Re: micro-optimizing json.c

On Thu, Dec 07, 2023 at 07:40:30PM -0500, Tom Lane wrote:
> 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?

I did both of these in v2, although I opted to test that the first
character after the optional '-' was a digit instead of testing that it was
_not_ an 'I' or 'N'.  I think that should be similar performance-wise, and
maybe it's a bit more future-proof in case someone invents some new
notation for a numeric data type (/shrug).  In any case, this seems to
speed up my test by another half a second or so.

I think there are some similar improvements that we can make for
JSONTYPE_BOOL and JSONTYPE_CAST, but I haven't tested them yet.

-- 
Nathan Bossart
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