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From: jian he <jian.universality@gmail.com>
Date: Thu, 29 Jan 2026 11:53:39 +0800
Message-ID: <CACJufxGEpQiMUOFcw5G67sNqgdyXxB7h-mpA_6neqNRf4qov=g@mail.gmail.com>
Subject: Re: ERROR: failed to find conversion function from unknown to text
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
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On Thu, Jan 29, 2026 at 9:24=E2=80=AFAM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> Is there a good reason why that shouldn't be an error?

at that time, i want
select cast('a'::text as unknown);

behave the same as

select cast('a' as unknown);

To make it an error, meaning it's not possible to coerce to the unknown typ=
e.
We can do it in the function find_coercion_pathway, just
after the ``if (sourceTypeId =3D=3D targetTypeId)`` check:

    if (targetTypeId =3D=3D UNKNOWNOID)
        return COERCION_PATH_NONE;

it's also doable in the function can_coerce_type,
right after the ``if (inputTypeId =3D=3D UNKNOWNOID)``:

        if (targetTypeId =3D=3D UNKNOWNOID)
            return false;


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