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From: David Rowley <dgrowleyml@gmail.com>
Date: Mon, 1 Jul 2024 12:47:31 +1200
Message-ID: <CAApHDvogKE8RrHx=qmYfvpD_AiVLX14nLvvSrPbFCXWgiYrKxg@mail.gmail.com>
Subject: Re: Should we document how column DEFAULT expressions work?
To: "David G. Johnston" <david.g.johnston@gmail.com>
Cc: Peter Eisentraut <peter@eisentraut.org>, Tom Lane <tgl@sss.pgh.pa.us>, 
	James Coleman <jtc331@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
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On Mon, 1 Jul 2024 at 12:16, David G. Johnston
<david.g.johnston@gmail.com> wrote:
>
> On Sun, Jun 30, 2024 at 4:55=E2=80=AFPM David Rowley <dgrowleyml@gmail.co=
m> wrote:
>>
>>
>> I'd like to know what led someone down the path of doing something
>> like DEFAULT 'now()'::timestamp in a CREATE TABLE. Could it be a
>> faulty migration tool that created these and people copy them thinking
>> it's a legitimate syntax?
>>
>
> My thought process on this used to be:  Provide a text string of the expr=
ession that is then stored within the catalog and eval'd during runtime.  I=
f the only thing you are providing is a single literal and not some compoun=
d expression it isn't that obvious that you are supposed to provide an unqu=
oted expression - which feels like it should be immediately evaluated - ver=
sus something that is a constant.  Kinda like dynamic SQL.

Thanks for sharing that.  Any idea where that thinking came from?

Maybe it was born from the fact that nothing complains when you do:
'now()'::timestamp? A quick test evaluation of that with a SELECT
statement might trick someone into thinking it'll work.

I wonder if there's anything else like this that might help fool
people into thinking this is some valid way of getting delayed
evaluation.

David