() calls that showed up
> prominently in perf for a COPY TO (FORMAT json) on 110M integers.  On
> my
> laptop, I see a 20% speedup from ~23.6s to ~18.9s for this test.

Nice improvement. The use of (len = ...) in a conditional is slightly
out of the ordinary, but it makes the conditionals a bit simpler and
you have a comment, so it's fine with me.

I wonder, if there were an efficient cast from numeric to text, then
perhaps you could avoid the strlen() entirely? Maybe there's a way to
use a static buffer to even avoid the palloc() in get_str_from_var()?
Not sure these are worth the effort; just brainstorming.

In any case, +1 to your simple change.

Regards,
	Jeff Davis

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From: Tom Lane <[email-masked]>
Date: Thu, 07 Dec 2023 19:40:30 -0500

> you have a comment, so it's fine with me.

Yeah.  It's a little not per project style, but I don't see a nice
way to do it differently, granting that we don't want to put the
strlen() ahead of the !key_scalar test.  More straightforward
coding would end up with two else-path calls of escape_json,
which doesn't seem all that much more straightforward.

> perhaps you could avoid the strlen() entirely?

Hmm ... I think that might not be the way to think about it.  What
I'm wondering is why we need a test as expensive as IsValidJsonNumber
in the first place, given that we know this is a numeric data type's
output.  ISTM we only need to reject "Inf"/"-Inf" and "NaN", which
surely does not require a full parse.  Skip over a sign, check for
"I"/"N", and you're done.

... and for that matter, why does quoting of Inf/NaN require
that we apply something as expensive as escape_json?  Several other
paths in this switch have no hesitation about assuming that they
can just plaster double quotes around what was emitted.  How is
that safe for timestamps but not Inf/NaN?

> In any case, +1 to your simple change.

If we end up not using IsValidJsonNumber then this strlen hackery
would become irrelevant, so maybe that idea should be looked at first.

			regards, tom lane