My recent fix for semijoin planning didn't actually work for a semijoin with a

RHS that can't be unique-ified --- join_is_legal has to check that before
deciding to build a join, else we'll have an unimplementable joinrel.
Per report from Greg Stark.

diff --git a/src/backend/optimizer/path/joinrels.c b/src/backend/optimizer/path/joinrels.c
index 5f49ee7e290136b631face5fd5ed16bd538b827b..5f2123defc548626ae38fcbc45a459b28a24b2b0 100644 (file)
--- a/src/backend/optimizer/path/joinrels.c
+++ b/src/backend/optimizer/path/joinrels.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *   $PostgreSQL: pgsql/src/backend/optimizer/path/joinrels.c,v 1.95 2008/11/22 22:47:06 tgl Exp $
+ *   $PostgreSQL: pgsql/src/backend/optimizer/path/joinrels.c,v 1.96 2008/11/28 19:29:07 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -420,11 +420,13 @@ join_is_legal(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2,
            reversed = true;
        }
        else if (sjinfo->jointype == JOIN_SEMI &&
-                bms_equal(sjinfo->syn_righthand, rel2->relids))
+                bms_equal(sjinfo->syn_righthand, rel2->relids) &&
+                create_unique_path(root, rel2, rel2->cheapest_total_path,
+                                   sjinfo) != NULL)
        {
            /*
             * For a semijoin, we can join the RHS to anything else by
-            * unique-ifying the RHS.
+            * unique-ifying the RHS (if the RHS can be unique-ified).
             */
            if (match_sjinfo)
                return false;   /* invalid join path */
@@ -432,7 +434,9 @@ join_is_legal(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2,
            reversed = false;
        }
        else if (sjinfo->jointype == JOIN_SEMI &&
-                bms_equal(sjinfo->syn_righthand, rel1->relids))
+                bms_equal(sjinfo->syn_righthand, rel1->relids) &&
+                create_unique_path(root, rel1, rel1->cheapest_total_path,
+                                   sjinfo) != NULL)
        {
            /* Reversed semijoin case */
            if (match_sjinfo)
@@ -664,7 +668,10 @@ make_join_rel(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2)
            /*
             * If we know how to unique-ify the RHS and one input rel is
             * exactly the RHS (not a superset) we can consider unique-ifying
-            * it and then doing a regular join.
+            * it and then doing a regular join.  (The create_unique_path
+            * check here is probably redundant with what join_is_legal did,
+            * but if so the check is cheap because it's cached.  So test
+            * anyway to be sure.)
             */
            if (bms_equal(sjinfo->syn_righthand, rel2->relids) &&
                create_unique_path(root, rel2, rel2->cheapest_total_path,