Restructure representation of join alias variables. An explicit JOIN

now has an RTE of its own, and references to its outputs now are Vars
referencing the JOIN RTE, rather than CASE-expressions.  This allows
reverse-listing in ruleutils.c to use the correct alias easily, rather
than painfully reverse-engineering the alias namespace as it used to do.
Also, nested FULL JOINs work correctly, because the result of the inner
joins are simple Vars that the planner can cope with.  This fixes a bug
reported a couple times now, notably by Tatsuo on 18-Nov-01.  The alias
Vars are expanded into COALESCE expressions where needed at the very end
of planning, rather than during parsing.
Also, beginnings of support for showing plan qualifier expressions in
EXPLAIN.  There are probably still cases that need work.
initdb forced due to change of stored-rule representation.

src/include/utils/builtins.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2001, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $Id: builtins.h,v 1.171 2001/11/05 17:46:36 momjian Exp $
+ * $Id: builtins.h,v 1.172 2002/03/12 00:52:06 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -15,7 +15,7 @@
 #define BUILTINS_H
 
 #include "fmgr.h"
-#include "nodes/primnodes.h"
+#include "nodes/parsenodes.h"
 #include "storage/itemptr.h"	/* for setLastTid() */
 
 /*
@@ -343,6 +343,10 @@ extern Datum pg_get_expr(PG_FUNCTION_ARGS);
 extern char *deparse_expression(Node *expr, List *dpcontext,
 				   bool forceprefix);
 extern List *deparse_context_for(char *relname, Oid relid);
+extern List *deparse_context_for_plan(int outer_varno, Node *outercontext,
+									  int inner_varno, Node *innercontext);
+extern Node *deparse_context_for_subplan(const char *name, List *tlist,
+										 List *rtable);
 
 /* tid.c */
 extern void setLastTid(const ItemPointer tid);