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/executor/spi.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/spi.c,v 1.170 2007/02/20 17:32:15 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/spi.c,v 1.171 2007/03/13 00:33:40 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -927,7 +927,7 @@ SPI_cursor_open(const char *name, void *plan,
 					  spiplan->query,
 					  CreateCommandTag(PortalListGetPrimaryStmt(stmt_list)),
 					  stmt_list,
-					  PortalGetHeapMemory(portal));
+					  NULL);
 
 	MemoryContextSwitchTo(oldcontext);
 
@@ -1471,7 +1471,12 @@ _SPI_execute_plan(_SPI_plan *plan, Datum *Values, const char *Nulls,
 				}
 				else
 				{
-					ProcessUtility(stmt, paramLI, dest, NULL);
+					ProcessUtility(stmt,
+								   NULL, /* XXX provide query string? */
+								   paramLI,
+								   false,				/* not top level */
+								   dest,
+								   NULL);
 					/* Update "processed" if stmt returned tuples */
 					if (_SPI_current->tuptable)
 						_SPI_current->processed = _SPI_current->tuptable->alloced - _SPI_current->tuptable->free;