MIME-Version: 1.0
From: SATYANARAYANA NARLAPURAM <satyanarlapuram@gmail.com>
Date: Mon, 20 Apr 2026 11:12:44 -0700
Message-ID: 
 <CAHg+QDeP=mTHTV48R23zKMy1SBmCKZ_L7-z5zKnYyw+K0x-gCg@mail.gmail.com>
Subject: [Bug][patch]: After dropping the last label from a property graph
 element, invoking pg_get_propgraphdef() triggers an assertion failure
To: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>,
	Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
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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?

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.

Thanks,
Satya

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<div dir=3D"ltr">Hi hackers,<br><div><br></div><div>ALTER PROPERTY GRAPH ..=
. ALTER ... DROP LABEL currently allows removing<br>the last label from an =
element, leaving it with zero labels.<br></div><div><br></div><div>On asser=
t-enabled builds, pg_get_propgraphdef() hits</div><div>TRAP: failed Assert(=
&quot;count &gt; 0&quot;), File: &quot;ruleutils.c&quot;, Line: 1837, PID: =
1821840</div><div><br></div><div>Repro:</div><div><br></div><div>CREATE TAB=
LE t (x int PRIMARY KEY, y int, z int);<br>CREATE PROPERTY GRAPH g VERTEX T=
ABLES (t KEY (x) LABEL l1 LABEL l2);<br>ALTER PROPERTY GRAPH g ALTER VERTEX=
 TABLE t DROP LABEL l2;<br>ALTER PROPERTY GRAPH g ALTER VERTEX TABLE t DROP=
 LABEL l1;<br>SELECT pg_get_propgraphdef(&#39;g&#39;::regclass); <br><br>We=
 can fix it two ways, (1) Prevent dropping the last label; (2) handle zero =
labels.<br>I feel it is easier to prevent dropping the last label than hand=
ling zero labels. Thoughts?<br></div><div><br></div><div>The attached patch=
 adds a check in AlterPropGraph() before<br>performDeletion(). It scans pg_=
propgraph_element_label to count labels<br>for the element, and raises an e=
rror if only one remains. A regression test is included=C2=A0</div><div>tha=
t drops labels down to the last one, verifies the error, then re-adds them =
back.</div><div><br></div><div>Thanks,</div><div>Satya</div><div><br></div>=
<div><br></div></div>

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