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/nodes/parsenodes.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: parsenodes.h,v 1.159 2002/03/08 04:37:18 tgl Exp $
+ * $Id: parsenodes.h,v 1.160 2002/03/12 00:52:01 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -430,9 +430,12 @@ typedef struct TargetEntry
  * RangeTblEntry -
  *	  A range table is a List of RangeTblEntry nodes.
  *
- *	  Currently we use the same node type for both plain relation references
- *	  and sub-selects in the FROM clause.  It might be cleaner to abstract
- *	  the common fields into a "superclass" nodetype.
+ *	  A range table entry may represent a plain relation, a sub-select in
+ *	  FROM, or the result of a JOIN clause.  (Only explicit JOIN syntax
+ *	  produces an RTE, not the implicit join resulting from multiple FROM
+ *	  items.  This is because we only need the RTE to deal with SQL features
+ *	  like outer joins and join-output-column aliasing.)  Other special
+ *	  RTE types also exist, as indicated by RTEKind.
  *
  *	  alias is an Attr node representing the AS alias-clause attached to the
  *	  FROM expression, or NULL if no clause.
@@ -445,7 +448,7 @@ typedef struct TargetEntry
  *
  *	  inh is TRUE for relation references that should be expanded to include
  *	  inheritance children, if the rel has any.  This *must* be FALSE for
- *	  subquery RTEs.
+ *	  RTEs other than RTE_RELATION entries.
  *
  *	  inFromCl marks those range variables that are listed in the FROM clause.
  *	  In SQL, the query can only refer to range variables listed in the
@@ -465,12 +468,28 @@ typedef struct TargetEntry
  *	  (This allows rules to act as setuid gateways.)
  *--------------------
  */
+typedef enum RTEKind
+{
+	RTE_RELATION,				/* ordinary relation reference */
+	RTE_SUBQUERY,				/* subquery in FROM */
+	RTE_JOIN,					/* join */
+	RTE_SPECIAL					/* special rule relation (NEW or OLD) */
+} RTEKind;
+
 typedef struct RangeTblEntry
 {
 	NodeTag		type;
 
+	RTEKind		rtekind;		/* see above */
+
 	/*
-	 * Fields valid for a plain relation RTE (else NULL/zero):
+	 * XXX the fields applicable to only some rte kinds should be merged
+	 * into a union.  I didn't do this yet because the diffs would impact
+	 * a lot of code that is being actively worked on.  FIXME later.
+	 */
+
+	/*
+	 * Fields valid for a plain relation or inh_relation RTE (else NULL/zero):
 	 */
 	char	   *relname;		/* real name of the relation */
 	Oid			relid;			/* OID of the relation */
@@ -480,6 +499,21 @@ typedef struct RangeTblEntry
 	 */
 	Query	   *subquery;		/* the sub-query */
 
+	/*
+	 * Fields valid for a join RTE (else NULL):
+	 *
+	 * joincoltypes/joincoltypmods identify the column datatypes of the
+	 * join result.  joinleftcols and joinrightcols identify the source
+	 * columns from the join's inputs: each entry is either a source column
+	 * AttrNumber or zero.  For normal columns exactly one is nonzero,
+	 * but both are nonzero for a column "merged" by USING or NATURAL.
+	 */
+	JoinType	jointype;		/* type of join */
+	List	   *joincoltypes;	/* integer list of column type OIDs */
+	List	   *joincoltypmods;	/* integer list of column typmods */
+	List	   *joinleftcols;	/* integer list of left-side column #s */
+	List	   *joinrightcols;	/* integer list of right-side column #s */
+
 	/*
 	 * Fields valid in all RTEs:
 	 */