From: Tom Lane <tgl@sss.pgh.pa.us>
To: =?utf-8?Q?=C3=81lvaro?= Herrera <alvherre@kurilemu.de>
cc: Tatsuo Ishii <ishii@postgresql.org>, ojford@gmail.com,
        peter@eisentraut.org, li.evan.chao@gmail.com, krasiyan@gmail.com,
        vik@postgresfriends.org, andrew@tao11.riddles.org.uk,
 david@fetter.org,
        pgsql-hackers@lists.postgresql.org
Subject: Re: Add RESPECT/IGNORE NULLS and FROM FIRST/LAST options
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Comments: In-reply-to =?utf-8?Q?=C3=81lvaro?= Herrera <alvherre@kurilemu.de>
	message dated "Sun, 05 Oct 2025 18:20:30 +0200"
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Date: Sun, 05 Oct 2025 12:51:45 -0400
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=3D?utf-8?Q?=3DC3=3D81lvaro?=3D Herrera <alvherre@kurilemu.de> writes:
> I just noticed this compiler warning in a CI run,

> [16:06:29.920] ../src/backend/executor/nodeWindowAgg.c:3820:16: warning:=
 =E2=80=98datum=E2=80=99 may be used uninitialized [-Wmaybe-uninitialized]
> [16:06:29.920]  3820 |         return datum;
> [16:06:29.920]       |                ^~~~~

Yeah, I can easily believe that a compiler running at relatively low
optimization level wouldn't make the connection that the NN_NOTNULL
case must perform the "prepare to exit this loop" bit if the loop
will be exited this time.  But there's another thing that is
confusing: the NN_NULL case certainly looks like it's expecting
to exit the loop, but that "break" will only get out of the switch
not the loop.  Moreover, the NN_NULL case looks like it'd fail
to notice end-of-frame.  And it's not entirely clear what the
default case thinks it's doing either.

In short, this loop is impossible to understand, and the lack of
comments doesn't help.  Even if it's not actually buggy, it
needs to be rewritten in a way that helps readers and compilers
see that it's not buggy.  I think it might help to separate the
detection of null-ness and fetching of the datum value (if required)
from the loop control logic.

			regards, tom lane