Major revision of sort-node handling: push knowledge of query

sort order down into planner, instead of handling it only at the very top
level of the planner.  This fixes many things.  An explicit sort is now
avoided if there is a cheaper alternative (typically an indexscan) not
only for ORDER BY, but also for the internal sort of GROUP BY.  It works
even when there is no other reason (such as a WHERE condition) to consider
the indexscan.  It works for indexes on functions.  It works for indexes
on functions, backwards.  It's just so cool...

CAUTION: I have changed the representation of SortClause nodes, therefore
THIS UPDATE BREAKS STORED RULES.  You will need to initdb.

src/include/nodes/parsenodes.h

@@ -6,7 +6,7 @@
  *
  * Copyright (c) 1994, Regents of the University of California
  *
- * $Id: parsenodes.h,v 1.77 1999/07/18 03:45:01 tgl Exp $
+ * $Id: parsenodes.h,v 1.78 1999/08/21 03:49:09 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -48,28 +48,29 @@ typedef struct Query
 	bool		hasAggs;		/* has aggregates in target list */
 	bool		hasSubLinks;	/* has subquery SubLink */
 
+	List	   *rtable;			/* list of range table entries */
+	List	   *targetList;		/* target list (of TargetEntry) */
+	Node	   *qual;			/* qualifications applied to tuples */
+	List	   *rowMark;		/* list of RowMark entries */
+
 	char	   *uniqueFlag;		/* NULL, '*', or Unique attribute name */
 	List	   *sortClause;		/* a list of SortClause's */
 
-	List	   *rtable;			/* list of range table entries */
-	List	   *targetList;		/* target list (of TargetEntry) */
-	Node	   *qual;			/* qualifications */
-	List	   *rowMark;		/* list of RowMark entries */
+	List	   *groupClause;	/* a list of GroupClause's */
 
-	List	   *groupClause;	/* list of columns to specified in GROUP
-								 * BY */
-	Node	   *havingQual;		/* qualification of each group */
+	Node	   *havingQual;		/* qualifications applied to groups */
 
 	List	   *intersectClause;
-
 	List	   *unionClause;	/* unions are linked under the previous
 								 * query */
+
 	Node	   *limitOffset;	/* # of result tuples to skip */
 	Node	   *limitCount;		/* # of result tuples to return */
 
 	/* internal to planner */
-	List	   *base_rel_list;	/* base relation list */
-	List	   *join_rel_list;	/* list of relation involved in joins */
+	List	   *base_rel_list;	/* list of base-relation RelOptInfos */
+	List	   *join_rel_list;	/* list of join-relation RelOptInfos */
+	List	   *query_pathkeys;	/* pathkeys for query_planner()'s result */
 } Query;
 
 
@@ -608,7 +609,7 @@ typedef struct InsertStmt
 	List	   *targetList;		/* the target list (of ResTarget) */
 	List	   *fromClause;		/* the from clause */
 	Node	   *whereClause;	/* qualifications */
-	List	   *groupClause;	/* group by clause */
+	List	   *groupClause;	/* GROUP BY clauses */
 	Node	   *havingClause;	/* having conditional-expression */
 	List	   *unionClause;	/* union subselect parameters */
 	bool		unionall;		/* union without unique sort */
@@ -652,7 +653,7 @@ typedef struct SelectStmt
 	List	   *targetList;		/* the target list (of ResTarget) */
 	List	   *fromClause;		/* the from clause */
 	Node	   *whereClause;	/* qualifications */
-	List	   *groupClause;	/* group by clause */
+	List	   *groupClause;	/* GROUP BY clauses */
 	Node	   *havingClause;	/* having conditional-expression */
 	List	   *intersectClause;
 	List	   *exceptClause;
@@ -950,25 +951,28 @@ typedef struct RangeTblEntry
 
 /*
  * SortClause -
- *	   used in the sort clause for retrieves and cursors
+ *	   representation of ORDER BY clauses
+ *
+ * tleSortGroupRef must match ressortgroupref of exactly one Resdom of the
+ * associated targetlist; that is the expression to be sorted (or grouped) by.
+ * sortop is the OID of the ordering operator.
  */
 typedef struct SortClause
 {
 	NodeTag		type;
-	Resdom	   *resdom;			/* attributes in tlist to be sorted */
-	Oid			opoid;			/* sort operators */
+	Index		tleSortGroupRef; /* reference into targetlist */
+	Oid			sortop;			/* the sort operator to use */
 } SortClause;
 
 /*
  * GroupClause -
- *	   used in the GROUP BY clause
+ *	   representation of GROUP BY clauses
+ *
+ * GroupClause is exactly like SortClause except for the nodetag value
+ * (and it's probably not even really necessary to have two different
+ * nodetags...).  We have routines that operate interchangeably on both.
  */
-typedef struct GroupClause
-{
-	NodeTag		type;
-	Oid			grpOpoid;		/* the sort operator to use */
-	Index		tleGroupref;	/* reference into targetlist */
-} GroupClause;
+typedef SortClause GroupClause;
 
 #define ROW_MARK_FOR_UPDATE		(1 << 0)
 #define ROW_ACL_FOR_UPDATE		(1 << 1)