From: Tom Lane <tgl@sss.pgh.pa.us>
To: David Rowley <dgrowleyml@gmail.com>
cc: PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Get rid of runtime handling of AlternativeSubPlan?
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Comments: In-reply-to David Rowley <dgrowleyml@gmail.com>
	message dated "Sun, 27 Sep 2020 21:48:20 +1300"
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Date: Sun, 27 Sep 2020 11:59:26 -0400
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David Rowley <dgrowleyml@gmail.com> writes:
> On Sun, 27 Sep 2020 at 10:03, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> And if it's true, why would we change all the call sites
>> rather than improving the pg_list.h macros?

> Maybe we should. Despite the non-NIL check and length check in
> list_head(), list_second_cell(), list_third_cell() functions, the
> corresponding macro will crash anyway if those functions were to
> return NULL.

Hm, good point.

> We might as well just use list_nth_cell() to get the
> ListCell without any checks to see if the cell exists.  I  can go off
> and fix those separately. I attached a 0004 patch to help explain what
> I'm talking about.

Yeah, that should be dealt with separately.

>> It seemed to me that dealing with the general case would make
>> fix_alternative_subplan() noticeably more complex and less obviously
>> correct.  I might be wrong though; what specific coding did you have in
>> mind?

> I had thought something like 0003 (attached). It's a net reduction of
> 3 entire lines, including the removal of the comment that explained
> that there's always two in the list.

Meh.  This seems to prove my point, as it's in fact wrong; you are only
nulling out the discarded subplans-list entry in one of the two cases.
Once you fix that it's not really shorter anymore, nor clearer.  Still,
I suppose there's some value in removing the assumption about exactly
two subplans.

I'll fix up fix_alternative_subplan and push this.  I think the other
topics should be raised in separate threads.

Thanks for reviewing!

			regards, tom lane