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Prevent dropping the last label from a property graph element

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Per SQL/PGQ standard, every graph element must have at least one
label.  When dropping a label from a graph element, ensure that there
exists at least one other label on the element.  If the label being
dropped is the only label on the element, raise an error.

We hold a ShareRowExclusiveLock when modifying a property graph.
Hence the label will not be dropped even when multiple labels are
being dropped concurrently.

Author: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Author: Satyanarayana Narlapuram <satyanarlapuram@gmail.com>
Reported-by: Satyanarayana Narlapuram <satyanarlapuram@gmail.com>
Discussion: https://www.postgresql.org/message-id/CAHg+QDeP=mTHTV48R23zKMy1SBmCKZ_L7-z5zKnYyw+K0x-gCg@mail.gmail.com
This commit is contained in:
Peter Eisentraut 2026-07-03 11:52:42 +02:00
parent 617c757405
commit 7afa11feca
4 changed files with 65 additions and 6 deletions
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4
doc/src/sgml/ref/alter_property_graph.sgml
View file

@ -99,7 +99,9 @@ ALTER PROPERTY GRAPH [ IF EXISTS ] <replaceable class="parameter">name</replacea
<term><literal>ALTER {VERTEX|NODE|EDGE|RELATIONSHIP} TABLE ... DROP LABEL</literal></term>
<listitem>
<para>
This form removes a label from an existing vertex or edge table.
This form removes a label from an existing vertex or edge table. The
last label on an element table cannot be dropped; every vertex or edge
table must have at least one label.
</para>
</listitem>
</varlistentry>

55
src/backend/commands/propgraphcmds.c
View file

@ -1294,6 +1294,11 @@ AlterPropGraph(ParseState *pstate, const AlterPropGraphStmt *stmt)
ListCell *lc;
ObjectAddress pgaddress;
/*
* ShareRowExclusiveLock is required because this command runs some
* graph-wide consistency checks that wouldn't work if more than one ALTER
* PROPERTY GRAPH could operate on the same graph at once.
*/
pgrelid = RangeVarGetRelidExtended(stmt->pgname,
ShareRowExclusiveLock,
stmt->missing_ok ? RVR_MISSING_OK : 0,
@ -1491,8 +1496,13 @@ AlterPropGraph(ParseState *pstate, const AlterPropGraphStmt *stmt)
{
Oid peoid;
Oid labeloid;
Oid ellabeloid;
Oid ellabeloid = InvalidOid;
ObjectAddress obj;
Relation ellabelrel;
SysScanDesc ellabelscan;
ScanKeyData ellabelkey[1];
int nlabels;
HeapTuple tuple;
if (stmt->element_kind == PROPGRAPH_ELEMENT_KIND_VERTEX)
peoid = get_vertex_oid(pstate, pgrelid, stmt->element_alias, -1);
@ -1510,10 +1520,34 @@ AlterPropGraph(ParseState *pstate, const AlterPropGraphStmt *stmt)
get_rel_name(pgrelid), stmt->element_alias, stmt->drop_label),
parser_errposition(pstate, -1));
ellabeloid = GetSysCacheOid2(PROPGRAPHELEMENTLABELELEMENTLABEL,
Anum_pg_propgraph_element_label_oid,
ObjectIdGetDatum(peoid),
ObjectIdGetDatum(labeloid));
/*
* Is the given label associated with the element? Is this the only
* label associated with the element? Scan the
* pg_propgraph_element_label table to find answers to these
* questions. Stop scanning when we know both answers.
*/
ellabelrel = table_open(PropgraphElementLabelRelationId, AccessShareLock);
ScanKeyInit(&ellabelkey[0],
Anum_pg_propgraph_element_label_pgelelid,
BTEqualStrategyNumber, F_OIDEQ,
ObjectIdGetDatum(peoid));
ellabelscan = systable_beginscan(ellabelrel, PropgraphElementLabelElementLabelIndexId,
true, NULL, 1, ellabelkey);
nlabels = 0;
while (HeapTupleIsValid(tuple = systable_getnext(ellabelscan)))
{
Form_pg_propgraph_element_label ellabelform = (Form_pg_propgraph_element_label) GETSTRUCT(tuple);
nlabels++;
if (ellabelform->pgellabelid == labeloid)
ellabeloid = ellabelform->oid;
if (nlabels > 1 && ellabeloid)
break;
}
systable_endscan(ellabelscan);
table_close(ellabelrel, AccessShareLock);
if (!ellabeloid)
ereport(ERROR,
@ -1522,6 +1556,17 @@ 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 associated with it.
*/
if (nlabels == 1)
ereport(ERROR,
(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
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);

7
src/test/regress/expected/create_property_graph.out
View file

@ -57,6 +57,13 @@ 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.
-- Restore the dropped label for further tests
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

5
src/test/regress/sql/create_property_graph.sql
View file

@ -52,6 +52,11 @@ 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
-- Restore the dropped label for further tests
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);