Switch the planner over to treating qualifications of a JOIN_SEMI join as

though it is an inner rather than outer join type.  This essentially means
that we don't bother to separate "pushed down" qual conditions from actual
join quals at a semijoin plan node; which is okay because the restrictions of
SQL syntax make it impossible to have a pushed-down qual that references the
inner side of a semijoin.  This allows noticeably better optimization of
IN/EXISTS cases than we had before, since the equivalence-class machinery can
now use those quals.  Also fix a couple of other mistakes that had essentially
disabled the ability to unique-ify the inner relation and then join it to just
a subset of the left-hand relations.  An example case using the regression
database is

select * from tenk1 a, tenk1 b
where (a.unique1,b.unique2) in (select unique1,unique2 from tenk1 c);

which is planned reasonably well by 8.3 and earlier but had been forcing a
cartesian join of a/b in CVS HEAD.

src/include/nodes/nodes.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/nodes/nodes.h,v 1.214 2008/10/21 20:42:53 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/nodes/nodes.h,v 1.215 2008/11/22 22:47:06 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -542,15 +542,23 @@ typedef enum JoinType
 
 /*
  * OUTER joins are those for which pushed-down quals must behave differently
- * from the join's own quals.  This is in fact everything except INNER joins.
- * However, this macro must also exclude the JOIN_UNIQUE symbols since those
- * are temporary proxies for what will eventually be an INNER join.
+ * from the join's own quals.  This is in fact everything except INNER and
+ * SEMI joins.  However, this macro must also exclude the JOIN_UNIQUE symbols
+ * since those are temporary proxies for what will eventually be an INNER
+ * join.
  *
- * Note: in some places it is preferable to treat JOIN_SEMI as not being
- * an outer join, since it doesn't produce null-extended rows.  Be aware
- * of that distinction when deciding whether to use this macro.
+ * Note: semijoins are a hybrid case, but we choose to treat them as not
+ * being outer joins.  This is okay principally because the SQL syntax makes
+ * it impossible to have a pushed-down qual that refers to the inner relation
+ * of a semijoin; so there is no strong need to distinguish join quals from
+ * pushed-down quals.  This is convenient because for almost all purposes,
+ * quals attached to a semijoin can be treated the same as innerjoin quals.
  */
 #define IS_OUTER_JOIN(jointype) \
-	((jointype) > JOIN_INNER && (jointype) < JOIN_UNIQUE_OUTER)
+	(((1 << (jointype)) & \
+	  ((1 << JOIN_LEFT) | \
+	   (1 << JOIN_FULL) | \
+	   (1 << JOIN_RIGHT) | \
+	   (1 << JOIN_ANTI))) != 0)
 
 #endif   /* NODES_H */