public inbox for [email protected]  

help / color / mirror / Atom feed

From: Tom Lane <[email protected]>
To: Robert Haas <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Subject: Re: Dubious usage of TYPCATEGORY_STRING
Date: Tue, 07 Dec 2021 12:19:41 -0500
Message-ID: <[email protected]> (raw)
In-Reply-To: <CA+TgmobEsWPvF4=yiL40cKySGDZ1bErHVei47PnpGuOTp86dww@mail.gmail.com>
References: <[email protected]>
	<CA+TgmobEsWPvF4=yiL40cKySGDZ1bErHVei47PnpGuOTp86dww@mail.gmail.com>

Robert Haas <[email protected]> writes:
> On Thu, Dec 2, 2021 at 4:22 PM Tom Lane <[email protected]> wrote:
>> An example of the reasons not to treat these types as being
>> general-purpose strings can be seen at [1], where the "char"
>> type has acquired some never-intended cast behaviors.  Taking
>> that to an extreme, we currently accept
>> 
>> regression=# select '(1,2)'::point::"char";
>> char
>> ------
>> (
>> (1 row)

> What's wrong with that?

Well, we don't allow things like

regression=# select '(1,2)'::point::float8;
ERROR:  cannot cast type point to double precision
LINE 1: select '(1,2)'::point::float8;
                             ^

It's not very clear to me why "char" should get a pass on that.
We allow such cases when the target is text/varchar/etc, but
the assumption is that the textual representation is sufficient
for your purposes.  It's hard to claim that just the first
byte is a useful textual representation.

Worse, PG is actually treating this as an assignment-level cast,
so we accept this:

regression=# create table t(f1 "char");
CREATE TABLE
regression=# insert into t values ('(1,2)'::point);
INSERT 0 1
regression=# table t;
 f1 
----
 (
(1 row)

I definitely don't think that should have worked without
any complaint.

			regards, tom lane

view thread (11+ messages)  latest in thread

reply

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Reply to all the recipients using the --to and --cc options:
  reply via email

  To: [email protected]
  Cc: [email protected], [email protected], [email protected]
  Subject: Re: Dubious usage of TYPCATEGORY_STRING
  In-Reply-To: <[email protected]>

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

This inbox is served by agora; see mirroring instructions
for how to clone and mirror all data and code used for this inbox