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From: Merlin Moncure <mmoncure@gmail.com>
Date: Thu, 21 Aug 2025 11:03:17 -0600
Message-ID: <CAHyXU0wHFNY=N8T4pdWFMpBiKeZmCYjr4cmF7ycxE0o_Ozp1QQ@mail.gmail.com>
Subject: Re: Domains vs data types
To: Dominique Devienne <ddevienne@gmail.com>
Cc: Florents Tselai <florents.tselai@gmail.com>, 
	=?UTF-8?B?RXJ0YW4gS8O8w6fDvGtvZ2x1?= <ertan.kucukoglu@gmail.com>, 
	pgsql-general@lists.postgresql.org
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On Thu, Aug 21, 2025 at 2:11=E2=80=AFAM Dominique Devienne <ddevienne@gmail=
.com> wrote:
> On Wed, Aug 20, 2025 at 7:37=E2=80=AFPM Florents Tselai
> <florents.tselai@gmail.com> wrote:
> > > On 20 Aug 2025, at 7:47=E2=80=AFAM, Ertan K=C3=BC=C3=A7=C3=BCkoglu <e=
rtan.kucukoglu@gmail.com> wrote:
> > > I would like to learn if there is any benefit of using domains over d=
ata types for table column definitions in terms of performance gain/loss.
>
> > I know that this doesn=E2=80=99t answer your question, but before explo=
ring custom types / domains,
> > and based on experience, I=E2=80=99d strongly recommend exploring jsonb=
 instead as an alternative.
>
> I stayed out of that thread, but this makes me step out and react.
> domains are typically out constraining the value space of a type.
> I.e. a type with a CHECK constraint. W/o one, it's pretty useless IMHO.

+1 this.

The main use for domains is to allow for standard constraints.   If
you find yourself writing the same constraint over and over, that's
when you might consider using them.

For example, in the USA, vehicle identification numbers must be
exactly 17 characters long.

postgres@postgres=3D# create domain vin as text check (length(VALUE) =3D 17=
);
CREATE DOMAIN
postgres@postgres=3D# select 'abc'::TEXT::VIN;
ERROR:  value for domain vin violates check constraint "vin_check"

The other use for domains I see is global sequences where you have
many tables pulling from the same sequence.

postgres@postgres=3D# create sequence global_id_seq;
CREATE SEQUENCE
postgres@postgres=3D# create domain global_id as bigint default
nextval('global_id_seq');
CREATE DOMAIN

Using domains is not really a performance thing, it's more regarding
establishing rigor around type rules.  In general, I tend not to use
length constraints for tex columns anymore for most cases (just use
text!), but I could get behind setting some standard length
constraints, say, capped at 1k characters as a safety precaution.

merlin

merlin