Re: CAST Within EXCLUSION constraint

On Tue, Aug 20, 2013 at 8:53 PM, David E. Wheeler <david(at)justatheory(dot)com>wrote:

> On Aug 20, 2013, at 6:50 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> > You need more parentheses -- (source::text) would've worked.
>
> Alas, no, same problem as for CAST():
>
> ERROR: functions in index expression must be marked IMMUTABLE
>
> >> No problem, I can use CAST(), right? So I try:
> >> EXCLUDE USING gist (CAST(source AS text) WITH =, within WITH &&)
> >> Not so much:
> >> try.sql:13: ERROR: functions in index expression must be marked
> IMMUTABLE
> >> I guess it's because locale settings might change, and therefore change
> the text representation? Seems unlikely, though.
> >
> > Not locale, just renaming one of the values would be enough to break
> that.
> > Admittedly we don't provide an official way to do that ATM, but you can
> do
> > an UPDATE on pg_enum.
>
> Ah, right. Maybe if there was a way to get at some immutable numeric value…
>

It seems reasonable to me to cast enum to oid. However, creating casts
without function isn't allowed for enums.

test=# create cast (source as oid) without function;
ERROR: enum data types are not binary-compatible

However, this restriction can be avoided either by writing dummy C-function
or touching catalog directly:

test=# insert into pg_cast values ((select oid from pg_type where typname =
'source'), (select oid from pg_type where typname = 'oid'), 0, 'e', 'b');
INSERT 341001 1

Then you can define desired restriction.

CREATE TABLE things (
source source NOT NULL,
within tstzrange NOT NULL,
EXCLUDE USING gist ((source::oid) WITH =, within WITH &&)
);

Probably, I'm missing something and casting enum to oid is somehow unsafe?

------
With best regards,
Alexander Korotkov.