Re: [Bug][patch]: After dropping the last label from a property graph element, invoking pg_get_propgraphdef() triggers an assertion failure

On Tue, Apr 21, 2026 at 1:29 PM SATYANARAYANA NARLAPURAM
<satyanarlapuram(at)gmail(dot)com> wrote:
>
> Hi,
>
> On Mon, Apr 20, 2026 at 11:58 PM Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com> wrote:
>>
>> On Tue, Apr 21, 2026 at 11:28 AM SATYANARAYANA NARLAPURAM
>> <satyanarlapuram(at)gmail(dot)com> wrote:
>> >
>> > HI Ashutosh,
>> >
>> > On Mon, Apr 20, 2026 at 10:34 PM Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com> wrote:
>> >>
>> >> On Mon, Apr 20, 2026 at 11:42 PM SATYANARAYANA NARLAPURAM
>> >> <satyanarlapuram(at)gmail(dot)com> wrote:
>> >> >
>> >> > Hi hackers,
>> >> >
>> >> > ALTER PROPERTY GRAPH ... ALTER ... DROP LABEL currently allows removing
>> >> > 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) handle zero labels.
>> >> > I feel it is easier to prevent dropping the last label than handling zero 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-adds 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.
>
>
> Addressed this in v3 patch.

Thanks.

I questioned whether it makes sense to move the code to check the
existence of at least one other label into its own function so the
code is more readable. AlterPropGraph is already about 500 lines and
this code increases it further. There are two possibilities:
a. The function returns true if the given element has more than two
labels associated with it. The choice of 2 makes sense only in the
drop label context but not otherwise.
b. The function returns true if it finds one label other than the one
with the given label oid. That looks a bit more elegant. But still
seems too tied to drop label context.

So I decided against it. But instead removed the extra indentation.
The earliest variable declaration block is not farther from that code.

I changed the errcode to ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE
since we expect the element to be in a specific state to drop the
label. ERRCODE_INVALID_OBJECT_DEFINITION would mean that the
definition of the element or the label is itself invalid. That's not
the case.

Here's a patch with some minor edits.

--
Best Wishes,
Ashutosh Bapat