Re: Cast jsonb to numeric, int, float, bool

Nikita Glukhov <n(dot)gluhov(at)postgrespro(dot)ru> writes:
> On 01.03.2018 11:19, Darafei "Komяpa" Praliaskouski wrote:
>> I would expect some casts to be implicit, so that chaining with other
>> functions is possible:

> I think that only cast to a numeric type can be made implicit, because
> it does not lose precision.

I hadn't been following this thread particularly, but I happened to notice
this bit, and I thought I'd better pop up to say No Way. There will be
*no* implicit casts from json to any numeric type. We have learned the
hard way that implicit cross-category casts are dangerous. See the
advice in the CREATE CAST man page:

It is wise to be conservative about marking casts as implicit. An
overabundance of implicit casting paths can cause PostgreSQL to choose
surprising interpretations of commands, or to be unable to resolve
commands at all because there are multiple possible interpretations. A
good rule of thumb is to make a cast implicitly invokable only for
information-preserving transformations between types in the same
general type category. For example, the cast from int2 to int4 can
reasonably be implicit, but the cast from float8 to int4 should
probably be assignment-only. Cross-type-category casts, such as text
to int4, are best made explicit-only.

regards, tom lane