Tweak joinlist creation to avoid generating useless one-element subproblems

when collapsing of JOIN trees is stopped by join_collapse_limit.  For instance
a list of 11 LEFT JOINs with limit 8 now produces something like
	((1 2 3 4 5 6 7 8) 9 10 11 12)
instead of
	(((1 2 3 4 5 6 7 8) (9)) 10 11 12)
The latter structure is really only required for a FULL JOIN.
Noted while studying an example from Shane Ambler.

diff --git a/src/backend/optimizer/plan/initsplan.c b/src/backend/optimizer/plan/initsplan.c
index e52015d6dc92024f8c32f034fc5e3d1d82c819a7..3dc7229cf3557740b1ef61c190dd76d001e1da2f 100644 (file)
--- a/src/backend/optimizer/plan/initsplan.c
+++ b/src/backend/optimizer/plan/initsplan.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *   $PostgreSQL: pgsql/src/backend/optimizer/plan/initsplan.c,v 1.126 2007/01/05 22:19:31 momjian Exp $
+ *   $PostgreSQL: pgsql/src/backend/optimizer/plan/initsplan.c,v 1.127 2007/01/08 16:47:30 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -402,13 +402,34 @@ deconstruct_recurse(PlannerInfo *root, Node *jtnode, bool below_outer_join,
         * except at a FULL JOIN or where join_collapse_limit would be
         * exceeded.
         */
-       if (j->jointype != JOIN_FULL &&
-           (list_length(leftjoinlist) + list_length(rightjoinlist) <=
-            join_collapse_limit))
+       if (j->jointype == JOIN_FULL)
+       {
+           /* force the join order exactly at this node */
+           joinlist = list_make1(list_make2(leftjoinlist, rightjoinlist));
+       }
+       else if (list_length(leftjoinlist) + list_length(rightjoinlist) <=
+                join_collapse_limit)
+       {
+           /* OK to combine subproblems */
            joinlist = list_concat(leftjoinlist, rightjoinlist);
+       }
        else
-           /* force the join order at this node */
-           joinlist = list_make1(list_make2(leftjoinlist, rightjoinlist));
+       {
+           /* can't combine, but needn't force join order above here */
+           Node   *leftpart,
+                  *rightpart;
+
+           /* avoid creating useless 1-element sublists */
+           if (list_length(leftjoinlist) == 1)
+               leftpart = (Node *) linitial(leftjoinlist);
+           else
+               leftpart = (Node *) leftjoinlist;
+           if (list_length(rightjoinlist) == 1)
+               rightpart = (Node *) linitial(rightjoinlist);
+           else
+               rightpart = (Node *) rightjoinlist;
+           joinlist = list_make2(leftpart, rightpart);
+       }
    }
    else
    {