Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions - Mailing list pgsql-hackers

On Tue, Aug 5, 2025 at 12:10 PM Corey Huinker <corey.huinker@gmail.com> wrote:
>>
>> In the end, it seems we need to make all these functions in the below
>> query error safe.
>> select castsource::regtype, casttarget::regtype, castfunc,
>> castcontext,castmethod, pp.prosrc, pp.proname from pg_cast pc join pg_proc pp on
>> pp.oid = pc.castfunc and pc.castfunc > 0
>> order by castsource::regtype;
>> It's a lot of work, but seems doable, after playing around with it.
>
> It is do-able. But that's just the cast functions that are part of core postgres.
>
hi.
it's doable for most of the data types.
but I found geometric type related cast function refactoring to error
safe is quite challenging,
so I skip that part refactoring.

>>
>> I don't think we need to change the pg_cast catalog entry,
>> we just need to make these function (pg_cast.castmethod) errors safe.
>
> That would break any user-defined cast functions that were not also error safe, which is to say all of them.
>
> We need a way for user-defined cast functions to indicate whether or not they are error safe, and handle both
situationsaccordingly (i.e. fail a CAST ON DEFAULT when the user-defined cast is not error-safe). 
>

I understand what you mean now.
CREATE CAST can use built-in functions too, we have no way to check
whether these CREATE CAST
associated functions are error safe or not.
for example:
CREATE CAST (jsonpath  AS bytea) WITH FUNCTION jsonpath_send (jsonpath
) AS ASSIGNMENT;
select '$'::jsonpath::bytea;

To avoid this situation, we have to add a new column to pg_cast to
indicate whether a cast function is error-safe.
It's unlikely a pg_cast entry has two functions, one is error safe, one is not,
adding pg_cast.casterrorsafefunc would not be that appropriate.

so I choose pg_cast.casterrorsafe would be fine.
pg_cast.casterrorsafe true means castfunc function is error safe, we
can use it as safe cast evaluation
(CAST... DEFAULT defexpr ON CONVERSION ERROR)

please check the attached V6 script:

v6-0001 to v6-0016 is about making existing pg_cast.castfunc function
error safe.
(commit message have associated query to explain the refactoring, as mentioned
above, geometric and money associated type not refactored yet)

v6-0017-make-ArrayCoerceExpr-error-safe.patch
v6-0018-CAST-expr-AS-newtype-DEFAULT-ON-ERROR.patch
is about (CAST... DEFAULT defexpr ON CONVERSION ERROR).