IN clauses appearing at top level of WHERE can now be handled as joins.

There are two implementation techniques: the executor understands a new
JOIN_IN jointype, which emits at most one matching row per left-hand row,
or the result of the IN's sub-select can be fed through a DISTINCT filter
and then joined as an ordinary relation.
Along the way, some minor code cleanup in the optimizer; notably, break
out most of the jointree-rearrangement preprocessing in planner.c and
put it in a new file prep/prepjointree.c.

src/include/optimizer/planner.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2002, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $Id: planner.h,v 1.24 2002/06/20 20:29:51 momjian Exp $
+ * $Id: planner.h,v 1.25 2003/01/20 18:55:05 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -21,7 +21,4 @@
 extern Plan *planner(Query *parse);
 extern Plan *subquery_planner(Query *parse, double tuple_fraction);
 
-extern Plan *make_sortplan(Query *parse, List *tlist,
-			  Plan *plannode, List *sortcls);
-
 #endif   /* PLANNER_H */