From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
cc: Andrew Dunstan <andrew@dunslane.net>,
        "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Subject: Re: json/jsonb cleanup + FmgrInfo caching
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Comments: In-reply-to Robert Haas <robertmhaas@gmail.com>
	message dated "Thu, 09 Jul 2026 11:15:46 -0400"
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Date: Thu, 09 Jul 2026 11:22:12 -0400
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Robert Haas <robertmhaas@gmail.com> writes:
> Right. I think it is a niche case, but I also don't think this is
> really costing us anything. As far as I can tell from my benchmarking
> to date, fmgr_info() is kind of expensive, but fmgr_info_copy() is
> pretty cheap. So, we could arrange to share a single flinfo for every
> argument of the VARIADIC list, but the savings are pretty limited
> because we wouldn't save fmgr_info() calls, only fmgr_info_copy()
> calls.

The hole in that argument is that fmgr_info_copy() does

	dstinfo->fn_extra = NULL;

so that if the function-to-be-called caches anything in fn_extra,
it will have to populate that cache for each argument.  That could be
quite expensive.  It might still not justify complicating the setup
code, but I don't think it's as clear-cut as you suggest.  Perhaps
some benchmarking is in order.

			regards, tom lane