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

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?

>
> While investigating this, I also looked at the DROP PROPERTIES
> specification. It's allowed to drop even the last property.
>

Thanks for checking this.

Thanks,
Satya