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

>
> >> I'm fine with it. I can see having 'f' and 's' both mean cast
> functions, but 's' means safe, but the extra boolean works too and we'll be
> fine with 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 CAST
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.

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
the DEFAULT clause altogether and fall back to being an ordinary CAST().

> Currently pg_cast.casterrorsafe works just fine.
> if the cast function is not applicable, the castfunc would be InvalidOid.
> also the cast function is either error safe or not, I don't see a
> usage case for the third value.
>

Agreed, it's fine. A committer may want a char with 's'/'u' values to keep
all the options char types, but even if they do that's a very minor change.