From: Tom Lane <tgl@sss.pgh.pa.us>
To: "David G. Johnston" <david.g.johnston@gmail.com>
cc: Damian Lukowski <pgsql-bugs@arcsin.de>,
        "pgsql-bugs@lists.postgresql.org" <pgsql-bugs@lists.postgresql.org>
Subject: Re: BUG #19428: Lazy evaluation of type checking in CASE in SQL functions including subqueries no longer works in 18
In-reply-to: <CAKFQuwYUnr+fnd_3eoPxxyZc9Yiv4zQ3Pc0-cDHcBFerZj+qPA@mail.gmail.com>
References: <19428-d9ac6c4d84c0bc81@postgresql.org> <3708.1773251497@sss.pgh.pa.us> <fac48759-8d95-41c2-a52f-19c6cb0c837a@arcsin.de> <CAKFQuwZN90TwSJJ=1zu88pkkC1e5c_wKDMsO5_849uwa2zDmhQ@mail.gmail.com> <0c78c92b-e3f5-42b6-a698-48855a1bd23b@arcsin.de> <CAKFQuwYUnr+fnd_3eoPxxyZc9Yiv4zQ3Pc0-cDHcBFerZj+qPA@mail.gmail.com>
Comments: In-reply-to "David G. Johnston" <david.g.johnston@gmail.com>
	message dated "Thu, 12 Mar 2026 06:29:31 -0700"
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Date: Thu, 12 Mar 2026 11:46:34 -0400
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"David G. Johnston" <david.g.johnston@gmail.com> writes:
> On Thursday, March 12, 2026, Damian Lukowski <pgsql-bugs@arcsin.de> wrote:
>> The `data` table could have millions of rows. Is the planner invoked for
>> every actual call to `func`, or only once before any pages are read?

> IIRC the first five invocations will get replanned using the parameter as a
> constant; then after either every call will be replanned or none will.

Read up on custom vs. generic plans for some more background on this.

That behavior used to apply only to plpgsql functions, but since v18
SQL-language functions do it too.

			regards, tom lane