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

>
> so we need to handle numeric source types with fractional points with
> special care.
> currently, this applies only to numeric, float4, and float8.
> (hope this is all the corner case we need to catch...)
>

I'm fairly certain that the committers won't like us special-casing the
internal cast functions, as we would have to maintain these special cases
as new core types are added, and it still bypasses the defined cast
function for user-defined types, which could have similar issues similar to
the rounding issue.

I think the way forward here is either to:

1. add a second function definition to CAST. The potential syntax forr a
second function gets clumsy, but might look something like this:

CREATE CAST (source_type AS target_type)
WITH FUNCTION function_name [ (argument_type [, ...]) ]
[ AS ASSIGNMENT | AS IMPLICIT ]
[
WITH SAFE FUNCTION function_name [ (argument_type [, ...]) ]
[ AS ASSIGNMENT | AS IMPLICIT ]
]

That doesn't seem right to me, it seems easier to:

2. Modify the CAST definition to indicate whether the existing cast
function has the regular function signature or a -Safe one. In cases where
a CAST has a defined function but the safe flag is turned off, we would
have to fail the query with an error like "Defined CAST function from
srctype to desttype is not error-safe".

This would involve changing the syntax of CREATE CAST by adding an option
SAFE, or ERROR SAFE, or similar:

CREATE CAST (source_type AS target_type)
WITH [SAFE] FUNCTION function_name [ (argument_type [, ...]) ]
[ AS ASSIGNMENT | AS IMPLICIT ]

We would add a new value to pg_cast.castmethod, 's' for "safe".

We could refactor all the numeric types to use the modified functions, so
no special-case code there anymore, and it gives extension writers an
incentive to (eventually) make their own cast functions error-safe.

While method 2 seems a lot cleaner, there may be a performance regression
in the now error-safe typecast functions. If so, that might tip the balance
to having two functions defined.