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/include/commands/explain.h

@@ -6,7 +6,7 @@
  * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994-5, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/commands/explain.h,v 1.29 2007/01/05 22:19:53 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/commands/explain.h,v 1.30 2007/03/13 00:33:43 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -16,11 +16,16 @@
 #include "executor/executor.h"
 
 
-extern void ExplainQuery(ExplainStmt *stmt, ParamListInfo params,
-			 DestReceiver *dest);
+extern void ExplainQuery(ExplainStmt *stmt, const char *queryString,
+						 ParamListInfo params, DestReceiver *dest);
 
 extern TupleDesc ExplainResultDesc(ExplainStmt *stmt);
 
+extern void ExplainOneUtility(Node *utilityStmt, ExplainStmt *stmt,
+							  const char *queryString,
+							  ParamListInfo params,
+							  TupOutputState *tstate);
+
 extern void ExplainOnePlan(QueryDesc *queryDesc, ExplainStmt *stmt,
 			   TupOutputState *tstate);