Re: Dubious usage of TYPCATEGORY_STRING

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Thu, Dec 2, 2021 at 4:22 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> 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