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

On Tue, Apr 21, 2026 at 5:49 PM Ashutosh Bapat
<ashutosh.bapat.oss@gmail.com> wrote:
>
> On Tue, Apr 21, 2026 at 1:29 PM SATYANARAYANA NARLAPURAM
> <satyanarlapuram@gmail.com> wrote:
> >
> > Hi,
> >
> > On Mon, Apr 20, 2026 at 11:58 PM Ashutosh Bapat <ashutosh.bapat.oss@gmail.com> wrote:
> >>
> >> On Tue, Apr 21, 2026 at 11:28 AM SATYANARAYANA NARLAPURAM
> >> <satyanarlapuram@gmail.com> wrote:
> >> >
> >> > HI Ashutosh,
> >> >
> >> > On Mon, Apr 20, 2026 at 10:34 PM Ashutosh Bapat <ashutosh.bapat.oss@gmail.com> wrote:
> >> >>
> >> >> On Mon, Apr 20, 2026 at 11:42 PM SATYANARAYANA NARLAPURAM
> >> >> <satyanarlapuram@gmail.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.
>

We are looking up element label catalogs twice in this patch - first
to find the label to be dropped and then to find the number of labels
associated with the given element. I combined these two into a single
while loop.

This patch causes a conflict with the patch posted at [1]. Hence
posting both the patches together here, so that they can be committed
without conflict.

[1] https://www.postgresql.org/message-id/E3D6552E-DB26-47BC-9C7C-0F9CB936BF5B@gmail.com



--
Best Wishes,
Ashutosh Bapat