Fix an old error in clause_selectivity: the default selectivity estimate

for unhandled clause types ought to be 0.5, not 1.0.  I fear I introduced
this silliness due to misreading the intent of the very-poorly-structured
code that was there when we inherited the file from Berkeley.  The lack
of sanity in this behavior was exposed by an example from Sim Zacks.
(Arguably this is a bug fix and should be back-patched, but I'm a bit
hesitant to introduce a possible planner behavior change in the back
branches; it might detune queries that worked acceptably in the past.)

While at it, make estimation for DistinctExpr do something marginally
realistic, rather than just defaulting.

diff --git a/src/backend/optimizer/path/clausesel.c b/src/backend/optimizer/path/clausesel.c
index 6a1dfc229d283ccdc0750f37443c6ddcf2386646..e358c6990e248e0c3d895c57baf40679b0c60e07 100644 (file)
--- a/src/backend/optimizer/path/clausesel.c
+++ b/src/backend/optimizer/path/clausesel.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *   $PostgreSQL: pgsql/src/backend/optimizer/path/clausesel.c,v 1.89 2008/01/01 19:45:50 momjian Exp $
+ *   $PostgreSQL: pgsql/src/backend/optimizer/path/clausesel.c,v 1.90 2008/01/11 17:00:45 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -428,7 +428,7 @@ clause_selectivity(PlannerInfo *root,
                   int varRelid,
                   JoinType jointype)
 {
-   Selectivity s1 = 1.0;       /* default for any unhandled clause type */
+   Selectivity s1 = 0.5;       /* default for any unhandled clause type */
    RestrictInfo *rinfo = NULL;
    bool        cacheable = false;
 
@@ -450,7 +450,7 @@ clause_selectivity(PlannerInfo *root,
        if (rinfo->pseudoconstant)
        {
            if (!IsA(rinfo->clause, Const))
-               return s1;
+               return (Selectivity) 1.0;
        }
 
        /*
@@ -517,9 +517,8 @@ clause_selectivity(PlannerInfo *root,
            {
                /*
                 * XXX not smart about subquery references... any way to do
-                * better?
+                * better than default?
                 */
-               s1 = 0.5;
            }
            else
            {
@@ -560,8 +559,7 @@ clause_selectivity(PlannerInfo *root,
        }
        else
        {
-           /* XXX any way to do better? */
-           s1 = (Selectivity) 0.5;
+           /* XXX any way to do better than default? */
        }
    }
    else if (not_clause(clause))
@@ -601,7 +599,7 @@ clause_selectivity(PlannerInfo *root,
            s1 = s1 + s2 - s1 * s2;
        }
    }
-   else if (is_opclause(clause))
+   else if (is_opclause(clause) || IsA(clause, DistinctExpr))
    {
        Oid         opno = ((OpExpr *) clause)->opno;
        bool        is_join_clause;
@@ -642,6 +640,15 @@ clause_selectivity(PlannerInfo *root,
                                         ((OpExpr *) clause)->args,
                                         varRelid);
        }
+
+       /*
+        * DistinctExpr has the same representation as OpExpr, but the
+        * contained operator is "=" not "<>", so we must negate the result.
+        * This estimation method doesn't give the right behavior for nulls,
+        * but it's better than doing nothing.
+        */
+       if (IsA(clause, DistinctExpr))
+           s1 = 1.0 - s1;
    }
    else if (is_funcclause(clause))
    {
@@ -652,6 +659,7 @@ clause_selectivity(PlannerInfo *root,
         */
        s1 = (Selectivity) 0.3333333;
    }
+#ifdef NOT_USED
    else if (is_subplan(clause))
    {
        /*
@@ -659,11 +667,7 @@ clause_selectivity(PlannerInfo *root,
         */
        s1 = (Selectivity) 0.5;
    }
-   else if (IsA(clause, DistinctExpr))
-   {
-       /* can we do better? */
-       s1 = (Selectivity) 0.5;
-   }
+#endif
    else if (IsA(clause, ScalarArrayOpExpr))
    {
        /* First, decide if it's a join clause, same as for OpExpr */