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From: jian he <jian.universality@gmail.com>
Date: Tue, 9 Dec 2025 11:39:11 +0800
Message-ID: 
 <CACJufxHrE0s7G0xg1frWo2+tFLTLaikKCObixH-4p9zMYKtHFw@mail.gmail.com>
Subject: Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User
 Functions
To: Corey Huinker <corey.huinker@gmail.com>
Cc: Vik Fearing <vik@postgresfriends.org>,
 Isaac Morland <isaac.morland@gmail.com>,
	pgsql-hackers@lists.postgresql.org
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On Mon, Dec 1, 2025 at 1:41=E2=80=AFPM Corey Huinker <corey.huinker@gmail.c=
om> wrote:
>>
>> >> I'm fine with it. I can see having 'f' and 's' both mean cast functio=
ns, but 's' means safe, but the extra boolean works too and we'll be fine w=
ith either method.
>> >
>> >
>> > I can work on this part if you don't have time.
>>
>> Do you mean change pg_cast.casterrorsafe from boolean to char?
>
>
> No, I meant implementing the syntax for being able to declare a custom CA=
ST function as safe (or not). Basically adding the [SAFE] to
>
> CREATE CAST (source_type AS target_type)
>     WITH [SAFE] FUNCTION function_name [ (argument_type [, ...]) ]
>
> I'm not tied to this syntax choice, but this one seemed the most obvious =
and least invasive.
>

hi.
In v14, I have removed pg_cast.casterrorsafe, but for user-defined CREATE C=
AST,
castfunc can be built-in function or user-defined function, we do need a wa=
y to
distinguish if the cast function is error safe or not.

I=E2=80=99ll incorporate pg_cast.casterrorsafe along with the implementatio=
n of the
user-defined CREATE CAST syntax into one patch.

> But this brings up an interesting point: if a cast is declared as WITHOUT=
 FUNCTION aka COERCION_METHOD_BINARY, then the cast can never fail, and we =
should probably check for that because a cast that cannot fail can ignore t=
he DEFAULT clause altogether and fall back to being an ordinary CAST().
>
>>

integer and oid are binary coercible, but the following should fail.
SELECT CAST(11 as oid DEFAULT 'a' ON CONVERSION ERROR);

if you mean that skip
+ ExecInitExprRec((Expr *) stcstate->stcexpr->default_expr,
+ state, resv, resnull);
For binary-coercible types, this approach seems fine. We=E2=80=99ve done so=
mething
similar in ExecInitExprRec for T_ArrayCoerceExpr.
```
                if (elemstate->steps_len =3D=3D 1 &&
                    elemstate->steps[0].opcode =3D=3D EEOP_CASE_TESTVAL)
                {
                    /* Trivial, so we need no per-element work at runtime *=
/
                    elemstate =3D NULL;
                }
```
in V14, I didn't do this part, I'll keep this in mind.


--
jian
https://www.enterprisedb.com/