On 28/07/2025 10:41, jian he wrote:
select oid, castsource::regtype, casttarget::regtype,
castfunc::regproc, castcontext, castmethod
from pg_cast
where casttarget::regtype::text in ('text') or
castsource::regtype::text in ('text');
As you can see from the query output, cast from other type to text or
cast from text to other type is not in the pg_cast catalog entry.
there are in type input/output functions. it will be represented as a
CoerceViaIO node.
see function find_coercion_pathway (src/backend/parser/parse_coerce.c
line:3577).
This is the same issue I came across when I tried to implement it
several years ago.
adding these pg_cast entries seems tricky.
for example:
(assume castsource as numeric, casttarget as text)
will
(castsource as numeric, casttarget as text, castfunc as numeric_out,
castformatfunc as numeric_to_char)
ever work?
but numeric_out' result type is cstring.
I had been imagining another castcontext that would only specify the
castfunc when the FORMAT claused is used, otherwise the current method
of passing through IO would be used.
so I tend to think adding castformatfunc to pg_cast will not work.
Perhaps not, but we need to find a way to make this generic so that
custom types can define formatting rules for themselves.
--
Vik Fearing
