Re: BUG #19736: `JSON_OBJECT ... ABSENT ON NULL` accepts invalid keys when the value is NULL

Hi,

CFbot reported that v2 needs a rebase after b58a9a97fe reverted the
json_check_mutability() changes. V3 is rebased onto master at 553262aff40d.
It updates the insertion context for the shared key validation helper
without restoring the reverted function; there are no behavior changes from
v2.

I also verified that the attached patch applies with git am to a clean
checkout of the stated master commit.

Thanks,
Yeongjae Min

On Sat, 17 Oct 2026 23:53:49 +0900, 민영재 <dudwo990413(at)gmail(dot)com> wrote:

Hi Dmitry, Thanks for the suggestions. V2 is attached. I found the same
issue in json_object_agg_strict() and jsonb_object_agg_strict() and
extended the fix to those workers, also covering JSON_OBJECTAGG with ABSENT
ON NULL. V2 shares the category checks through json_validate_key_category()
and clarifies that key restrictions apply even to omitted pairs. For
JSONTYPE_JSONB, json_categorize_type(..., false, ...) maps jsonb to
JSONTYPE_JSON, so v1 already covered it in the json path. The shared helper
explicitly handles both categories. All 239 core regression tests pass on
macOS/arm64 with assertions enabled, on master at 7787aec8ab00, including
the added aggregate tests. I also verified that omitted keys still avoid
output conversion without uniqueness checking, while WITH UNIQUE KEYS
retains its behavior. Thanks, Yeongjae Min On Fri, 16 Oct 2026 22:41:27
+0200, Dmitry Dolgov <9erthalion6(at)gmail(dot)com> wrote: > > On Sun, Oct 11,
2026 at 07:26:02PM +0900, 민영재 wrote: > > I reproduced both examples on
master at 9e7d152488f1. > > > > In json_build_object_worker(), the ABSENT
ON NULL skip happens before > > NULL-key validation. Both json and jsonb
paths also skip the scalar-key > > checks when omitting an entry without a
uniqueness check. > > > > The attached patch moves the json NULL-key check
before the skip and > > validates the key type before omitting an entry in
either output path. > > It avoids calling output or cast functions for
omitted keys and preserves > > the existing uniqueness checks. > > > >
Regression tests cover NULL, array, composite, json and jsonb keys with > >
NULL values. The new tests fail without the fix; with the patch, make > >
check passes all 239 tests on macOS/arm64 with assertions enabled. > >
Thanks for investigating. I think it's worth to clarify the > documentation
a bit -- one can argue that we don't need to verify keys > if the pair is
going to be skipped, and only the wording happen to > insist on it.
Currently it says: > > <replaceable>key_expression</replaceable> is a
scalar expression > defining the <acronym>JSON</acronym> key, which is >
converted to the <type>text</type> type. > It cannot be
<literal>NULL</literal> nor can it > belong to a type that has a cast to
the <type>json</type> type. > > Maybe extending it to "It cannot be ...,
even if the pair is going to be > eventually omitted due to ABSENT ON
NULL". > > It also probably makes sense to verify if there are more such
examples. > E.g. looks like jsonb_object_agg_strict exhibit something
similar: > > -- jsonb_object_agg_transfn_worker tests if the key is NULL >
select jsonb_object_agg_strict(NULL, NULL); > ERROR: 22023: field name must
not be null > > -- it also checks if the key is not a composite or json, if
the > -- value is not NULL > select jsonb_object_agg_strict('{"a":
1}'::jsonb, 'value'); > ERROR: 22023: key value must be scalar, not array,
composite, or json > > -- but it doesn't do so if the value is NULL >
select jsonb_object_agg_strict('{"a": 1}'::jsonb, NULL); >
jsonb_object_agg_strict > ------------------------- > {} > (1 row) > >
Regarding the proposed patch, maybe the condition on tcategory could be >
factored out to not repeat it few times (including one in the >
datum_to_jsonb_internal). It's also not clear to me why JSONTYPE_JSONB > is
not included in json_build_object_worker, shouldn't it be included > since
it has a cast to json?