First phase of plan-invalidation project: create a plan cache management

module and teach PREPARE and protocol-level prepared statements to use it.
In service of this, rearrange utility-statement processing so that parse
analysis does not assume table schemas can't change before execution for
utility statements (necessary because we don't attempt to re-acquire locks
for utility statements when reusing a stored plan).  This requires some
refactoring of the ProcessUtility API, but it ends up cleaner anyway,
for instance we can get rid of the QueryContext global.

Still to do: fix up SPI and related code to use the plan cache; I'm tempted to
try to make SQL functions use it too.  Also, there are at least some aspects
of system state that we want to ensure remain the same during a replan as in
the original processing; search_path certainly ought to behave that way for
instance, and perhaps there are others.

src/backend/commands/cluster.c

@@ -11,7 +11,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/cluster.c,v 1.156 2007/02/01 19:10:25 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/cluster.c,v 1.157 2007/03/13 00:33:39 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -82,7 +82,7 @@ static List *get_tables_to_cluster(MemoryContext cluster_context);
  *---------------------------------------------------------------------------
  */
 void
-cluster(ClusterStmt *stmt)
+cluster(ClusterStmt *stmt, bool isTopLevel)
 {
 	if (stmt->relation != NULL)
 	{
@@ -173,7 +173,7 @@ cluster(ClusterStmt *stmt)
 		 * We cannot run this form of CLUSTER inside a user transaction block;
 		 * we'd be holding locks way too long.
 		 */
-		PreventTransactionChain((void *) stmt, "CLUSTER");
+		PreventTransactionChain(isTopLevel, "CLUSTER");
 
 		/*
 		 * Create special memory context for cross-transaction storage.