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From: jian he <jian.universality@gmail.com>
Date: Wed, 22 Apr 2026 14:33:01 +0800
Message-ID: 
 <CACJufxF7iSeRF2+bprERSwBe5zShVch1EJK-KcxQCcLT-UHBbg@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 11:53=E2=80=AFAM jian he <jian.universality@gmail.c=
om> wrote:
>
> To make it an error, meaning it's not possible to coerce to the unknown t=
ype.
> 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;

I choose to disallow UNKNOWN target types in find_coercion_pathway.

do $$ declare a int;
begin a :=3D '1'::text::unknown;
end$$;

This DO statement does not cause any error in the HEAD, because of
function find_coercion_pathway:
```
    /*
     * When parsing PL/pgSQL assignments, allow an I/O cast to be used
     * whenever no normal coercion is available.
     */
    if (result =3D=3D COERCION_PATH_NONE &&
        ccontext =3D=3D COERCION_PLPGSQL)
        result =3D COERCION_PATH_COERCEVIAIO;
```

but will result error with the attached V2:
ERROR:  cannot cast type text to unknown


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

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