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/backend/executor/nodeHashjoin.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/executor/nodeHashjoin.c,v 1.46 2002/12/30 15:21:20 tgl Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/executor/nodeHashjoin.c,v 1.47 2003/01/20 18:54:45 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -95,6 +95,15 @@ ExecHashJoin(HashJoinState *node)
 		node->js.ps.ps_TupFromTlist = false;
 	}
 
+	/*
+	 * If we're doing an IN join, we want to return at most one row per
+	 * outer tuple; so we can stop scanning the inner scan if we matched on
+	 * the previous try.
+	 */
+	if (node->js.jointype == JOIN_IN && 
+		node->hj_MatchedOuter)
+		node->hj_NeedNewOuter = true;
+
 	/*
 	 * Reset per-tuple memory context to free any expression evaluation
 	 * storage allocated in the previous tuple cycle.  Note this can't
@@ -353,6 +362,7 @@ ExecInitHashJoin(HashJoin *node, EState *estate)
 	switch (node->join.jointype)
 	{
 		case JOIN_INNER:
+		case JOIN_IN:
 			break;
 		case JOIN_LEFT:
 			hjstate->hj_NullInnerTupleSlot =