Fix brain fade in join-removal patch: a pushed-down clause in the outer join's

restrict list is not just something to ignore, it's actually grounds to
abandon the optimization entirely.  Per bug #5255 from Matteo Beccati.

diff --git a/src/backend/optimizer/path/joinpath.c b/src/backend/optimizer/path/joinpath.c
index 53fac037a636fb95d9cb1c5139e93d38cc9b5c01..43f7eac192c6f712dc37b5599bb1050a41ac7f9c 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 1.126 2009/09/19 17:48:09 tgl Exp $
+ *   $PostgreSQL: pgsql/src/backend/optimizer/path/joinpath.c,v 1.127 2009/12/25 17:11:32 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -228,6 +228,11 @@ join_is_removable(PlannerInfo *root,
     * We can't remove the join if any inner-rel attributes are used above
     * the join.
     *
+    * Note that this test only detects use of inner-rel attributes in
+    * higher join conditions and the target list.  There might be such
+    * attributes in pushed-down conditions at this join, too.  We check
+    * that case below.
+    *
     * As a micro-optimization, it seems better to start with max_attr and
     * count down rather than starting with min_attr and counting up, on the
     * theory that the system attributes are somewhat less likely to be wanted
@@ -253,13 +258,16 @@ join_is_removable(PlannerInfo *root,
        RestrictInfo *restrictinfo = (RestrictInfo *) lfirst(l);
 
        /*
-        * We are always considering an outer join here, so ignore pushed-down
-        * clauses.  Also ignore anything that doesn't have a mergejoinable
-        * operator.
+        * If we find a pushed-down clause, it must have come from above the
+        * outer join and it must contain references to the inner rel.  (If
+        * it had only outer-rel variables, it'd have been pushed down into
+        * the outer rel.)  Therefore, we can conclude that join removal
+        * is unsafe without any examination of the clause contents.
         */
        if (restrictinfo->is_pushed_down)
-           continue;
+           return false;
 
+       /* Ignore if it's not a mergejoinable clause */
        if (!restrictinfo->can_join ||
            restrictinfo->mergeopfamilies == NIL)
            continue;           /* not mergejoinable */