Fix longstanding bug that would sometimes let the planner generate a bad plan

for an outer join; symptom is bogus error "RIGHT JOIN is only supported with
merge-joinable join conditions".  Problem was that select_mergejoin_clauses
did its tests in the wrong order.  We need to force left join not right join
for a merge join when there are non-mergeable join clauses; but the test for
this only accounted for mergejoinability of the clause operator, and not
whether the left and right Vars were of the proper relations.  Per report
from Jean-Pierre Pelletier.

diff --git a/src/backend/optimizer/path/joinpath.c b/src/backend/optimizer/path/joinpath.c
index ab3f902f02b7bb6e8d2d2f0230272819aa97deae..3d6b333e31eb82ac779764b26f4cfed705f98f0e 100644 (file)
--- a/src/backend/optimizer/path/joinpath.c
+++ b/src/backend/optimizer/path/joinpath.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *       $PostgreSQL: pgsql/src/backend/optimizer/path/joinpath.c,v 1.96 2005/10/15 02:49:20 momjian Exp $
+ *       $PostgreSQL: pgsql/src/backend/optimizer/path/joinpath.c,v 1.97 2005/10/25 20:30:30 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -795,6 +795,7 @@ select_mergejoin_clauses(RelOptInfo *joinrel,
 {
        List       *result_list = NIL;
        bool            isouterjoin = IS_OUTER_JOIN(jointype);
+       bool            have_nonmergeable_joinclause = false;
        ListCell   *l;
 
        foreach(l, restrictlist)
@@ -803,42 +804,19 @@ select_mergejoin_clauses(RelOptInfo *joinrel,
 
                /*
                 * If processing an outer join, only use its own join clauses in the
-                * merge.  For inner joins we need not be so picky.
-                *
-                * Furthermore, if it is a right/full join then *all* the explicit join
-                * clauses must be mergejoinable, else the executor will fail. If we
-                * are asked for a right join then just return NIL to indicate no
-                * mergejoin is possible (we can handle it as a left join instead). If
-                * we are asked for a full join then emit an error, because there is
-                * no fallback.
+                * merge.  For inner joins we can use pushed-down clauses too.
+                * (Note: we don't set have_nonmergeable_joinclause here because
+                * pushed-down clauses will become otherquals not joinquals.)
                 */
-               if (isouterjoin)
-               {
-                       if (restrictinfo->is_pushed_down)
-                               continue;
-                       switch (jointype)
-                       {
-                               case JOIN_RIGHT:
-                                       if (!restrictinfo->can_join ||
-                                               restrictinfo->mergejoinoperator == InvalidOid)
-                                               return NIL;             /* not mergejoinable */
-                                       break;
-                               case JOIN_FULL:
-                                       if (!restrictinfo->can_join ||
-                                               restrictinfo->mergejoinoperator == InvalidOid)
-                                               ereport(ERROR,
-                                                               (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-                                                                errmsg("FULL JOIN is only supported with merge-joinable join conditions")));
-                                       break;
-                               default:
-                                       /* otherwise, it's OK to have nonmergeable join quals */
-                                       break;
-                       }
-               }
+               if (isouterjoin && restrictinfo->is_pushed_down)
+                       continue;
 
                if (!restrictinfo->can_join ||
                        restrictinfo->mergejoinoperator == InvalidOid)
+               {
+                       have_nonmergeable_joinclause = true;
                        continue;                       /* not mergejoinable */
+               }
 
                /*
                 * Check if clause is usable with these input rels.  All the vars
@@ -856,10 +834,37 @@ select_mergejoin_clauses(RelOptInfo *joinrel,
                        /* lefthand side is inner */
                }
                else
+               {
+                       have_nonmergeable_joinclause = true;
                        continue;                       /* no good for these input relations */
+               }
 
                result_list = lcons(restrictinfo, result_list);
        }
 
+       /*
+        * If it is a right/full join then *all* the explicit join clauses must be
+        * mergejoinable, else the executor will fail. If we are asked for a right
+        * join then just return NIL to indicate no mergejoin is possible (we can
+        * handle it as a left join instead). If we are asked for a full join then
+        * emit an error, because there is no fallback.
+        */
+       if (have_nonmergeable_joinclause)
+       {
+               switch (jointype)
+               {
+                       case JOIN_RIGHT:
+                               return NIL;             /* not mergejoinable */
+                       case JOIN_FULL:
+                               ereport(ERROR,
+                                               (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+                                                errmsg("FULL JOIN is only supported with merge-joinable join conditions")));
+                               break;
+                       default:
+                               /* otherwise, it's OK to have nonmergeable join quals */
+                               break;
+               }
+       }
+
        return result_list;
 }