Minor additional refactoring of planner.c's PathTarget handling.

Teach make_group_input_target() and make_window_input_target() to work
entirely with the PathTarget representation of tlists, rather than
constructing a tlist and immediately deconstructing it into PathTarget
format.  In itself this only saves a few palloc's; the bigger picture is
that it opens the door for sharing cost_qual_eval work across all of
planner.c's constructions of PathTargets.  I'll come back to that later.

In support of this, flesh out tlist.c's infrastructure for PathTargets
a bit more.

src/backend/optimizer/util/tlist.c

@@ -623,6 +623,17 @@ copy_pathtarget(PathTarget *src)
 	return dst;
 }
 
+/*
+ * create_empty_pathtarget
+ *	  Create an empty (zero columns, zero cost) PathTarget.
+ */
+PathTarget *
+create_empty_pathtarget(void)
+{
+	/* This is easy, but we don't want callers to hard-wire this ... */
+	return (PathTarget *) palloc0(sizeof(PathTarget));
+}
+
 /*
  * add_column_to_pathtarget
  *		Append a target column to the PathTarget.
@@ -655,6 +666,41 @@ add_column_to_pathtarget(PathTarget *target, Expr *expr, Index sortgroupref)
 	}
 }
 
+/*
+ * add_new_column_to_pathtarget
+ *		Append a target column to the PathTarget, but only if it's not
+ *		equal() to any pre-existing target expression.
+ *
+ * The caller cannot specify a sortgroupref, since it would be unclear how
+ * to merge that with a pre-existing column.
+ *
+ * As with make_pathtarget_from_tlist, we leave it to the caller to update
+ * the cost and width fields.
+ */
+void
+add_new_column_to_pathtarget(PathTarget *target, Expr *expr)
+{
+	if (!list_member(target->exprs, expr))
+		add_column_to_pathtarget(target, expr, 0);
+}
+
+/*
+ * add_new_columns_to_pathtarget
+ *		Apply add_new_column_to_pathtarget() for each element of the list.
+ */
+void
+add_new_columns_to_pathtarget(PathTarget *target, List *exprs)
+{
+	ListCell   *lc;
+
+	foreach(lc, exprs)
+	{
+		Expr	   *expr = (Expr *) lfirst(lc);
+
+		add_new_column_to_pathtarget(target, expr);
+	}
+}
+
 /*
  * apply_pathtarget_labeling_to_tlist
  *		Apply any sortgrouprefs in the PathTarget to matching tlist entries

src/include/optimizer/tlist.h

@@ -55,8 +55,11 @@ extern bool grouping_is_hashable(List *groupClause);
 extern PathTarget *make_pathtarget_from_tlist(List *tlist);
 extern List *make_tlist_from_pathtarget(PathTarget *target);
 extern PathTarget *copy_pathtarget(PathTarget *src);
+extern PathTarget *create_empty_pathtarget(void);
 extern void add_column_to_pathtarget(PathTarget *target,
 						 Expr *expr, Index sortgroupref);
+extern void add_new_column_to_pathtarget(PathTarget *target, Expr *expr);
+extern void add_new_columns_to_pathtarget(PathTarget *target, List *exprs);
 extern void apply_pathtarget_labeling_to_tlist(List *tlist, PathTarget *target);
 
 /* Convenience macro to get a PathTarget with valid cost/width fields */