Feedback-ID: ia2694551:Fastmail
Date: Tue, 20 Sep 2022 16:39:46 +0200
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Isaac Morland <isaac.morland@gmail.com>
Cc: Matthias van de Meent <boekewurm+postgres@gmail.com>,
	Robert Haas <robertmhaas@gmail.com>,
	Pg Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: cataloguing NOT NULL constraints
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On 2022-Sep-20, Isaac Morland wrote:

> On Tue, 20 Sept 2022 at 06:56, Alvaro Herrera <alvherre@alvh.no-ip.org>
> wrote:
> 
> > .. though I'm now wondering if there's additional overhead from checking
> > the constraint twice on each row: first the attnotnull bit, then the
> > CHECK itself.  Hmm.  That's probably quite bad.
> 
> Another reason to treat NOT NULL-implementing constraints differently.

Yeah.

> My thinking is that pg_constraint entries for NOT NULL columns are mostly
> an implementation detail. I've certainly never cared whether I had an
> actual constraint corresponding to my NOT NULL columns.

Naturally, all catalog entries are implementation details; a user never
really cares if an entry exists or not, only that the desired semantics
are provided.  In this case, we want the constraint row because it gives
us some additional features, such as the ability to mark NOT NULL
constraints NOT VALID and validating them later, which is a useful thing
to do in large production databases.  We have some hacks to provide part
of that functionality using straight CHECK constraints, but you cannot
cleanly get the `attnotnull` flag set for a column (which means it's
hard to add a primary key, for example).

It is also supposed to fix some inconsistencies such as disallowing to
remove a constraint on a table when it is implied from a constraint on
an ancestor table.  Right now we have ad-hoc protections for partitions,
but we don't do that for legacy inheritance.

That said, the patch I posted for this ~10 years ago used a separate
contype and was simpler than what I ended up with now, but amusingly
enough it was returned at the time with the argument that it would be
better to treat them as normal CHECK constraints; so I want to be very
sure that we're not just going around in circles.

-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/