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From: James Coleman <jtc331@gmail.com>
Date: Mon, 1 Jul 2024 09:37:17 -0400
Message-ID: <CAAaqYe-e9Np89M0+NV1ovjX+By-+w3T0MSnUB_EY__jQaKtAnw@mail.gmail.com>
Subject: Re: Should we document how column DEFAULT expressions work?
To: "David G. Johnston" <david.g.johnston@gmail.com>
Cc: David Rowley <dgrowleyml@gmail.com>, Peter Eisentraut <peter@eisentraut.org>, 
	Tom Lane <tgl@sss.pgh.pa.us>, pgsql-hackers <pgsql-hackers@postgresql.org>
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On Sun, Jun 30, 2024 at 8:16=E2=80=AFPM 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.

I have a similar story to tell: I've honestly never thought about it
deeply until I started this thread, but just through experimentation a
few things were obvious:

- now() as a function call gives you the current timestamp in a query
- now() as a function call in a DDL DEFAULT clause sets that as a
default function call
- Quoting that function call (using the function call syntax is the
natural thing to try, I think, if you've already done the first two)
-- because some examples online show quoting it -- gives you DDL time
evaluation.

So I suspect -- though I've been doing this for so long I couldn't
tell you for certain -- that I largely intuitive the behavior by
observation.

And similarly to David J. I'd then assumed -- but never had a need to
test it -- that this was generalized.

I think DDL is also different conceptually from SQL/DML here in a kind
of insidious way: the "bare" function call in DEFAULT is *not*
executed as part of the query for DDL like it is with other queries.

Hope this helps explain things.

James Coleman