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From: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Date: Tue, 21 Apr 2026 12:28:08 +0530
Message-ID: 
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Subject: Re: [Bug][patch]: After dropping the last label from a property graph
 element, invoking pg_get_propgraphdef() triggers an assertion failure
To: SATYANARAYANA NARLAPURAM <satyanarlapuram@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
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On Tue, Apr 21, 2026 at 11:28=E2=80=AFAM SATYANARAYANA NARLAPURAM
<satyanarlapuram@gmail.com> wrote:
>
> HI Ashutosh,
>
> On Mon, Apr 20, 2026 at 10:34=E2=80=AFPM Ashutosh Bapat <ashutosh.bapat.o=
ss@gmail.com> wrote:
>>
>> On Mon, Apr 20, 2026 at 11:42=E2=80=AFPM SATYANARAYANA NARLAPURAM
>> <satyanarlapuram@gmail.com> wrote:
>> >
>> > Hi hackers,
>> >
>> > ALTER PROPERTY GRAPH ... ALTER ... DROP LABEL currently allows removin=
g
>> > the last label from an element, leaving it with zero labels.
>> >
>> > On assert-enabled builds, pg_get_propgraphdef() hits
>> > TRAP: failed Assert("count > 0"), File: "ruleutils.c", Line: 1837, PID=
: 1821840
>> >
>> > Repro:
>> >
>> > CREATE TABLE t (x int PRIMARY KEY, y int, z int);
>> > CREATE PROPERTY GRAPH g VERTEX TABLES (t KEY (x) LABEL l1 LABEL l2);
>> > ALTER PROPERTY GRAPH g ALTER VERTEX TABLE t DROP LABEL l2;
>> > ALTER PROPERTY GRAPH g ALTER VERTEX TABLE t DROP LABEL l1;
>> > SELECT pg_get_propgraphdef('g'::regclass);
>> >
>> > We can fix it two ways, (1) Prevent dropping the last label; (2) handl=
e zero labels.
>> > I feel it is easier to prevent dropping the last label than handling z=
ero labels. Thoughts?
>> >
>>
>> SQL/PGQ standard section 11.25 syntax rule 6 says
>> "Element table descriptor shall include two or more labels, one of
>> which has an <identifier> that is equivalent to the <label name>
>> simply contained in the <drop element table label clause>."
>>
>> IIUC this simply means that the last label can not be dropped. That
>> agrees with your chosen option.
>>
>> In the patch,
>> + while (HeapTupleIsValid(systable_getnext(elscan)))
>> + nlabels++;
>>
>> It's better to break from the while loop after incrementing nlabels
>> and avoid scanning the entire table in the worst case. All we want to
>> check is whether another label exists and not all the labels.
>
>
> Please find the attached v2 patch.
>
>>
>>
>> > The attached patch adds a check in AlterPropGraph() before
>> > performDeletion(). It scans pg_propgraph_element_label to count labels
>> > for the element, and raises an error if only one remains. A regression=
 test is included
>> > that drops labels down to the last one, verifies the error, then re-ad=
ds them back.
>>
>> I would add a test to make sure ADD LABEL ... DROP LABEL .. is allowed.
>
>
> +ALTER PROPERTY GRAPH g3 ALTER VERTEX TABLE t3 DROP LABEL t3l1;  -- error=
: last label
> +ALTER PROPERTY GRAPH g3 ALTER VERTEX TABLE t3 ADD LABEL t3l2 PROPERTIES =
ALL COLUMNS;
>
> Are you looking for any additional coverage?

I thought specifying ADD LABEL and DROP LABEL is supported in the same
DDL. But that's not the case. Sorry.

Will review the patch soon.

Additionally, the patch should update the DDL document to mention that
the DDL does not allow dropping the last label on the given graph
element table.

--=20
Best Wishes,
Ashutosh Bapat