micro-optimizing json.c

Moving this to a new thread...

On Thu, Dec 07, 2023 at 07:15:28AM -0500, Joe Conway wrote:
> On 12/6/23 21:56, Nathan Bossart wrote:
>> On Wed, Dec 06, 2023 at 03:20:46PM -0500, Tom Lane wrote:
>> > If Nathan's perf results hold up elsewhere, it seems like some
>> > micro-optimization around the text-pushing (appendStringInfoString)
>> > might be more useful than caching. The 7% spent in cache lookups
>> > could be worth going after later, but it's not the top of the list.
>>
>> Hah, it turns out my benchmark of 110M integers really stresses the
>> JSONTYPE_NUMERIC path in datum_to_json_internal(). That particular path
>> calls strlen() twice: once for IsValidJsonNumber(), and once in
>> appendStringInfoString(). If I save the result from IsValidJsonNumber()
>> and give it to appendBinaryStringInfo() instead, the COPY goes ~8% faster.
>> It's probably worth giving datum_to_json_internal() a closer look in a new
>> thread.
>
> Yep, after looking through that code I was going to make the point that your
> 11 integer test was over indexing on that one type. I am sure there are
> other micro-optimizations to be made here, but I also think that it is
> outside the scope of the COPY TO JSON patch.

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.

I plan to test the other types as well, and I'd also like to look into the
caching mentioned above if/when COPY TO (FORMAT json) is committed.

--
Nathan Bossart
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