Subject: Re: ERROR: failed to find conversion function from unknown to text

From: jian he <[email-masked]>
Date: Wed, 22 Apr 2026 14:33:01 +0800

On Thu, Jan 29, 2026 at 11:53 AM jian he <[email-masked]> wrote:
>
> To make it an error, meaning it's not possible to coerce to the unknown type.
> We can do it in the function find_coercion_pathway, just
> after the ``if (sourceTypeId == targetTypeId)`` check:
>
>     if (targetTypeId == UNKNOWNOID)
>         return COERCION_PATH_NONE;
>
> it's also doable in the function can_coerce_type,
> right after the ``if (inputTypeId == UNKNOWNOID)``:
>
>         if (targetTypeId == UNKNOWNOID)
>             return false;

I choose to disallow UNKNOWN target types in find_coercion_pathway.

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

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 == COERCION_PATH_NONE &&
        ccontext == COERCION_PLPGSQL)
        result = COERCION_PATH_COERCEVIAIO;
```

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

--
jian
https://www.enterprisedb.com/
From 9f3d8471db578d63729064d117ca2fed1abec112 Mon Sep 17 00:00:00 2001
From: jian he <[email-masked]>
Date: Wed, 22 Apr 2026 14:12:25 +0800
Subject: [PATCH v2 1/1] Disallow UNKNOWN target types

discussion: https://postgr.es/m/[email-masked]
commitfest entry: https://commitfest.postgresql.org/patch/
---
 src/backend/parser/parse_coerce.c  |  4 ++++
 src/test/regress/expected/misc.out | 12 ++++++++++++
 src/test/regress/sql/misc.sql      |  6 ++++++
 3 files changed, 22 insertions(+)

diff --git a/src/backend/parser/parse_coerce.c b/src/backend/parser/parse_coerce.c
index 913ca53666f..aec75612ca1 100644
--- a/src/backend/parser/parse_coerce.c
+++ b/src/backend/parser/parse_coerce.c
@@ -3166,6 +3166,10 @@ find_coercion_pathway(Oid targetTypeId, Oid sourceTypeId,
 	if (OidIsValid(targetTypeId))
 		targetTypeId = getBaseType(targetTypeId);
 
+	/* Disallow UNKNOWN target types. */
+	if (targetTypeId == UNKNOWNOID)
+		return COERCION_PATH_NONE;
+
 	/* Domains are always coercible to and from their base type */
 	if (sourceTypeId == targetTypeId)
 		return COERCION_PATH_RELABELTYPE;
diff --git a/src/test/regress/expected/misc.out b/src/test/regress/expected/misc.out
index 6e816c57f1f..1c6ae0a3bbc 100644
--- a/src/test/regress/expected/misc.out
+++ b/src/test/regress/expected/misc.out
@@ -396,3 +396,15 @@ SELECT *, (equipment(CAST((h.*) AS hobbies_r))).name FROM hobbies_r h;
 --
 -- rewrite rules
 --
+do $$ declare a text;
+begin a := 's'::text::unknown;
+end$$;
+ERROR:  cannot cast type text to unknown
+LINE 1: a := 's'::text::unknown
+                      ^
+QUERY:  a := 's'::text::unknown
+CONTEXT:  PL/pgSQL function inline_code_block line 2 at assignment
+select 's'::text::unknown;
+ERROR:  cannot cast type text to unknown
+LINE 1: select 's'::text::unknown;
+                        ^
diff --git a/src/test/regress/sql/misc.sql b/src/test/regress/sql/misc.sql
index 165a2e175fb..c568b4239f8 100644
--- a/src/test/regress/sql/misc.sql
+++ b/src/test/regress/sql/misc.sql
@@ -273,3 +273,9 @@ SELECT *, (equipment(CAST((h.*) AS hobbies_r))).name FROM hobbies_r h;
 --
 -- rewrite rules
 --
+
+do $$ declare a text;
+begin a := 's'::text::unknown;
+end$$;
+
+select 's'::text::unknown;
-- 
2.34.1