Support parenthesized syntax for CLUSTER without a table name.

b5913f6120 added a parenthesized syntax for CLUSTER, but it
requires specifying a table name.  This is unlike commands such as
VACUUM and ANALYZE, which do not require specifying a table in the
parenthesized syntax.  This change resolves this inconsistency.
This is preparatory work for a follow-up commit that will move the
unparenthesized syntax to the "Compatibility" section of the
CLUSTER documentation.

Reviewed-by: Melanie Plageman, Michael Paquier
Discussion: https://postgr.es/m/CAAKRu_bc5uHieG1976kGqJKxyWtyQt9yvktjsVX%2Bi7NOigDjOA%40mail.gmail.com

doc/src/sgml/ref/cluster.sgml

@@ -21,7 +21,7 @@ PostgreSQL documentation
 
  <refsynopsisdiv>
 <synopsis>
-CLUSTER [ ( <replaceable class="parameter">option</replaceable> [, ...] ) ] <replaceable class="parameter">table_name</replaceable> [ USING <replaceable class="parameter">index_name</replaceable> ]
+CLUSTER [ ( <replaceable class="parameter">option</replaceable> [, ...] ) ] [ <replaceable class="parameter">table_name</replaceable> [ USING <replaceable class="parameter">index_name</replaceable> ] ]
 CLUSTER [ VERBOSE ] [ <replaceable class="parameter">table_name</replaceable> [ USING <replaceable class="parameter">index_name</replaceable> ] ]
 
 <phrase>where <replaceable class="parameter">option</replaceable> can be one of:</phrase>

src/backend/parser/gram.y

@@ -11553,7 +11553,7 @@ CreateConversionStmt:
 /*****************************************************************************
  *
  *		QUERY:
- *				CLUSTER (options) <qualified_name> [ USING <index_name> ]
+ *				CLUSTER (options) [ <qualified_name> [ USING <index_name> ] ]
  *				CLUSTER [VERBOSE] [ <qualified_name> [ USING <index_name> ] ]
  *				CLUSTER [VERBOSE] <index_name> ON <qualified_name> (for pre-8.3)
  *
@@ -11569,6 +11569,15 @@ ClusterStmt:
 					n->params = $3;
 					$$ = (Node *) n;
 				}
+			| CLUSTER '(' utility_option_list ')'
+				{
+					ClusterStmt *n = makeNode(ClusterStmt);
+
+					n->relation = NULL;
+					n->indexname = NULL;
+					n->params = $3;
+					$$ = (Node *) n;
+				}
 			/* unparenthesized VERBOSE kept for pre-14 compatibility */
 			| CLUSTER opt_verbose qualified_name cluster_index_specification
 				{
@@ -11581,6 +11590,7 @@ ClusterStmt:
 						n->params = lappend(n->params, makeDefElem("verbose", NULL, @2));
 					$$ = (Node *) n;
 				}
+			/* unparenthesized VERBOSE kept for pre-17 compatibility */
 			| CLUSTER opt_verbose
 				{
 					ClusterStmt *n = makeNode(ClusterStmt);