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From: Robert Haas <robertmhaas@gmail.com>
Date: Thu, 25 Apr 2024 13:12:39 -0400
Message-ID: <CA+TgmoZ9pGaY7MzdAYbOZ1Sab3be-kpW+muDw68ro5va3-yEfA@mail.gmail.com>
Subject: Re: Why don't we support external input/output functions for the
 composite types
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Dilip Kumar <dilipbalaut@gmail.com>, 
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
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On Thu, Apr 25, 2024 at 12:34=E2=80=AFPM Tom Lane <tgl@sss.pgh.pa.us> wrote=
:
> Yes.  We've heard that argument about "this only affects extensions
> that choose to use it" before, and it's nonsense.  As soon as you
> extend system-wide APIs, the consequences are system-wide: everybody
> has to cope with the new definition.

Sure. Any new feature has this problem to some extent.

> Uh, no.  My point is that if you make a custom output function
> for "type complex (real float8, imaginary float8)", that function
> will probably crash pretty hard if what it's handed is something
> other than two float8s.  But there is nothing to stop somebody
> from trying to ALTER the type to be two numerics or whatever.
> Conversely, the type's custom input function would likely keep on
> producing two float8s, yielding corrupt data so far as the rest
> of the system is concerned.

I'm not sure I really buy this. Changing the column definitions
amounts to changing the on-disk format, and no data type can survive a
change to the on-disk format without updating the I/O functions to
match.

> I think that to the extent that there's a need for custom I/O
> of something like this, it should be handled by bespoke types,
> similar to (say) type point.

I somewhat agree with this. The main disadvantage of that approach is
that you lose the ability to directly refer to the members, which in
some cases would be quite nice. I bet a lot of people would enjoy
being able to write my_point.x and my_point.y instead of my_point[0]
and my_point[1], for example. But maybe the solution to that is not
$SUBJECT.

A related problem is that, even if my_point behaved like a composite
type, you'd have to write (my_point).x and (my_point).y to avoid
something like:

ERROR:  missing FROM-clause entry for table "my_point"

I think it's confusing and counterintuitive that putting parentheses
around a subexpression completely changes the meaning. I don't know of
any other programming language that behaves that way, and I find the
way the "indirection" productions are coded in gram.y to be highly
questionable. I suspect everything we currently treat as an
indirection_el should instead be a way of constructing a new a_expr or
c_expr or something like that, but I strongly suspect if I try to make
the work I'll discover horrible problems I can't fix. Still, it's
awful.

--=20
Robert Haas
EDB: http://www.enterprisedb.com