(SQL/PGQ) cache lookup failed for label

Hi all,

I noticed that the following SQL statement triggers the error message `cache lookup failed for label`.

```sql
CREATE TABLE vt (id text PRIMARY KEY, name text, age int);
CREATE TABLE et (id text PRIMARY KEY, src text REFERENCES vt(id), dst text REFERENCES vt(id));
INSERT INTO vt VALUES ('a', 'Alice', 30), ('b', 'Bob', 25);
INSERT INTO et VALUES ('e1', 'a', 'b');

CREATE PROPERTY GRAPH g
VERTEX TABLES (vt LABEL l1 PROPERTIES (name) LABEL l2 PROPERTIES (name, age))
EDGE TABLES (et SOURCE KEY (src) REFERENCES vt(id) DESTINATION KEY (dst) REFERENCES vt(id));

CREATE VIEW v1 AS SELECT * FROM GRAPH_TABLE(g MATCH (a IS l2)-[e IS et]->(b IS l2) COLUMNS (a.name, a.age, b.name AS bname));

ALTER PROPERTY GRAPH g ALTER VERTEX TABLE vt DROP LABEL l2;
SELECT * FROM v1;
```

Here are the actual test results; it appears to be caused by missing dependency information.

```sql
test=# CREATE TABLE vt (id text PRIMARY KEY, name text, age int);
CREATE TABLE
test=# CREATE TABLE et (id text PRIMARY KEY, src text REFERENCES vt(id), dst text REFERENCES vt(id));
CREATE TABLE
test=# INSERT INTO vt VALUES ('a', 'Alice', 30), ('b', 'Bob', 25);
INSERT 0 2
test=# INSERT INTO et VALUES ('e1', 'a', 'b');
INSERT 0 1
test=# CREATE PROPERTY GRAPH g
VERTEX TABLES (vt LABEL l1 PROPERTIES (name) LABEL l2 PROPERTIES (name, age))
EDGE TABLES (et SOURCE KEY (src) REFERENCES vt(id) DESTINATION KEY (dst) REFERENCES vt(id));
CREATE PROPERTY GRAPH
test=# CREATE VIEW v1 AS SELECT * FROM GRAPH_TABLE(g MATCH (a IS l2)-[e IS et]->(b IS l2) COLUMNS (a.name, a.age, b.name AS bname));
CREATE VIEW
test=# ALTER PROPERTY GRAPH g ALTER VERTEX TABLE vt DROP LABEL l2;
ALTER PROPERTY GRAPH
test=# SELECT * FROM v1;
2026-05-08 15:38:37.121 CST [175953] ERROR: cache lookup failed for label 16472
2026-05-08 15:38:37.121 CST [175953] STATEMENT: SELECT * FROM v1;
ERROR: cache lookup failed for label 16472
test=#
```

I've made some minor modifications; this is my diffs file. I'm not sure if anything is missing, so feel free to add to or supplement it.

```c
diff --git a/src/backend/catalog/dependency.c b/src/backend/catalog/dependency.c
index fdb8e67e1f5..6a73b74fc9b 100644
--- a/src/backend/catalog/dependency.c
+++ b/src/backend/catalog/dependency.c
@@ -2247,6 +2247,22 @@ find_expr_references_walker(Node *node,
context->addrs);
/* fall through to examine substructure */
}
+ if (IsA(node, GraphLabelRef))
+ {
+ GraphLabelRef *lref = (GraphLabelRef *) node;
+
+ add_object_address(PropgraphLabelRelationId, lref->labelid, 0,
+ context->addrs);
+ return false;
+ }
+ if (IsA(node, GraphPropertyRef))
+ {
+ GraphPropertyRef *gpr = (GraphPropertyRef *) node;
+
+ add_object_address(PropgraphPropertyRelationId, gpr->propid, 0,
+ context->addrs);
+ return false;
+ }
else if (IsA(node, Query))
{
/* Recurse into RTE subquery or not-yet-planned sublink subquery */
@@ -2277,9 +2293,31 @@ find_expr_references_walker(Node *node,
switch (rte->rtekind)
{
case RTE_RELATION:
+ add_object_address(RelationRelationId, rte->relid, 0,
+ context->addrs);
+ break;
case RTE_GRAPH_TABLE:
add_object_address(RelationRelationId, rte->relid, 0,
context->addrs);
+
+ if (rte->graph_pattern)
+ {
+ GraphPattern *gp = rte->graph_pattern;
+ ListCell *lc1;
+
+ foreach(lc1, gp->path_pattern_list)
+ {
+ List *path_term = lfirst_node(List, lc1);
+ ListCell *lc2;
+
+ foreach(lc2, path_term)
+ {
+ GraphElementPattern *gep = lfirst_node(GraphElementPattern, lc2);
+
+ find_expr_references_walker(gep->labelexpr, context);
+ }
+ }
+ }
break;
case RTE_JOIN:

```

Final running results

```sql
test=# CREATE TABLE vt (id text PRIMARY KEY, name text, age int);
CREATE TABLE et (id text PRIMARY KEY, src text REFERENCES vt(id), dst text REFERENCES vt(id));
INSERT INTO vt VALUES ('a', 'Alice', 30), ('b', 'Bob', 25);
INSERT INTO et VALUES ('e1', 'a', 'b');

CREATE PROPERTY GRAPH g
VERTEX TABLES (vt LABEL l1 PROPERTIES (name) LABEL l2 PROPERTIES (name, age))
EDGE TABLES (et SOURCE KEY (src) REFERENCES vt(id) DESTINATION KEY (dst) REFERENCES vt(id));

CREATE VIEW v1 AS SELECT * FROM GRAPH_TABLE(g MATCH (a IS l2)-[e IS et]->(b IS l2) COLUMNS (a.name, a.age, b.name AS bname));

ALTER PROPERTY GRAPH g ALTER VERTEX TABLE vt DROP LABEL l2;
SELECT * FROM v1;
CREATE TABLE
CREATE TABLE
INSERT 0 2
INSERT 0 1
CREATE PROPERTY GRAPH
CREATE VIEW
2026-05-08 16:24:59.938 CST [182833] ERROR: cannot drop label l2 of property graph g because other objects depend on it
2026-05-08 16:24:59.938 CST [182833] DETAIL: view v1 depends on label l2 of property graph g
2026-05-08 16:24:59.938 CST [182833] HINT: Use DROP ... CASCADE to drop the dependent objects too.
2026-05-08 16:24:59.938 CST [182833] STATEMENT: ALTER PROPERTY GRAPH g ALTER VERTEX TABLE vt DROP LABEL l2;
ERROR: cannot drop label l2 of property graph g because other objects depend on it
DETAIL: view v1 depends on label l2 of property graph g
HINT: Use DROP ... CASCADE to drop the dependent objects too.
name | age | bname
-------+-----+-------
Alice | 30 | Bob
(1 row)
```

--
regards,
Man Zeng