Re: BUG #19442: PL/pgSQL: domain over composite type bypasses type validation when assigning NULL (PostgreSQL 18.3)

On Mon, Mar 30, 2026 at 7:52 PM PG Bug reporting form
<noreply(at)postgresql(dot)org> wrote:
>
> The following bug has been logged on the website:
>
> Bug reference: 19442
> Logged by: Xianghang Zheng
> Email address: zheng_xianghang(at)163(dot)com
> PostgreSQL version: 18.3
> Operating system: Linux x86_64
> Description:
>
> I found an issue on PostgreSQL 18.3 related to domain handling over
> composite types in PL/pgSQL.
> After the fix for BUG #18735, the "cache lookup failed for type 0" error no
> longer occurs, but a new correctness problem arises:
> When assigning a NULL composite variable to a domain over composite type in
> PL/pgSQL,
> the domain's type validation and coercion logic is bypassed.
> This indicates an incomplete fix and may lead to incorrect enforcement of
> domain constraints (CHECK, NOT NULL, etc.) in the future.
> Version: PostgreSQL 18.3
> Test case:
> CREATE TYPE comp AS (a int, b text);
> CREATE DOMAIN dcomp AS comp;
> CREATE OR REPLACE FUNCTION test_domain_bug()
> RETURNS dcomp
> AS $$
> DECLARE
> v_comp comp := NULL;
> v_dcomp dcomp;
> BEGIN
> v_dcomp := v_comp;
> RETURN v_dcomp;
> END;
> $$ LANGUAGE plpgsql;
> SELECT test_domain_bug();
> Actual result: NULL (no error)
> Expected result: Domain type coercion should be properly executed even for
> NULL values.

dcomp doesn't have the not null constraint, so I think this is ok?

I tried `CREATE DOMAIN dcomp AS comp not null;`, it does report error:

[local] zhjwpku(at)postgres:5432-20309=# SELECT test_domain_bug();
ERROR: domain dcomp does not allow null values
CONTEXT: PL/pgSQL function test_domain_bug() line 4 during statement
block local variable initialization
Time: 2.730 ms

> This is a regression caused by incomplete backpatch of BUG #18735.
>
>
>
>

--
Regards
Junwang Zhao