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

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,
Satya