v2-0001-Prevent-dropping-the-last-label-from-a-property-grap.patch

From 16f02ae5d8f6854fe4f9dd318151b4e2e4e5b861 Mon Sep 17 00:00:00 2001
From: satyanarayana narlapuram <satyanarlapuram@gmail.com>
Date: Tue, 21 Apr 2026 05:49:51 +0000
Subject: [PATCH] Prevent dropping the last label from a property graph element

ALTER PROPERTY GRAPH ... DROP LABEL allowed removing the last label
from an element, leaving it with zero labels. This causes
pg_get_propgraphdef() to crash.

Add a check in AlterPropGraph() that counts the element's labels
before deletion and errors out if only one remains.  The label count
loop breaks early once it finds more than one label, avoiding a full
scan of the element_label catalog.

---
 src/backend/commands/propgraphcmds.c          | 34 +++++++++++++++++++
 .../expected/create_property_graph.out        |  6 ++++
 .../regress/sql/create_property_graph.sql     |  4 +++
 3 files changed, 44 insertions(+)

diff --git a/src/backend/commands/propgraphcmds.c b/src/backend/commands/propgraphcmds.c
index 45d2ff1b..b1c5e8ae 100644
--- a/src/backend/commands/propgraphcmds.c
+++ b/src/backend/commands/propgraphcmds.c
@@ -1522,6 +1522,40 @@ AlterPropGraph(ParseState *pstate, const AlterPropGraphStmt *stmt)
 						   get_rel_name(pgrelid), stmt->element_alias, stmt->drop_label),
 					parser_errposition(pstate, -1));
 
+		/*
+		 * Prevent dropping the last label from an element. Every element
+		 * must have at least one label.
+		 */
+		{
+			Relation	elrel;
+			SysScanDesc elscan;
+			ScanKeyData elkey[1];
+			int			nlabels = 0;
+
+			elrel = table_open(PropgraphElementLabelRelationId, AccessShareLock);
+			ScanKeyInit(&elkey[0],
+						Anum_pg_propgraph_element_label_pgelelid,
+						BTEqualStrategyNumber, F_OIDEQ,
+						ObjectIdGetDatum(peoid));
+			elscan = systable_beginscan(elrel, PropgraphElementLabelElementLabelIndexId,
+										true, NULL, 1, elkey);
+			while (HeapTupleIsValid(systable_getnext(elscan)))
+			{
+				nlabels++;
+				if (nlabels > 1)
+					break;
+			}
+			systable_endscan(elscan);
+			table_close(elrel, AccessShareLock);
+
+			if (nlabels <= 1)
+				ereport(ERROR,
+						(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+						 errmsg("cannot drop the last label from element \"%s\"",
+								stmt->element_alias),
+						 errhint("Every element must have at least one label.")));
+		}
+
 		ObjectAddressSet(obj, PropgraphElementLabelRelationId, ellabeloid);
 		performDeletion(&obj, stmt->drop_behavior, 0);
 
diff --git a/src/test/regress/expected/create_property_graph.out b/src/test/regress/expected/create_property_graph.out
index bc9a596e..740f886c 100644
--- a/src/test/regress/expected/create_property_graph.out
+++ b/src/test/regress/expected/create_property_graph.out
@@ -57,6 +57,12 @@ ALTER PROPERTY GRAPH g3
 ALTER PROPERTY GRAPH g3 ALTER VERTEX TABLE t3 DROP LABEL t3l3x;  -- error
 ERROR:  property graph "g3" element "t3" has no label "t3l3x"
 ALTER PROPERTY GRAPH g3 ALTER VERTEX TABLE t3 DROP LABEL t3l3;
+-- Test that the last label on an element cannot be dropped
+ALTER PROPERTY GRAPH g3 ALTER VERTEX TABLE t3 DROP LABEL t3l2;
+ALTER PROPERTY GRAPH g3 ALTER VERTEX TABLE t3 DROP LABEL t3l1;  -- error: last label
+ERROR:  cannot drop the last label from element "t3"
+HINT:  Every element must have at least one label.
+ALTER PROPERTY GRAPH g3 ALTER VERTEX TABLE t3 ADD LABEL t3l2 PROPERTIES ALL COLUMNS;
 ALTER PROPERTY GRAPH g3 DROP VERTEX TABLES (t2);  -- fail
 ERROR:  cannot drop vertex t2 of property graph g3 because other objects depend on it
 DETAIL:  edge e1 of property graph g3 depends on vertex t2 of property graph g3
diff --git a/src/test/regress/sql/create_property_graph.sql b/src/test/regress/sql/create_property_graph.sql
index 241f93df..4cf77159 100644
--- a/src/test/regress/sql/create_property_graph.sql
+++ b/src/test/regress/sql/create_property_graph.sql
@@ -52,6 +52,10 @@ ALTER PROPERTY GRAPH g3
         ADD LABEL t3l3 PROPERTIES ALL COLUMNS;
 ALTER PROPERTY GRAPH g3 ALTER VERTEX TABLE t3 DROP LABEL t3l3x;  -- error
 ALTER PROPERTY GRAPH g3 ALTER VERTEX TABLE t3 DROP LABEL t3l3;
+-- Test that the last label on an element cannot be dropped
+ALTER PROPERTY GRAPH g3 ALTER VERTEX TABLE t3 DROP LABEL t3l2;
+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;
 ALTER PROPERTY GRAPH g3 DROP VERTEX TABLES (t2);  -- fail
 ALTER PROPERTY GRAPH g3 DROP VERTEX TABLES (t2) CASCADE;
 ALTER PROPERTY GRAPH g3 DROP EDGE TABLES (e2);
-- 
2.43.0